[ASTImporter] Added error handling for AST import.

[balazske]

Summary

The goal of this change is to make the ASTImporter::Import functions return
llvm::Expected instead of the imported type.
As first part the ASTNodeImporter visit functions are updated to return with
llvm::Expected. Various import functions are added to ASTNodeImporter to
simplify the code and have a common place for interface towards ASTImporter
(from ASTNodeImporter). There is some temporary code that is needed before
ASTImporter is updated.

[martong]

This patch is huge, but we change here almost all implementation functions of ASTNodeImporter to return with either Error or with Expected<T>.
We could not really come up with a cohesive but smaller patch because of the recursive nature of the importer. (We are open to ideas about how to cut this patch into smaller parts.)
Most of the changes are trivial: we always have to check the return value of any import function and pass on the error to the caller if there was any.
In my opinion one great simplification is the introduction of the auxiliary importSeq function. It imports each entity one-by-one after each other and in case of any error it returns with that error and does not continue with the subsequent import operations.

[a_sidorin]

Hi Gabor,

To make the review proces faster, you can split the review on separate parts for Stmts, Decls, Types, etc. Otherwise, the review can last for too long.

lib/AST/ASTImporter.cpp

162 ↗

(On Diff #163819)

Can we get rid from namespace specifier usages on Error and None? As I see, even in this patch Error is used without it sometimes.

417 ↗

(On Diff #163819)

The changes for argument indentation look redundant. Is it a result of clang-format?

840 ↗

(On Diff #163819)

Could you please add some newlines to this function? Its control flow is not trivial so some blocks need to be separated.

[balazske]

Hi,
Did you mean to split the review only or the patches?
It is not trivial how to make incremental patches for this change, every part is connected to the other. The import functions for a type (Decl or Stmt) call imports of other types (Expr for example) too. If the import is changed for a single type (only Stmt) to the new way every other visit (for Decl and others) needs to be updated. And then again with the next type (Stmt for example). The importSeq function calls work with any type and the visit functions that use importSeq can not work if a part of the imports is the old way, another the new.
A solution can be to have the new and old import interface together for every type (with different names), then a part of the visit functions can be updated. The first change would be to introduce the new interface (that returns Expected) for every type, but do not change the visit functions (but a new version of every other "helper" function like importDefinition is needed). If a new version of such a function is introduced and the old remains, we do not have a diff view (with relative changes) for it.

lib/AST/ASTImporter.cpp

417 ↗

(On Diff #163819)

clang-format aligns start of new argument lines to the ( character above. Indentations are not always consistent in this file, I can reformat the whole file with clang-format.
(But I do not like the formatting like

Error ImportDeclParts(NamedDecl *D, DeclContext *&DC, DeclContext *&LexicalDC,
                      DeclarationName &Name, NamedDecl *&ToD,
                      SourceLocation &Loc);

if there is a LongNamedClass::LongReturnType LongNamedClass::LongNamedFunction.)

[a_sidorin]

Hi Balasz,
It's going to take some time to review the whole patch.

lib/AST/ASTImporter.cpp

194 ↗

(On Diff #163819)

Do I understand correctly that we have to use the upper variant because ASTImporter API is unchanged?

417 ↗

(On Diff #163819)

No need to reformat the whole file. As I see, the indentation changes are not breaking and too wide to roll them back so we can leave them as-is.

1087 ↗

(On Diff #163819)

Nice catch!

1658 ↗

(On Diff #163819)

Space before '*' is needed.

1663 ↗

(On Diff #163819)

Silent fail doesn't look correct in case of structures. Should we add a FIXME?

1953 ↗

(On Diff #163819)

Should we add a FIXME for removing this method?

2266 ↗

(On Diff #163819)

Space after comma is lost.

2656 ↗

(On Diff #163819)

This move looks like an additional functional change. Could you please explain the reason?

2683 ↗

(On Diff #163819)

Same here.

2706 ↗

(On Diff #163819)

We usually don't qualify casts.

4136 ↗

(On Diff #163819)

// Stopped here.

[balazske]

lib/AST/ASTImporter.cpp

194 ↗

(On Diff #163819)

The "upper variant" (importInto(ImportT &To, const ImportT &From) form) is the final one. It is only to make the code more simple, you can call importInto in ASTImporter instead of Importer.importInto. This second variant is needed because the ASTImporter API is not changed (yet), the Import functions still return pointer (later they will return Expected).
But this special version for pointer is still needed to have type cast for pointers. The commented-out code should be used later.

1663 ↗

(On Diff #163819)

Maybe yes. But aborting the whole import of the DeclContext could cause lots of not imported things. A warning can be printed but maybe it is already printed at the called failed import. There was no FIXME in the old code either.

[a_sidorin]

Hi Balázs,

Finally, I have completed the review. Despite the fact that this patch is huge, I found only a few issues.

lib/AST/ASTImporter.cpp

4604 ↗

(On Diff #163819)

I think we can just return std::move(Err); as it was done below.

5657 ↗

(On Diff #163819)

Shouldn't we return an error here?

7339 ↗

(On Diff #163819)

That's true, this copying can be removed.

[balazske]

lib/AST/ASTImporter.cpp

2683 ↗

(On Diff #163819)

I do not know exactly why this was made, it looks like a bugfix for a problem encountered during the tests. Maybe I can undo this change (there are other related problem fixes so this is probably not needed).

4604 ↗

(On Diff #163819)

Changed to return the error.

5657 ↗

(On Diff #163819)

Good catch, this looks like the change was missed.

[balazske]

• Fix in ASTNodeImporter::VisitCXXDependentScopeMemberExpr.
• Removed trailing spaces in ASTImporter.cpp.
• Corrected formatting and small problem fixes.

[balazske]

Rebase to current master.

[balazske]

• ASTImporter::ImportDefinition compatibility rename.

[balazske]

In the current version all clang tests do pass and most of the LLDB tests. Some LLDB tests do not work on my machine, the problems exist on master too.
This code is still a temporary version (possible to commit now or wait until every change is made?). Later the Import functions should return Expected and the remaining functions in ASTImporter.cpp should be updated. For compatibility with old code in LLDB the Import functions can not be changed without LLDB code changes so a Import_New will be created instead. Code in clang will only use the Import_New. When LLDB was updated, Import_New should be renamed to Import.

[balazske]

To the reviewers: Please accept this patch formally if you do not find any problems. This is an intermediate state of the code and there is more work that is dependent on this change.

[a_sidorin]

Hi Balasz,
I cannot find any problems with the latest changes. I suggest you to commit the patch to make further reviews easier.

lib/AST/ASTImporter.cpp

8197 ↗

(On Diff #169975)

ImportDefinitionOrError?

[balazske]

lib/AST/ASTImporter.cpp

8197 ↗

(On Diff #169975)

The intention was to make the _New name only temporary until only this new function is used therefore it is not a nice name (requires changes in LLDB). Then it should be renamed back to ImportDefinition. It is probably not needed to indicate in the name that the function can fail, it can be seen from the return value. (This will be the case with other Import functions later.)

[balazske]

Rebase again to last master.

• Fix in ASTNodeImporter::VisitCXXDependentScopeMemberExpr.
• Removed trailing spaces in ASTImporter.cpp.
• Corrected formatting and small problem fixes.
• Removed llvm namespace qualifier from Error and None.
• ASTImporter::ImportDefinition compatibility rename.

cfe/trunk/include/clang/AST/ASTImporter.h

 #include "clang/AST/TemplateName.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/SourceLocation.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/Error.h"
 #include <utility>
 
 namespace clang {
 
 class ASTContext;
 class CXXBaseSpecifier;
 class CXXCtorInitializer;
 class Decl;
 class DeclContext;
 class Expr;
 class FileManager;
 class NamedDecl;
 class Stmt;
 class TagDecl;
 class TypeSourceInfo;
 class Attr;
 
+  class ImportError : public llvm::ErrorInfo<ImportError> {
+  public:
+    /// \brief Kind of error when importing an AST component.
+    enum ErrorKind {
+        NameConflict, /// Naming ambiguity (likely ODR violation).
+        UnsupportedConstruct, /// Not supported node or case.
+        Unknown /// Other error.
+    };
+
+    ErrorKind Error;
+
+    static char ID;
+
+    ImportError() : Error(Unknown) { }
+    ImportError(const ImportError &Other) : Error(Other.Error) { }
+    ImportError(ErrorKind Error) : Error(Error) { }
+
+    std::string toString() const;
+
+    void log(raw_ostream &OS) const override;
+    std::error_code convertToErrorCode() const override;
+  };
+
   // \brief Returns with a list of declarations started from the canonical decl
   // then followed by subsequent decls in the translation unit.
   // This gives a canonical list for each entry in the redecl chain.
   // `Decl::redecls()` gives a list of decls which always start from the
   // previous decl and the next item is actually the previous item in the order
   // of source locations.  Thus, `Decl::redecls()` gives different lists for
   // the different entries in a given redecl chain.
   llvm::SmallVector<Decl*, 2> getCanonicalForwardRedeclChain(Decl* D);
ASTImporter(ASTContext &ToContext, FileManager &ToFileManager,
                 bool MinimalImport);
 
     virtual ~ASTImporter();
 
     /// Whether the importer will perform a minimal import, creating
     /// to-be-completed forward declarations when possible.
     bool isMinimalImport() const { return Minimal; }
 
+    /// \brief Import the given object, returns the result.
+    ///
+    /// \param To Import the object into this variable.
+    /// \param From Object to import.
+    /// \return Error information (success or error).
+    template <typename ImportT>
+    LLVM_NODISCARD llvm::Error importInto(ImportT &To, const ImportT &From) {
+      To = Import(From);
+      if (From && !To)
+          return llvm::make_error<ImportError>();
+      return llvm::Error::success();
+      // FIXME: this should be the final code
+      //auto ToOrErr = Import(From);
+      //if (ToOrErr)
+      //  To = *ToOrErr;
+      //return ToOrErr.takeError();
+    }
+
     /// Import the given type from the "from" context into the "to"
     /// context.
     ///
     /// \returns the equivalent type in the "to" context, or a NULL type if
     /// an error occurred.
     QualType Import(QualType FromT);
 
     /// Import the given type source information from the
public:
     /// it has already been imported from the "from" context.  Otherwise return
     /// NULL.
     Decl *GetAlreadyImportedOrNull(Decl *FromD);
 
     /// Import the given declaration context from the "from"
     /// AST context into the "to" AST context.
     ///
     /// \returns the equivalent declaration context in the "to"
-    /// context, or a NULL type if an error occurred.
-    DeclContext *ImportContext(DeclContext *FromDC);
+    /// context, or error value.
+    llvm::Expected<DeclContext *> ImportContext(DeclContext *FromDC);
 
     /// Import the given expression from the "from" context into the
     /// "to" context.
     ///
     /// \returns the equivalent expression in the "to" context, or NULL if
     /// an error occurred.
     Expr *Import(Expr *FromE);
 
public:
     ///
     /// \returns the equivalent declaration name in the "to" context,
     /// or an empty declaration name if an error occurred.
     DeclarationName Import(DeclarationName FromName);
 
     /// Import the given identifier from the "from" context
     /// into the "to" context.
     ///
-    /// \returns the equivalent identifier in the "to" context.
+    /// \returns the equivalent identifier in the "to" context. Note: It
+    /// returns nullptr only if the FromId was nullptr.
     IdentifierInfo *Import(const IdentifierInfo *FromId);
 
     /// Import the given Objective-C selector from the "from"
     /// context into the "to" context.
     ///
     /// \returns the equivalent selector in the "to" context.
     Selector Import(Selector FromSel);
 
public:
     /// the "to" context.
     ///
     /// \returns the equivalent CXXBaseSpecifier in the source manager of the
     /// "to" context.
     CXXBaseSpecifier *Import(const CXXBaseSpecifier *FromSpec);
 
     /// Import the definition of the given declaration, including all of
     /// the declarations it contains.
-    ///
-    /// This routine is intended to be used
+    LLVM_NODISCARD llvm::Error ImportDefinition_New(Decl *From);
+
+    // FIXME: Compatibility function.
+    // Usages of this should be changed to ImportDefinition_New.
     void ImportDefinition(Decl *From);
 
     /// Cope with a name conflict when importing a declaration into the
     /// given context.
     ///
     /// This routine is invoked whenever there is a name conflict while
     /// importing a declaration. The returned name will become the name of the
     /// imported declaration. By default, the returned name is the same as the
public:
 
     /// Determine whether the given types are structurally
     /// equivalent.
     bool IsStructurallyEquivalent(QualType From, QualType To,
                                   bool Complain = true);
 
     /// Determine the index of a field in its parent record.
     /// F should be a field (or indirect field) declaration.
-    /// \returns The index of the field in its parent context, starting from 1.
-    /// 0 is returned on error (parent context is non-record).
-    static unsigned getFieldIndex(Decl *F);
+    /// \returns The index of the field in its parent context (starting from 0).
+    /// On error `None` is returned (parent context is non-record).
+    static llvm::Optional<unsigned> getFieldIndex(Decl *F);
 
   };
 
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_ASTIMPORTER_H

cfe/trunk/lib/AST/ASTImporter.cpp

 #include <cassert>
 #include <cstddef>
 #include <memory>
 #include <type_traits>
 #include <utility>
 
 namespace clang {
 
-  unsigned ASTImporter::getFieldIndex(Decl *F) {
-    assert(F && (isa<FieldDecl>(*F) || isa<IndirectFieldDecl>(*F)) &&
-        "Try to get field index for non-field.");
-
-    auto *Owner = dyn_cast<RecordDecl>(F->getDeclContext());
-    if (!Owner)
-      return 0;
-
-    unsigned Index = 1;
-    for (const auto *D : Owner->decls()) {
-      if (D == F)
-        return Index;
-
-      if (isa<FieldDecl>(*D) || isa<IndirectFieldDecl>(*D))
-        ++Index;
-    }
+  using llvm::make_error;
+  using llvm::Error;
+  using llvm::Expected;
+  using ExpectedType = llvm::Expected<QualType>;
+  using ExpectedStmt = llvm::Expected<Stmt *>;
+  using ExpectedExpr = llvm::Expected<Expr *>;
+  using ExpectedDecl = llvm::Expected<Decl *>;
+  using ExpectedSLoc = llvm::Expected<SourceLocation>;
+
+  std::string ImportError::toString() const {
+    // FIXME: Improve error texts.
+    switch (Error) {
+    case NameConflict:
+      return "NameConflict";
+    case UnsupportedConstruct:
+      return "UnsupportedConstruct";
+    case Unknown:
+      return "Unknown error";
+    default:
+      llvm_unreachable("Invalid error code.");
+    }
 
-    llvm_unreachable("Field was not found in its parent context.");
+  void ImportError::log(raw_ostream &OS) const {
+    OS << toString();
+  }
 
-    return 0;
+  std::error_code ImportError::convertToErrorCode() const {
+    llvm_unreachable("Function not implemented.");
   }
 
+  char ImportError::ID;
+
   template <class T>
-  SmallVector<Decl*, 2>
+  SmallVector<Decl *, 2>
   getCanonicalForwardRedeclChain(Redeclarable<T>* D) {
-    SmallVector<Decl*, 2> Redecls;
+    SmallVector<Decl *, 2> Redecls;
     for (auto *R : D->getFirstDecl()->redecls()) {
       if (R != D->getFirstDecl())
         Redecls.push_back(R);
     }
     Redecls.push_back(D->getFirstDecl());
     std::reverse(Redecls.begin(), Redecls.end());
     return Redecls;
   }
 
   SmallVector<Decl*, 2> getCanonicalForwardRedeclChain(Decl* D) {
     if (auto *FD = dyn_cast<FunctionDecl>(D))
       return getCanonicalForwardRedeclChain<FunctionDecl>(FD);
     if (auto *VD = dyn_cast<VarDecl>(D))
       return getCanonicalForwardRedeclChain<VarDecl>(VD);
     llvm_unreachable("Bad declaration kind");
   }
 
   void updateFlags(const Decl *From, Decl *To) {
     // Check if some flags or attrs are new in 'From' and copy into 'To'.
     // FIXME: Other flags or attrs?
     if (From->isUsed(false) && !To->isUsed(false))
       To->setIsUsed();
   }
 
-  class ASTNodeImporter : public TypeVisitor<ASTNodeImporter, QualType>,
-                          public DeclVisitor<ASTNodeImporter, Decl *>,
-                          public StmtVisitor<ASTNodeImporter, Stmt *> {
+  Optional<unsigned> ASTImporter::getFieldIndex(Decl *F) {
+    assert(F && (isa<FieldDecl>(*F) || isa<IndirectFieldDecl>(*F)) &&
+        "Try to get field index for non-field.");
+
+    auto *Owner = dyn_cast<RecordDecl>(F->getDeclContext());
+    if (!Owner)
+      return None;
+
+    unsigned Index = 0;
+    for (const auto *D : Owner->decls()) {
+      if (D == F)
+        return Index;
+
+      if (isa<FieldDecl>(*D) || isa<IndirectFieldDecl>(*D))
+        ++Index;
+    }
+
+    llvm_unreachable("Field was not found in its parent context.");
+
+    return None;
+  }
+
+  // FIXME: Temporary until every import returns Expected.
+  template <>
+  LLVM_NODISCARD Error
+  ASTImporter::importInto(SourceLocation &To, const SourceLocation &From) {
+    To = Import(From);
+    if (From.isValid() && To.isInvalid())
+        return llvm::make_error<ImportError>();
+    return Error::success();
+  }
+  // FIXME: Temporary until every import returns Expected.
+  template <>
+  LLVM_NODISCARD Error
+  ASTImporter::importInto(QualType &To, const QualType &From) {
+    To = Import(From);
+    if (!From.isNull() && To.isNull())
+        return llvm::make_error<ImportError>();
+    return Error::success();
+  }
+
+  class ASTNodeImporter : public TypeVisitor<ASTNodeImporter, ExpectedType>,
+                          public DeclVisitor<ASTNodeImporter, ExpectedDecl>,
+                          public StmtVisitor<ASTNodeImporter, ExpectedStmt> {
     ASTImporter &Importer;
 
-    // Wrapper for an overload set.
+    // Use this instead of Importer.importInto .
+    template <typename ImportT>
+    LLVM_NODISCARD Error importInto(ImportT &To, const ImportT &From) {
+      return Importer.importInto(To, From);
+    }
+
+    // Use this to import pointers of specific type.
+    template <typename ImportT>
+    LLVM_NODISCARD Error importInto(ImportT *&To, ImportT *From) {
+      auto ToI = Importer.Import(From);
+      if (!ToI && From)
+        return make_error<ImportError>();
+      To = cast_or_null<ImportT>(ToI);
+      return Error::success();
+      // FIXME: This should be the final code.
+      //auto ToOrErr = Importer.Import(From);
+      //if (ToOrErr) {
+      //  To = cast_or_null<ImportT>(*ToOrErr);
+      //}
+      //return ToOrErr.takeError();
+    }
+
+    // Call the import function of ASTImporter for a baseclass of type `T` and
+    // cast the return value to `T`.
+    template <typename T>
+    Expected<T *> import(T *From) {
+      auto *To = Importer.Import(From);
+      if (!To && From)
+        return make_error<ImportError>();
+      return cast_or_null<T>(To);
+      // FIXME: This should be the final code.
+      //auto ToOrErr = Importer.Import(From);
+      //if (!ToOrErr)
+      //  return ToOrErr.takeError();
+      //return cast_or_null<T>(*ToOrErr);
+    }
+
+    template <typename T>
+    Expected<T *> import(const T *From) {
+      return import(const_cast<T *>(From));
+    }
+
+    // Call the import function of ASTImporter for type `T`.
+    template <typename T>
+    Expected<T> import(const T &From) {
+      T To = Importer.Import(From);
+      T DefaultT;
+      if (To == DefaultT && !(From == DefaultT))
+        return make_error<ImportError>();
+      return To;
+      // FIXME: This should be the final code.
+      //return Importer.Import(From);
+    }
+
+    template <class T>
+    Expected<std::tuple<T>>
+    importSeq(const T &From) {
+      Expected<T> ToOrErr = import(From);
+      if (!ToOrErr)
+        return ToOrErr.takeError();
+      return std::make_tuple<T>(std::move(*ToOrErr));
+    }
+
+    // Import multiple objects with a single function call.
+    // This should work for every type for which a variant of `import` exists.
+    // The arguments are processed from left to right and import is stopped on
+    // first error.
+    template <class THead, class... TTail>
+    Expected<std::tuple<THead, TTail...>>
+    importSeq(const THead &FromHead, const TTail &...FromTail) {
+      Expected<std::tuple<THead>> ToHeadOrErr = importSeq(FromHead);
+      if (!ToHeadOrErr)
+        return ToHeadOrErr.takeError();
+      Expected<std::tuple<TTail...>> ToTailOrErr = importSeq(FromTail...);
+      if (!ToTailOrErr)
+        return ToTailOrErr.takeError();
+      return std::tuple_cat(*ToHeadOrErr, *ToTailOrErr);
+    }
+
     template <typename ToDeclT> struct CallOverloadedCreateFun {
       template <typename... Args>
       auto operator()(Args &&... args)
           -> decltype(ToDeclT::Create(std::forward<Args>(args)...)) {
         return ToDeclT::Create(std::forward<Args>(args)...);
       }
     };
 
// Wrapper for an overload set.
     }
     // Use this version if a special create function must be
     // used, e.g. CXXRecordDecl::CreateLambda .
     template <typename ToDeclT, typename CreateFunT, typename FromDeclT,
               typename... Args>
     LLVM_NODISCARD bool
     GetImportedOrCreateSpecialDecl(ToDeclT *&ToD, CreateFunT CreateFun,
                                    FromDeclT *FromD, Args &&... args) {
+      // FIXME: This code is needed later.
+      //if (Importer.getImportDeclErrorIfAny(FromD)) {
+      //  ToD = nullptr;
+      //  return true; // Already imported but with error.
+      //}
       ToD = cast_or_null<ToDeclT>(Importer.GetAlreadyImportedOrNull(FromD));
       if (ToD)
         return true; // Already imported.
       ToD = CreateFun(std::forward<Args>(args)...);
       InitializeImportedDecl(FromD, ToD);
       return false; // A new Decl is created.
     }
 
     void InitializeImportedDecl(Decl *FromD, Decl *ToD) {
       Importer.MapImported(FromD, ToD);
       ToD->IdentifierNamespace = FromD->IdentifierNamespace;
       if (FromD->hasAttrs())
         for (const Attr *FromAttr : FromD->getAttrs())
           ToD->addAttr(Importer.Import(FromAttr));
       if (FromD->isUsed())
         ToD->setIsUsed();
       if (FromD->isImplicit())
         ToD->setImplicit();
     }
 
   public:
     explicit ASTNodeImporter(ASTImporter &Importer) : Importer(Importer) {}
 
-    using TypeVisitor<ASTNodeImporter, QualType>::Visit;
-    using DeclVisitor<ASTNodeImporter, Decl *>::Visit;
-    using StmtVisitor<ASTNodeImporter, Stmt *>::Visit;
+    using TypeVisitor<ASTNodeImporter, ExpectedType>::Visit;
+    using DeclVisitor<ASTNodeImporter, ExpectedDecl>::Visit;
+    using StmtVisitor<ASTNodeImporter, ExpectedStmt>::Visit;
 
     // Importing types
-    QualType VisitType(const Type *T);
-    QualType VisitAtomicType(const AtomicType *T);
-    QualType VisitBuiltinType(const BuiltinType *T);
-    QualType VisitDecayedType(const DecayedType *T);
-    QualType VisitComplexType(const ComplexType *T);
-    QualType VisitPointerType(const PointerType *T);
-    QualType VisitBlockPointerType(const BlockPointerType *T);
-    QualType VisitLValueReferenceType(const LValueReferenceType *T);
-    QualType VisitRValueReferenceType(const RValueReferenceType *T);
-    QualType VisitMemberPointerType(const MemberPointerType *T);
-    QualType VisitConstantArrayType(const ConstantArrayType *T);
-    QualType VisitIncompleteArrayType(const IncompleteArrayType *T);
-    QualType VisitVariableArrayType(const VariableArrayType *T);
-    QualType VisitDependentSizedArrayType(const DependentSizedArrayType *T);
+    ExpectedType VisitType(const Type *T);
+    ExpectedType VisitAtomicType(const AtomicType *T);
+    ExpectedType VisitBuiltinType(const BuiltinType *T);
+    ExpectedType VisitDecayedType(const DecayedType *T);
+    ExpectedType VisitComplexType(const ComplexType *T);
+    ExpectedType VisitPointerType(const PointerType *T);
+    ExpectedType VisitBlockPointerType(const BlockPointerType *T);
+    ExpectedType VisitLValueReferenceType(const LValueReferenceType *T);
+    ExpectedType VisitRValueReferenceType(const RValueReferenceType *T);
+    ExpectedType VisitMemberPointerType(const MemberPointerType *T);
+    ExpectedType VisitConstantArrayType(const ConstantArrayType *T);
+    ExpectedType VisitIncompleteArrayType(const IncompleteArrayType *T);
+    ExpectedType VisitVariableArrayType(const VariableArrayType *T);
+    ExpectedType VisitDependentSizedArrayType(const DependentSizedArrayType *T);
     // FIXME: DependentSizedExtVectorType
-    QualType VisitVectorType(const VectorType *T);
-    QualType VisitExtVectorType(const ExtVectorType *T);
-    QualType VisitFunctionNoProtoType(const FunctionNoProtoType *T);
-    QualType VisitFunctionProtoType(const FunctionProtoType *T);
-    QualType VisitUnresolvedUsingType(const UnresolvedUsingType *T);
-    QualType VisitParenType(const ParenType *T);
-    QualType VisitTypedefType(const TypedefType *T);
-    QualType VisitTypeOfExprType(const TypeOfExprType *T);
+    ExpectedType VisitVectorType(const VectorType *T);
+    ExpectedType VisitExtVectorType(const ExtVectorType *T);
+    ExpectedType VisitFunctionNoProtoType(const FunctionNoProtoType *T);
+    ExpectedType VisitFunctionProtoType(const FunctionProtoType *T);
+    ExpectedType VisitUnresolvedUsingType(const UnresolvedUsingType *T);
+    ExpectedType VisitParenType(const ParenType *T);
+    ExpectedType VisitTypedefType(const TypedefType *T);
+    ExpectedType VisitTypeOfExprType(const TypeOfExprType *T);
     // FIXME: DependentTypeOfExprType
-    QualType VisitTypeOfType(const TypeOfType *T);
-    QualType VisitDecltypeType(const DecltypeType *T);
-    QualType VisitUnaryTransformType(const UnaryTransformType *T);
-    QualType VisitAutoType(const AutoType *T);
-    QualType VisitInjectedClassNameType(const InjectedClassNameType *T);
+    ExpectedType VisitTypeOfType(const TypeOfType *T);
+    ExpectedType VisitDecltypeType(const DecltypeType *T);
+    ExpectedType VisitUnaryTransformType(const UnaryTransformType *T);
+    ExpectedType VisitAutoType(const AutoType *T);
+    ExpectedType VisitInjectedClassNameType(const InjectedClassNameType *T);
     // FIXME: DependentDecltypeType
-    QualType VisitRecordType(const RecordType *T);
-    QualType VisitEnumType(const EnumType *T);
-    QualType VisitAttributedType(const AttributedType *T);
-    QualType VisitTemplateTypeParmType(const TemplateTypeParmType *T);
-    QualType VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T);
-    QualType VisitTemplateSpecializationType(const TemplateSpecializationType *T);
-    QualType VisitElaboratedType(const ElaboratedType *T);
-    QualType VisitDependentNameType(const DependentNameType *T);
-    QualType VisitPackExpansionType(const PackExpansionType *T);
-    QualType VisitDependentTemplateSpecializationType(
+    ExpectedType VisitRecordType(const RecordType *T);
+    ExpectedType VisitEnumType(const EnumType *T);
+    ExpectedType VisitAttributedType(const AttributedType *T);
+    ExpectedType VisitTemplateTypeParmType(const TemplateTypeParmType *T);
+    ExpectedType VisitSubstTemplateTypeParmType(
+        const SubstTemplateTypeParmType *T);
+    ExpectedType VisitTemplateSpecializationType(
+        const TemplateSpecializationType *T);
+    ExpectedType VisitElaboratedType(const ElaboratedType *T);
+    ExpectedType VisitDependentNameType(const DependentNameType *T);
+    ExpectedType VisitPackExpansionType(const PackExpansionType *T);
+    ExpectedType VisitDependentTemplateSpecializationType(
         const DependentTemplateSpecializationType *T);
-    QualType VisitObjCInterfaceType(const ObjCInterfaceType *T);
-    QualType VisitObjCObjectType(const ObjCObjectType *T);
-    QualType VisitObjCObjectPointerType(const ObjCObjectPointerType *T);
+    ExpectedType VisitObjCInterfaceType(const ObjCInterfaceType *T);
+    ExpectedType VisitObjCObjectType(const ObjCObjectType *T);
+    ExpectedType VisitObjCObjectPointerType(const ObjCObjectPointerType *T);
 
     // Importing declarations
-    bool ImportDeclParts(NamedDecl *D, DeclContext *&DC,
-                         DeclContext *&LexicalDC, DeclarationName &Name,
-                         NamedDecl *&ToD, SourceLocation &Loc);
-    void ImportDefinitionIfNeeded(Decl *FromD, Decl *ToD = nullptr);
-    void ImportDeclarationNameLoc(const DeclarationNameInfo &From,
-                                  DeclarationNameInfo& To);
-    void ImportDeclContext(DeclContext *FromDC, bool ForceImport = false);
-    void ImportImplicitMethods(const CXXRecordDecl *From, CXXRecordDecl *To);
+    Error ImportDeclParts(
+        NamedDecl *D, DeclContext *&DC, DeclContext *&LexicalDC,
+        DeclarationName &Name, NamedDecl *&ToD, SourceLocation &Loc);
+    Error ImportDefinitionIfNeeded(Decl *FromD, Decl *ToD = nullptr);
+    Error ImportDeclarationNameLoc(
+        const DeclarationNameInfo &From, DeclarationNameInfo &To);
+    Error ImportDeclContext(DeclContext *FromDC, bool ForceImport = false);
+    Error ImportDeclContext(
+        Decl *From, DeclContext *&ToDC, DeclContext *&ToLexicalDC);
+    Error ImportImplicitMethods(const CXXRecordDecl *From, CXXRecordDecl *To);
 
-    bool ImportCastPath(CastExpr *E, CXXCastPath &Path);
+    Expected<CXXCastPath> ImportCastPath(CastExpr *E);
 
     using Designator = DesignatedInitExpr::Designator;
 
-    Designator ImportDesignator(const Designator &D);
-
-    Optional<LambdaCapture> ImportLambdaCapture(const LambdaCapture &From);
-
     /// What we should import from the definition.
     enum ImportDefinitionKind {
       /// Import the default subset of the definition, which might be
       /// nothing (if minimal import is set) or might be everything (if minimal
       /// import is not set).
       IDK_Default,
-
       /// Import everything.
       IDK_Everything,
-
       /// Import only the bare bones needed to establish a valid
       /// DeclContext.
       IDK_Basic
     };
 
     bool shouldForceImportDeclContext(ImportDefinitionKind IDK) {
       return IDK == IDK_Everything ||
              (IDK == IDK_Default && !Importer.isMinimalImport());
     }
 
-    bool ImportInitializer(VarDecl *From, VarDecl *To);
-    bool ImportDefinition(RecordDecl *From, RecordDecl *To,
-                          ImportDefinitionKind Kind = IDK_Default);
-    bool ImportDefinition(EnumDecl *From, EnumDecl *To,
-                          ImportDefinitionKind Kind = IDK_Default);
-    bool ImportDefinition(ObjCInterfaceDecl *From, ObjCInterfaceDecl *To,
-                          ImportDefinitionKind Kind = IDK_Default);
-    bool ImportDefinition(ObjCProtocolDecl *From, ObjCProtocolDecl *To,
-                          ImportDefinitionKind Kind = IDK_Default);
-    TemplateParameterList *ImportTemplateParameterList(
+    Error ImportInitializer(VarDecl *From, VarDecl *To);
+    Error ImportDefinition(
+        RecordDecl *From, RecordDecl *To,
+    Error ImportDefinition(
+        EnumDecl *From, EnumDecl *To,
+    Error ImportDefinition(
+        ObjCInterfaceDecl *From, ObjCInterfaceDecl *To,
+    Error ImportDefinition(
+        ObjCProtocolDecl *From, ObjCProtocolDecl *To,
+    Expected<TemplateParameterList *> ImportTemplateParameterList(
         TemplateParameterList *Params);
-    TemplateArgument ImportTemplateArgument(const TemplateArgument &From);
-    Optional<TemplateArgumentLoc> ImportTemplateArgumentLoc(
-        const TemplateArgumentLoc &TALoc);
-    bool ImportTemplateArguments(const TemplateArgument *FromArgs,
-                                 unsigned NumFromArgs,
-                                 SmallVectorImpl<TemplateArgument> &ToArgs);
+    Error ImportTemplateArguments(
+        const TemplateArgument *FromArgs, unsigned NumFromArgs,
+    Expected<TemplateArgument>
+    ImportTemplateArgument(const TemplateArgument &From);
 
     template <typename InContainerTy>
-    bool ImportTemplateArgumentListInfo(const InContainerTy &Container,
-                                        TemplateArgumentListInfo &ToTAInfo);
+    Error ImportTemplateArgumentListInfo(
+        const InContainerTy &Container, TemplateArgumentListInfo &ToTAInfo);
 
     template<typename InContainerTy>
-    bool ImportTemplateArgumentListInfo(SourceLocation FromLAngleLoc,
-                                        SourceLocation FromRAngleLoc,
-                                        const InContainerTy &Container,
-                                        TemplateArgumentListInfo &Result);
+    Error ImportTemplateArgumentListInfo(
+      SourceLocation FromLAngleLoc, SourceLocation FromRAngleLoc,
+      const InContainerTy &Container, TemplateArgumentListInfo &Result);
 
     using TemplateArgsTy = SmallVector<TemplateArgument, 8>;
-    using OptionalTemplateArgsTy = Optional<TemplateArgsTy>;
-    std::tuple<FunctionTemplateDecl *, OptionalTemplateArgsTy>
+    using FunctionTemplateAndArgsTy =
+        std::tuple<FunctionTemplateDecl *, TemplateArgsTy>;
+    Expected<FunctionTemplateAndArgsTy>
     ImportFunctionTemplateWithTemplateArgsFromSpecialization(
         FunctionDecl *FromFD);
 
-    bool ImportTemplateInformation(FunctionDecl *FromFD, FunctionDecl *ToFD);
+    Error ImportTemplateInformation(FunctionDecl *FromFD, FunctionDecl *ToFD);
 
     bool IsStructuralMatch(Decl *From, Decl *To, bool Complain);
     bool IsStructuralMatch(RecordDecl *FromRecord, RecordDecl *ToRecord,
                            bool Complain = true);
     bool IsStructuralMatch(VarDecl *FromVar, VarDecl *ToVar,
                            bool Complain = true);
     bool IsStructuralMatch(EnumDecl *FromEnum, EnumDecl *ToRecord);
     bool IsStructuralMatch(EnumConstantDecl *FromEC, EnumConstantDecl *ToEC);
     bool IsStructuralMatch(FunctionTemplateDecl *From,
                            FunctionTemplateDecl *To);
     bool IsStructuralMatch(FunctionDecl *From, FunctionDecl *To);
     bool IsStructuralMatch(ClassTemplateDecl *From, ClassTemplateDecl *To);
     bool IsStructuralMatch(VarTemplateDecl *From, VarTemplateDecl *To);
-    Decl *VisitDecl(Decl *D);
-    Decl *VisitEmptyDecl(EmptyDecl *D);
-    Decl *VisitAccessSpecDecl(AccessSpecDecl *D);
-    Decl *VisitStaticAssertDecl(StaticAssertDecl *D);
-    Decl *VisitTranslationUnitDecl(TranslationUnitDecl *D);
-    Decl *VisitNamespaceDecl(NamespaceDecl *D);
-    Decl *VisitNamespaceAliasDecl(NamespaceAliasDecl *D);
-    Decl *VisitTypedefNameDecl(TypedefNameDecl *D, bool IsAlias);
-    Decl *VisitTypedefDecl(TypedefDecl *D);
-    Decl *VisitTypeAliasDecl(TypeAliasDecl *D);
-    Decl *VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D);
-    Decl *VisitLabelDecl(LabelDecl *D);
-    Decl *VisitEnumDecl(EnumDecl *D);
-    Decl *VisitRecordDecl(RecordDecl *D);
-    Decl *VisitEnumConstantDecl(EnumConstantDecl *D);
-    Decl *VisitFunctionDecl(FunctionDecl *D);
-    Decl *VisitCXXMethodDecl(CXXMethodDecl *D);
-    Decl *VisitCXXConstructorDecl(CXXConstructorDecl *D);
-    Decl *VisitCXXDestructorDecl(CXXDestructorDecl *D);
-    Decl *VisitCXXConversionDecl(CXXConversionDecl *D);
-    Decl *VisitFieldDecl(FieldDecl *D);
-    Decl *VisitIndirectFieldDecl(IndirectFieldDecl *D);
-    Decl *VisitFriendDecl(FriendDecl *D);
-    Decl *VisitObjCIvarDecl(ObjCIvarDecl *D);
-    Decl *VisitVarDecl(VarDecl *D);
-    Decl *VisitImplicitParamDecl(ImplicitParamDecl *D);
-    Decl *VisitParmVarDecl(ParmVarDecl *D);
-    Decl *VisitObjCMethodDecl(ObjCMethodDecl *D);
-    Decl *VisitObjCTypeParamDecl(ObjCTypeParamDecl *D);
-    Decl *VisitObjCCategoryDecl(ObjCCategoryDecl *D);
-    Decl *VisitObjCProtocolDecl(ObjCProtocolDecl *D);
-    Decl *VisitLinkageSpecDecl(LinkageSpecDecl *D);
-    Decl *VisitUsingDecl(UsingDecl *D);
-    Decl *VisitUsingShadowDecl(UsingShadowDecl *D);
-    Decl *VisitUsingDirectiveDecl(UsingDirectiveDecl *D);
-    Decl *VisitUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *D);
-    Decl *VisitUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D);
-
-    ObjCTypeParamList *ImportObjCTypeParamList(ObjCTypeParamList *list);
-    Decl *VisitObjCInterfaceDecl(ObjCInterfaceDecl *D);
-    Decl *VisitObjCCategoryImplDecl(ObjCCategoryImplDecl *D);
-    Decl *VisitObjCImplementationDecl(ObjCImplementationDecl *D);
-    Decl *VisitObjCPropertyDecl(ObjCPropertyDecl *D);
-    Decl *VisitObjCPropertyImplDecl(ObjCPropertyImplDecl *D);
-    Decl *VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D);
-    Decl *VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
-    Decl *VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D);
-    Decl *VisitClassTemplateDecl(ClassTemplateDecl *D);
-    Decl *VisitClassTemplateSpecializationDecl(
+    ExpectedDecl VisitDecl(Decl *D);
+    ExpectedDecl VisitImportDecl(ImportDecl *D);
+    ExpectedDecl VisitEmptyDecl(EmptyDecl *D);
+    ExpectedDecl VisitAccessSpecDecl(AccessSpecDecl *D);
+    ExpectedDecl VisitStaticAssertDecl(StaticAssertDecl *D);
+    ExpectedDecl VisitTranslationUnitDecl(TranslationUnitDecl *D);
+    ExpectedDecl VisitNamespaceDecl(NamespaceDecl *D);
+    ExpectedDecl VisitNamespaceAliasDecl(NamespaceAliasDecl *D);
+    ExpectedDecl VisitTypedefNameDecl(TypedefNameDecl *D, bool IsAlias);
+    ExpectedDecl VisitTypedefDecl(TypedefDecl *D);
+    ExpectedDecl VisitTypeAliasDecl(TypeAliasDecl *D);
+    ExpectedDecl VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D);
+    ExpectedDecl VisitLabelDecl(LabelDecl *D);
+    ExpectedDecl VisitEnumDecl(EnumDecl *D);
+    ExpectedDecl VisitRecordDecl(RecordDecl *D);
+    ExpectedDecl VisitEnumConstantDecl(EnumConstantDecl *D);
+    ExpectedDecl VisitFunctionDecl(FunctionDecl *D);
+    ExpectedDecl VisitCXXMethodDecl(CXXMethodDecl *D);
+    ExpectedDecl VisitCXXConstructorDecl(CXXConstructorDecl *D);
+    ExpectedDecl VisitCXXDestructorDecl(CXXDestructorDecl *D);
+    ExpectedDecl VisitCXXConversionDecl(CXXConversionDecl *D);
+    ExpectedDecl VisitFieldDecl(FieldDecl *D);
+    ExpectedDecl VisitIndirectFieldDecl(IndirectFieldDecl *D);
+    ExpectedDecl VisitFriendDecl(FriendDecl *D);
+    ExpectedDecl VisitObjCIvarDecl(ObjCIvarDecl *D);
+    ExpectedDecl VisitVarDecl(VarDecl *D);
+    ExpectedDecl VisitImplicitParamDecl(ImplicitParamDecl *D);
+    ExpectedDecl VisitParmVarDecl(ParmVarDecl *D);
+    ExpectedDecl VisitObjCMethodDecl(ObjCMethodDecl *D);
+    ExpectedDecl VisitObjCTypeParamDecl(ObjCTypeParamDecl *D);
+    ExpectedDecl VisitObjCCategoryDecl(ObjCCategoryDecl *D);
+    ExpectedDecl VisitObjCProtocolDecl(ObjCProtocolDecl *D);
+    ExpectedDecl VisitLinkageSpecDecl(LinkageSpecDecl *D);
+    ExpectedDecl VisitUsingDecl(UsingDecl *D);
+    ExpectedDecl VisitUsingShadowDecl(UsingShadowDecl *D);
+    ExpectedDecl VisitUsingDirectiveDecl(UsingDirectiveDecl *D);
+    ExpectedDecl VisitUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *D);
+    ExpectedDecl VisitUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D);
+
+    Expected<ObjCTypeParamList *>
+    ImportObjCTypeParamList(ObjCTypeParamList *list);
+
+    ExpectedDecl VisitObjCInterfaceDecl(ObjCInterfaceDecl *D);
+    ExpectedDecl VisitObjCCategoryImplDecl(ObjCCategoryImplDecl *D);
+    ExpectedDecl VisitObjCImplementationDecl(ObjCImplementationDecl *D);
+    ExpectedDecl VisitObjCPropertyDecl(ObjCPropertyDecl *D);
+    ExpectedDecl VisitObjCPropertyImplDecl(ObjCPropertyImplDecl *D);
+    ExpectedDecl VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D);
+    ExpectedDecl VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
+    ExpectedDecl VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D);
+    ExpectedDecl VisitClassTemplateDecl(ClassTemplateDecl *D);
+    ExpectedDecl VisitClassTemplateSpecializationDecl(
                                             ClassTemplateSpecializationDecl *D);
-    Decl *VisitVarTemplateDecl(VarTemplateDecl *D);
-    Decl *VisitVarTemplateSpecializationDecl(VarTemplateSpecializationDecl *D);
-    Decl *VisitFunctionTemplateDecl(FunctionTemplateDecl *D);
+    ExpectedDecl VisitVarTemplateDecl(VarTemplateDecl *D);
+    ExpectedDecl VisitVarTemplateSpecializationDecl(VarTemplateSpecializationDecl *D);
+    ExpectedDecl VisitFunctionTemplateDecl(FunctionTemplateDecl *D);
 
     // Importing statements
-    DeclGroupRef ImportDeclGroup(DeclGroupRef DG);
-
-    Stmt *VisitStmt(Stmt *S);
-    Stmt *VisitGCCAsmStmt(GCCAsmStmt *S);
-    Stmt *VisitDeclStmt(DeclStmt *S);
-    Stmt *VisitNullStmt(NullStmt *S);
-    Stmt *VisitCompoundStmt(CompoundStmt *S);
-    Stmt *VisitCaseStmt(CaseStmt *S);
-    Stmt *VisitDefaultStmt(DefaultStmt *S);
-    Stmt *VisitLabelStmt(LabelStmt *S);
-    Stmt *VisitAttributedStmt(AttributedStmt *S);
-    Stmt *VisitIfStmt(IfStmt *S);
-    Stmt *VisitSwitchStmt(SwitchStmt *S);
-    Stmt *VisitWhileStmt(WhileStmt *S);
-    Stmt *VisitDoStmt(DoStmt *S);
-    Stmt *VisitForStmt(ForStmt *S);
-    Stmt *VisitGotoStmt(GotoStmt *S);
-    Stmt *VisitIndirectGotoStmt(IndirectGotoStmt *S);
-    Stmt *VisitContinueStmt(ContinueStmt *S);
-    Stmt *VisitBreakStmt(BreakStmt *S);
-    Stmt *VisitReturnStmt(ReturnStmt *S);
+    ExpectedStmt VisitStmt(Stmt *S);
+    ExpectedStmt VisitGCCAsmStmt(GCCAsmStmt *S);
+    ExpectedStmt VisitDeclStmt(DeclStmt *S);
+    ExpectedStmt VisitNullStmt(NullStmt *S);
+    ExpectedStmt VisitCompoundStmt(CompoundStmt *S);
+    ExpectedStmt VisitCaseStmt(CaseStmt *S);
+    ExpectedStmt VisitDefaultStmt(DefaultStmt *S);
+    ExpectedStmt VisitLabelStmt(LabelStmt *S);
+    ExpectedStmt VisitAttributedStmt(AttributedStmt *S);
+    ExpectedStmt VisitIfStmt(IfStmt *S);
+    ExpectedStmt VisitSwitchStmt(SwitchStmt *S);
+    ExpectedStmt VisitWhileStmt(WhileStmt *S);
+    ExpectedStmt VisitDoStmt(DoStmt *S);
+    ExpectedStmt VisitForStmt(ForStmt *S);
+    ExpectedStmt VisitGotoStmt(GotoStmt *S);
+    ExpectedStmt VisitIndirectGotoStmt(IndirectGotoStmt *S);
+    ExpectedStmt VisitContinueStmt(ContinueStmt *S);
+    ExpectedStmt VisitBreakStmt(BreakStmt *S);
+    ExpectedStmt VisitReturnStmt(ReturnStmt *S);
     // FIXME: MSAsmStmt
     // FIXME: SEHExceptStmt
     // FIXME: SEHFinallyStmt
     // FIXME: SEHTryStmt
     // FIXME: SEHLeaveStmt
     // FIXME: CapturedStmt
-    Stmt *VisitCXXCatchStmt(CXXCatchStmt *S);
-    Stmt *VisitCXXTryStmt(CXXTryStmt *S);
-    Stmt *VisitCXXForRangeStmt(CXXForRangeStmt *S);
+    ExpectedStmt VisitCXXCatchStmt(CXXCatchStmt *S);
+    ExpectedStmt VisitCXXTryStmt(CXXTryStmt *S);
+    ExpectedStmt VisitCXXForRangeStmt(CXXForRangeStmt *S);
     // FIXME: MSDependentExistsStmt
-    Stmt *VisitObjCForCollectionStmt(ObjCForCollectionStmt *S);
-    Stmt *VisitObjCAtCatchStmt(ObjCAtCatchStmt *S);
-    Stmt *VisitObjCAtFinallyStmt(ObjCAtFinallyStmt *S);
-    Stmt *VisitObjCAtTryStmt(ObjCAtTryStmt *S);
-    Stmt *VisitObjCAtSynchronizedStmt(ObjCAtSynchronizedStmt *S);
-    Stmt *VisitObjCAtThrowStmt(ObjCAtThrowStmt *S);
-    Stmt *VisitObjCAutoreleasePoolStmt(ObjCAutoreleasePoolStmt *S);
+    ExpectedStmt VisitObjCForCollectionStmt(ObjCForCollectionStmt *S);
+    ExpectedStmt VisitObjCAtCatchStmt(ObjCAtCatchStmt *S);
+    ExpectedStmt VisitObjCAtFinallyStmt(ObjCAtFinallyStmt *S);
+    ExpectedStmt VisitObjCAtTryStmt(ObjCAtTryStmt *S);
+    ExpectedStmt VisitObjCAtSynchronizedStmt(ObjCAtSynchronizedStmt *S);
+    ExpectedStmt VisitObjCAtThrowStmt(ObjCAtThrowStmt *S);
+    ExpectedStmt VisitObjCAutoreleasePoolStmt(ObjCAutoreleasePoolStmt *S);
 
     // Importing expressions
-    Expr *VisitExpr(Expr *E);
-    Expr *VisitVAArgExpr(VAArgExpr *E);
-    Expr *VisitGNUNullExpr(GNUNullExpr *E);
-    Expr *VisitPredefinedExpr(PredefinedExpr *E);
-    Expr *VisitDeclRefExpr(DeclRefExpr *E);
-    Expr *VisitImplicitValueInitExpr(ImplicitValueInitExpr *ILE);
-    Expr *VisitDesignatedInitExpr(DesignatedInitExpr *E);
-    Expr *VisitCXXNullPtrLiteralExpr(CXXNullPtrLiteralExpr *E);
-    Expr *VisitIntegerLiteral(IntegerLiteral *E);
-    Expr *VisitFloatingLiteral(FloatingLiteral *E);
-    Expr *VisitImaginaryLiteral(ImaginaryLiteral *E);
-    Expr *VisitCharacterLiteral(CharacterLiteral *E);
-    Expr *VisitStringLiteral(StringLiteral *E);
-    Expr *VisitCompoundLiteralExpr(CompoundLiteralExpr *E);
-    Expr *VisitAtomicExpr(AtomicExpr *E);
-    Expr *VisitAddrLabelExpr(AddrLabelExpr *E);
-    Expr *VisitParenExpr(ParenExpr *E);
-    Expr *VisitParenListExpr(ParenListExpr *E);
-    Expr *VisitStmtExpr(StmtExpr *E);
-    Expr *VisitUnaryOperator(UnaryOperator *E);
-    Expr *VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *E);
-    Expr *VisitBinaryOperator(BinaryOperator *E);
-    Expr *VisitConditionalOperator(ConditionalOperator *E);
-    Expr *VisitBinaryConditionalOperator(BinaryConditionalOperator *E);
-    Expr *VisitOpaqueValueExpr(OpaqueValueExpr *E);
-    Expr *VisitArrayTypeTraitExpr(ArrayTypeTraitExpr *E);
-    Expr *VisitExpressionTraitExpr(ExpressionTraitExpr *E);
-    Expr *VisitArraySubscriptExpr(ArraySubscriptExpr *E);
-    Expr *VisitCompoundAssignOperator(CompoundAssignOperator *E);
-    Expr *VisitImplicitCastExpr(ImplicitCastExpr *E);
-    Expr *VisitExplicitCastExpr(ExplicitCastExpr *E);
-    Expr *VisitOffsetOfExpr(OffsetOfExpr *OE);
-    Expr *VisitCXXThrowExpr(CXXThrowExpr *E);
-    Expr *VisitCXXNoexceptExpr(CXXNoexceptExpr *E);
-    Expr *VisitCXXDefaultArgExpr(CXXDefaultArgExpr *E);
-    Expr *VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *E);
-    Expr *VisitCXXBindTemporaryExpr(CXXBindTemporaryExpr *E);
-    Expr *VisitCXXTemporaryObjectExpr(CXXTemporaryObjectExpr *CE);
-    Expr *VisitMaterializeTemporaryExpr(MaterializeTemporaryExpr *E);
-    Expr *VisitPackExpansionExpr(PackExpansionExpr *E);
-    Expr *VisitSizeOfPackExpr(SizeOfPackExpr *E);
-    Expr *VisitCXXNewExpr(CXXNewExpr *CE);
-    Expr *VisitCXXDeleteExpr(CXXDeleteExpr *E);
-    Expr *VisitCXXConstructExpr(CXXConstructExpr *E);
-    Expr *VisitCXXMemberCallExpr(CXXMemberCallExpr *E);
-    Expr *VisitCXXDependentScopeMemberExpr(CXXDependentScopeMemberExpr *E);
-    Expr *VisitDependentScopeDeclRefExpr(DependentScopeDeclRefExpr *E);
-    Expr *VisitCXXUnresolvedConstructExpr(CXXUnresolvedConstructExpr *CE);
-    Expr *VisitUnresolvedLookupExpr(UnresolvedLookupExpr *E);
-    Expr *VisitUnresolvedMemberExpr(UnresolvedMemberExpr *E);
-    Expr *VisitExprWithCleanups(ExprWithCleanups *EWC);
-    Expr *VisitCXXThisExpr(CXXThisExpr *E);
-    Expr *VisitCXXBoolLiteralExpr(CXXBoolLiteralExpr *E);
-    Expr *VisitCXXPseudoDestructorExpr(CXXPseudoDestructorExpr *E);
-    Expr *VisitMemberExpr(MemberExpr *E);
-    Expr *VisitCallExpr(CallExpr *E);
-    Expr *VisitLambdaExpr(LambdaExpr *LE);
-    Expr *VisitInitListExpr(InitListExpr *E);
-    Expr *VisitCXXStdInitializerListExpr(CXXStdInitializerListExpr *E);
-    Expr *VisitCXXInheritedCtorInitExpr(CXXInheritedCtorInitExpr *E);
-    Expr *VisitArrayInitLoopExpr(ArrayInitLoopExpr *E);
-    Expr *VisitArrayInitIndexExpr(ArrayInitIndexExpr *E);
-    Expr *VisitCXXDefaultInitExpr(CXXDefaultInitExpr *E);
-    Expr *VisitCXXNamedCastExpr(CXXNamedCastExpr *E);
-    Expr *VisitSubstNonTypeTemplateParmExpr(SubstNonTypeTemplateParmExpr *E);
-    Expr *VisitTypeTraitExpr(TypeTraitExpr *E);
-    Expr *VisitCXXTypeidExpr(CXXTypeidExpr *E);
+    ExpectedStmt VisitExpr(Expr *E);
+    ExpectedStmt VisitVAArgExpr(VAArgExpr *E);
+    ExpectedStmt VisitGNUNullExpr(GNUNullExpr *E);
+    ExpectedStmt VisitPredefinedExpr(PredefinedExpr *E);
+    ExpectedStmt VisitDeclRefExpr(DeclRefExpr *E);
+    ExpectedStmt VisitImplicitValueInitExpr(ImplicitValueInitExpr *E);
+    ExpectedStmt VisitDesignatedInitExpr(DesignatedInitExpr *E);
+    ExpectedStmt VisitCXXNullPtrLiteralExpr(CXXNullPtrLiteralExpr *E);
+    ExpectedStmt VisitIntegerLiteral(IntegerLiteral *E);
+    ExpectedStmt VisitFloatingLiteral(FloatingLiteral *E);
+    ExpectedStmt VisitImaginaryLiteral(ImaginaryLiteral *E);
+    ExpectedStmt VisitCharacterLiteral(CharacterLiteral *E);
+    ExpectedStmt VisitStringLiteral(StringLiteral *E);
+    ExpectedStmt VisitCompoundLiteralExpr(CompoundLiteralExpr *E);
+    ExpectedStmt VisitAtomicExpr(AtomicExpr *E);
+    ExpectedStmt VisitAddrLabelExpr(AddrLabelExpr *E);
+    ExpectedStmt VisitParenExpr(ParenExpr *E);
+    ExpectedStmt VisitParenListExpr(ParenListExpr *E);
+    ExpectedStmt VisitStmtExpr(StmtExpr *E);
+    ExpectedStmt VisitUnaryOperator(UnaryOperator *E);
+    ExpectedStmt VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *E);
+    ExpectedStmt VisitBinaryOperator(BinaryOperator *E);
+    ExpectedStmt VisitConditionalOperator(ConditionalOperator *E);
+    ExpectedStmt VisitBinaryConditionalOperator(BinaryConditionalOperator *E);
+    ExpectedStmt VisitOpaqueValueExpr(OpaqueValueExpr *E);
+    ExpectedStmt VisitArrayTypeTraitExpr(ArrayTypeTraitExpr *E);
+    ExpectedStmt VisitExpressionTraitExpr(ExpressionTraitExpr *E);
+    ExpectedStmt VisitArraySubscriptExpr(ArraySubscriptExpr *E);
+    ExpectedStmt VisitCompoundAssignOperator(CompoundAssignOperator *E);
+    ExpectedStmt VisitImplicitCastExpr(ImplicitCastExpr *E);
+    ExpectedStmt VisitExplicitCastExpr(ExplicitCastExpr *E);
+    ExpectedStmt VisitOffsetOfExpr(OffsetOfExpr *OE);
+    ExpectedStmt VisitCXXThrowExpr(CXXThrowExpr *E);
+    ExpectedStmt VisitCXXNoexceptExpr(CXXNoexceptExpr *E);
+    ExpectedStmt VisitCXXDefaultArgExpr(CXXDefaultArgExpr *E);
+    ExpectedStmt VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *E);
+    ExpectedStmt VisitCXXBindTemporaryExpr(CXXBindTemporaryExpr *E);
+    ExpectedStmt VisitCXXTemporaryObjectExpr(CXXTemporaryObjectExpr *E);
+    ExpectedStmt VisitMaterializeTemporaryExpr(MaterializeTemporaryExpr *E);
+    ExpectedStmt VisitPackExpansionExpr(PackExpansionExpr *E);
+    ExpectedStmt VisitSizeOfPackExpr(SizeOfPackExpr *E);
+    ExpectedStmt VisitCXXNewExpr(CXXNewExpr *E);
+    ExpectedStmt VisitCXXDeleteExpr(CXXDeleteExpr *E);
+    ExpectedStmt VisitCXXConstructExpr(CXXConstructExpr *E);
+    ExpectedStmt VisitCXXMemberCallExpr(CXXMemberCallExpr *E);
+    ExpectedStmt VisitCXXDependentScopeMemberExpr(CXXDependentScopeMemberExpr *E);
+    ExpectedStmt VisitDependentScopeDeclRefExpr(DependentScopeDeclRefExpr *E);
+    ExpectedStmt VisitCXXUnresolvedConstructExpr(CXXUnresolvedConstructExpr *E);
+    ExpectedStmt VisitUnresolvedLookupExpr(UnresolvedLookupExpr *E);
+    ExpectedStmt VisitUnresolvedMemberExpr(UnresolvedMemberExpr *E);
+    ExpectedStmt VisitExprWithCleanups(ExprWithCleanups *E);
+    ExpectedStmt VisitCXXThisExpr(CXXThisExpr *E);
+    ExpectedStmt VisitCXXBoolLiteralExpr(CXXBoolLiteralExpr *E);
+    ExpectedStmt VisitCXXPseudoDestructorExpr(CXXPseudoDestructorExpr *E);
+    ExpectedStmt VisitMemberExpr(MemberExpr *E);
+    ExpectedStmt VisitCallExpr(CallExpr *E);
+    ExpectedStmt VisitLambdaExpr(LambdaExpr *LE);
+    ExpectedStmt VisitInitListExpr(InitListExpr *E);
+    ExpectedStmt VisitCXXStdInitializerListExpr(CXXStdInitializerListExpr *E);
+    ExpectedStmt VisitCXXInheritedCtorInitExpr(CXXInheritedCtorInitExpr *E);
+    ExpectedStmt VisitArrayInitLoopExpr(ArrayInitLoopExpr *E);
+    ExpectedStmt VisitArrayInitIndexExpr(ArrayInitIndexExpr *E);
+    ExpectedStmt VisitCXXDefaultInitExpr(CXXDefaultInitExpr *E);
+    ExpectedStmt VisitCXXNamedCastExpr(CXXNamedCastExpr *E);
+    ExpectedStmt VisitSubstNonTypeTemplateParmExpr(SubstNonTypeTemplateParmExpr *E);
+    ExpectedStmt VisitTypeTraitExpr(TypeTraitExpr *E);
+    ExpectedStmt VisitCXXTypeidExpr(CXXTypeidExpr *E);
 
     template<typename IIter, typename OIter>
-    void ImportArray(IIter Ibegin, IIter Iend, OIter Obegin) {
+    Error ImportArrayChecked(IIter Ibegin, IIter Iend, OIter Obegin) {
       using ItemT = typename std::remove_reference<decltype(*Obegin)>::type;
-
-      ASTImporter &ImporterRef = Importer;
-      std::transform(Ibegin, Iend, Obegin,
-                     [&ImporterRef](ItemT From) -> ItemT {
-                       return ImporterRef.Import(From);
-                     });
-    }
-
-    template<typename IIter, typename OIter>
-    bool ImportArrayChecked(IIter Ibegin, IIter Iend, OIter Obegin) {
-      using ItemT = typename std::remove_reference<decltype(**Obegin)>::type;
-
-      ASTImporter &ImporterRef = Importer;
-      bool Failed = false;
-      std::transform(Ibegin, Iend, Obegin,
-                     [&ImporterRef, &Failed](ItemT *From) -> ItemT * {
-                       auto *To = cast_or_null<ItemT>(ImporterRef.Import(From));
-                       if (!To && From)
-                         Failed = true;
-                       return To;
-                     });
-      return Failed;
+      for (; Ibegin != Iend; ++Ibegin, ++Obegin) {
+        Expected<ItemT> ToOrErr = import(*Ibegin);
+        if (!ToOrErr)
+          return ToOrErr.takeError();
+        *Obegin = *ToOrErr;
+      }
+      return Error::success();
     }
 
+    // Import every item from a container structure into an output container.
+    // If error occurs, stops at first error and returns the error.
+    // The output container should have space for all needed elements (it is not
+    // expanded, new items are put into from the beginning).
     template<typename InContainerTy, typename OutContainerTy>
-    bool ImportContainerChecked(const InContainerTy &InContainer,
-                                OutContainerTy &OutContainer) {
-      return ImportArrayChecked(InContainer.begin(), InContainer.end(),
-                                OutContainer.begin());
+    Error ImportContainerChecked(
+        const InContainerTy &InContainer, OutContainerTy &OutContainer) {
+      return ImportArrayChecked(
+          InContainer.begin(), InContainer.end(), OutContainer.begin());
     }
 
     template<typename InContainerTy, typename OIter>
-    bool ImportArrayChecked(const InContainerTy &InContainer, OIter Obegin) {
+    Error ImportArrayChecked(const InContainerTy &InContainer, OIter Obegin) {
       return ImportArrayChecked(InContainer.begin(), InContainer.end(), Obegin);
     }
 
-    // Importing overrides.
     void ImportOverrides(CXXMethodDecl *ToMethod, CXXMethodDecl *FromMethod);
 
-    FunctionDecl *FindFunctionTemplateSpecialization(FunctionDecl *FromFD);
+    Expected<FunctionDecl *> FindFunctionTemplateSpecialization(
+        FunctionDecl *FromFD);
   };
 
+// FIXME: Temporary until every import returns Expected.
+template <>
+Expected<TemplateName> ASTNodeImporter::import(const TemplateName &From) {
+  TemplateName To = Importer.Import(From);
+  if (To.isNull() && !From.isNull())
+    return make_error<ImportError>();
+  return To;
+}
+
 template <typename InContainerTy>
-bool ASTNodeImporter::ImportTemplateArgumentListInfo(
+Error ASTNodeImporter::ImportTemplateArgumentListInfo(
     SourceLocation FromLAngleLoc, SourceLocation FromRAngleLoc,
     const InContainerTy &Container, TemplateArgumentListInfo &Result) {
-  TemplateArgumentListInfo ToTAInfo(Importer.Import(FromLAngleLoc),
-                                    Importer.Import(FromRAngleLoc));
-  if (ImportTemplateArgumentListInfo(Container, ToTAInfo))
-    return true;
+  auto ToLAngleLocOrErr = import(FromLAngleLoc);
+  if (!ToLAngleLocOrErr)
+    return ToLAngleLocOrErr.takeError();
+  auto ToRAngleLocOrErr = import(FromRAngleLoc);
+  if (!ToRAngleLocOrErr)
+    return ToRAngleLocOrErr.takeError();
+
+  TemplateArgumentListInfo ToTAInfo(*ToLAngleLocOrErr, *ToRAngleLocOrErr);
+  if (auto Err = ImportTemplateArgumentListInfo(Container, ToTAInfo))
+    return Err;
   Result = ToTAInfo;
-  return false;
+  return Error::success();
 }
 
 template <>
-bool ASTNodeImporter::ImportTemplateArgumentListInfo<TemplateArgumentListInfo>(
+Error ASTNodeImporter::ImportTemplateArgumentListInfo<TemplateArgumentListInfo>(
     const TemplateArgumentListInfo &From, TemplateArgumentListInfo &Result) {
   return ImportTemplateArgumentListInfo(
       From.getLAngleLoc(), From.getRAngleLoc(), From.arguments(), Result);
 }
 
 template <>
-bool ASTNodeImporter::ImportTemplateArgumentListInfo<
-    ASTTemplateArgumentListInfo>(const ASTTemplateArgumentListInfo &From,
-                                 TemplateArgumentListInfo &Result) {
-  return ImportTemplateArgumentListInfo(From.LAngleLoc, From.RAngleLoc,
-                                        From.arguments(), Result);
+Error ASTNodeImporter::ImportTemplateArgumentListInfo<
+    ASTTemplateArgumentListInfo>(
+        const ASTTemplateArgumentListInfo &From,
+  return ImportTemplateArgumentListInfo(
+      From.LAngleLoc, From.RAngleLoc, From.arguments(), Result);
 }
 
-std::tuple<FunctionTemplateDecl *, ASTNodeImporter::OptionalTemplateArgsTy>
+Expected<ASTNodeImporter::FunctionTemplateAndArgsTy>
 ASTNodeImporter::ImportFunctionTemplateWithTemplateArgsFromSpecialization(
     FunctionDecl *FromFD) {
   assert(FromFD->getTemplatedKind() ==
-         FunctionDecl::TK_FunctionTemplateSpecialization);
+
+  FunctionTemplateAndArgsTy Result;
+
   auto *FTSInfo = FromFD->getTemplateSpecializationInfo();
-  auto *Template = cast_or_null<FunctionTemplateDecl>(
-      Importer.Import(FTSInfo->getTemplate()));
+  if (Error Err = importInto(std::get<0>(Result), FTSInfo->getTemplate()))
+    return std::move(Err);
 
   // Import template arguments.
   auto TemplArgs = FTSInfo->TemplateArguments->asArray();
-  TemplateArgsTy ToTemplArgs;
-  if (ImportTemplateArguments(TemplArgs.data(), TemplArgs.size(),
-                              ToTemplArgs)) // Error during import.
-    return std::make_tuple(Template, OptionalTemplateArgsTy());
+  if (Error Err = ImportTemplateArguments(TemplArgs.data(), TemplArgs.size(),
+      std::get<1>(Result)))
+    return std::move(Err);
+
+  return Result;
+}
+
+template <>
+Expected<TemplateParameterList *>
+ASTNodeImporter::import(TemplateParameterList *From) {
+  SmallVector<NamedDecl *, 4> To(From->size());
+  if (Error Err = ImportContainerChecked(*From, To))
+    return std::move(Err);
+
+  ExpectedExpr ToRequiresClause = import(From->getRequiresClause());
+  if (!ToRequiresClause)
+    return ToRequiresClause.takeError();
+
+  auto ToTemplateLocOrErr = import(From->getTemplateLoc());
+  if (!ToTemplateLocOrErr)
+    return ToTemplateLocOrErr.takeError();
+  auto ToLAngleLocOrErr = import(From->getLAngleLoc());
+  if (!ToLAngleLocOrErr)
+    return ToLAngleLocOrErr.takeError();
+  auto ToRAngleLocOrErr = import(From->getRAngleLoc());
+  if (!ToRAngleLocOrErr)
+    return ToRAngleLocOrErr.takeError();
+
+  return TemplateParameterList::Create(
+      Importer.getToContext(),
+      *ToTemplateLocOrErr,
+      *ToLAngleLocOrErr,
+      To,
+      *ToRAngleLocOrErr,
+      *ToRequiresClause);
+}
+
+template <>
+Expected<TemplateArgument>
+ASTNodeImporter::import(const TemplateArgument &From) {
+  switch (From.getKind()) {
+  case TemplateArgument::Null:
+    return TemplateArgument();
+
+  case TemplateArgument::Type: {
+    ExpectedType ToTypeOrErr = import(From.getAsType());
+    if (!ToTypeOrErr)
+      return ToTypeOrErr.takeError();
+    return TemplateArgument(*ToTypeOrErr);
+  }
+
+  case TemplateArgument::Integral: {
+    ExpectedType ToTypeOrErr = import(From.getIntegralType());
+    if (!ToTypeOrErr)
+      return ToTypeOrErr.takeError();
+    return TemplateArgument(From, *ToTypeOrErr);
+  }
+
+  case TemplateArgument::Declaration: {
+    Expected<ValueDecl *> ToOrErr = import(From.getAsDecl());
+    if (!ToOrErr)
+      return ToOrErr.takeError();
+    ExpectedType ToTypeOrErr = import(From.getParamTypeForDecl());
+    if (!ToTypeOrErr)
+      return ToTypeOrErr.takeError();
+    return TemplateArgument(*ToOrErr, *ToTypeOrErr);
+  }
+
+  case TemplateArgument::NullPtr: {
+    ExpectedType ToTypeOrErr = import(From.getNullPtrType());
+    if (!ToTypeOrErr)
+      return ToTypeOrErr.takeError();
+    return TemplateArgument(*ToTypeOrErr, /*isNullPtr*/true);
+  }
+
+  case TemplateArgument::Template: {
+    Expected<TemplateName> ToTemplateOrErr = import(From.getAsTemplate());
+    if (!ToTemplateOrErr)
+      return ToTemplateOrErr.takeError();
+
+    return TemplateArgument(*ToTemplateOrErr);
+  }
+
+  case TemplateArgument::TemplateExpansion: {
+    Expected<TemplateName> ToTemplateOrErr =
+        import(From.getAsTemplateOrTemplatePattern());
+    if (!ToTemplateOrErr)
+      return ToTemplateOrErr.takeError();
+
+    return TemplateArgument(
+        *ToTemplateOrErr, From.getNumTemplateExpansions());
+  }
+
+  case TemplateArgument::Expression:
+    if (ExpectedExpr ToExpr = import(From.getAsExpr()))
+      return TemplateArgument(*ToExpr);
+    else
+      return ToExpr.takeError();
+
+  case TemplateArgument::Pack: {
+    SmallVector<TemplateArgument, 2> ToPack;
+    ToPack.reserve(From.pack_size());
+    if (Error Err = ImportTemplateArguments(
+        From.pack_begin(), From.pack_size(), ToPack))
+      return std::move(Err);
+
+    return TemplateArgument(
+        llvm::makeArrayRef(ToPack).copy(Importer.getToContext()));
+  }
+  }
+
+  llvm_unreachable("Invalid template argument kind");
+}
+
+template <>
+Expected<TemplateArgumentLoc>
+ASTNodeImporter::import(const TemplateArgumentLoc &TALoc) {
+  Expected<TemplateArgument> ArgOrErr = import(TALoc.getArgument());
+  if (!ArgOrErr)
+    return ArgOrErr.takeError();
+  TemplateArgument Arg = *ArgOrErr;
+
+  TemplateArgumentLocInfo FromInfo = TALoc.getLocInfo();
+
+  TemplateArgumentLocInfo ToInfo;
+  if (Arg.getKind() == TemplateArgument::Expression) {
+    ExpectedExpr E = import(FromInfo.getAsExpr());
+    if (!E)
+      return E.takeError();
+    ToInfo = TemplateArgumentLocInfo(*E);
+  } else if (Arg.getKind() == TemplateArgument::Type) {
+    if (auto TSIOrErr = import(FromInfo.getAsTypeSourceInfo()))
+      ToInfo = TemplateArgumentLocInfo(*TSIOrErr);
+    else
+      return TSIOrErr.takeError();
+  } else {
+    auto ToTemplateQualifierLocOrErr =
+        import(FromInfo.getTemplateQualifierLoc());
+    if (!ToTemplateQualifierLocOrErr)
+      return ToTemplateQualifierLocOrErr.takeError();
+    auto ToTemplateNameLocOrErr = import(FromInfo.getTemplateNameLoc());
+    if (!ToTemplateNameLocOrErr)
+      return ToTemplateNameLocOrErr.takeError();
+    auto ToTemplateEllipsisLocOrErr =
+        import(FromInfo.getTemplateEllipsisLoc());
+    if (!ToTemplateEllipsisLocOrErr)
+      return ToTemplateEllipsisLocOrErr.takeError();
+
+    ToInfo = TemplateArgumentLocInfo(
+          *ToTemplateQualifierLocOrErr,
+          *ToTemplateNameLocOrErr,
+          *ToTemplateEllipsisLocOrErr);
+  }
+
+  return TemplateArgumentLoc(Arg, ToInfo);
+}
+
+template <>
+Expected<DeclGroupRef> ASTNodeImporter::import(const DeclGroupRef &DG) {
+  if (DG.isNull())
+    return DeclGroupRef::Create(Importer.getToContext(), nullptr, 0);
+  size_t NumDecls = DG.end() - DG.begin();
+  SmallVector<Decl *, 1> ToDecls;
+  ToDecls.reserve(NumDecls);
+  for (Decl *FromD : DG) {
+    if (auto ToDOrErr = import(FromD))
+      ToDecls.push_back(*ToDOrErr);
+    else
+      return ToDOrErr.takeError();
+  }
+  return DeclGroupRef::Create(Importer.getToContext(),
+                              ToDecls.begin(),
+                              NumDecls);
+}
+
+template <>
+Expected<ASTNodeImporter::Designator>
+ASTNodeImporter::import(const Designator &D) {
+  if (D.isFieldDesignator()) {
+    IdentifierInfo *ToFieldName = Importer.Import(D.getFieldName());
+
+    ExpectedSLoc ToDotLocOrErr = import(D.getDotLoc());
+    if (!ToDotLocOrErr)
+      return ToDotLocOrErr.takeError();
+
+    ExpectedSLoc ToFieldLocOrErr = import(D.getFieldLoc());
+    if (!ToFieldLocOrErr)
+      return ToFieldLocOrErr.takeError();
+
+    return Designator(ToFieldName, *ToDotLocOrErr, *ToFieldLocOrErr);
+  }
+
+  ExpectedSLoc ToLBracketLocOrErr = import(D.getLBracketLoc());
+  if (!ToLBracketLocOrErr)
+    return ToLBracketLocOrErr.takeError();
+
+  ExpectedSLoc ToRBracketLocOrErr = import(D.getRBracketLoc());
+  if (!ToRBracketLocOrErr)
+    return ToRBracketLocOrErr.takeError();
+
+  if (D.isArrayDesignator())
+    return Designator(D.getFirstExprIndex(),
+                      *ToLBracketLocOrErr, *ToRBracketLocOrErr);
+
+  ExpectedSLoc ToEllipsisLocOrErr = import(D.getEllipsisLoc());
+  if (!ToEllipsisLocOrErr)
+    return ToEllipsisLocOrErr.takeError();
+
+  assert(D.isArrayRangeDesignator());
+  return Designator(
+      D.getFirstExprIndex(), *ToLBracketLocOrErr, *ToEllipsisLocOrErr,
+      *ToRBracketLocOrErr);
+}
+
+template <>
+Expected<LambdaCapture> ASTNodeImporter::import(const LambdaCapture &From) {
+  VarDecl *Var = nullptr;
+  if (From.capturesVariable()) {
+    if (auto VarOrErr = import(From.getCapturedVar()))
+      Var = *VarOrErr;
+    else
+      return VarOrErr.takeError();
+  }
 
-  return std::make_tuple(Template, ToTemplArgs);
+  auto LocationOrErr = import(From.getLocation());
+  if (!LocationOrErr)
+    return LocationOrErr.takeError();
+
+  SourceLocation EllipsisLoc;
+  if (From.isPackExpansion())
+    if (Error Err = importInto(EllipsisLoc, From.getEllipsisLoc()))
+      return std::move(Err);
+
+  return LambdaCapture(
+      *LocationOrErr, From.isImplicit(), From.getCaptureKind(), Var,
+      EllipsisLoc);
 }
 
 } // namespace clang
 
 //----------------------------------------------------------------------------
 // Import Types
 //----------------------------------------------------------------------------
 
 using namespace clang;
 
-QualType ASTNodeImporter::VisitType(const Type *T) {
+ExpectedType ASTNodeImporter::VisitType(const Type *T) {
   Importer.FromDiag(SourceLocation(), diag::err_unsupported_ast_node)
     << T->getTypeClassName();
-  return {};
+  return make_error<ImportError>(ImportError::UnsupportedConstruct);
 }
 
-QualType ASTNodeImporter::VisitAtomicType(const AtomicType *T){
-  QualType UnderlyingType = Importer.Import(T->getValueType());
-  if(UnderlyingType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitAtomicType(const AtomicType *T){
+  ExpectedType UnderlyingTypeOrErr = import(T->getValueType());
+  if (!UnderlyingTypeOrErr)
+    return UnderlyingTypeOrErr.takeError();
 
-  return Importer.getToContext().getAtomicType(UnderlyingType);
+  return Importer.getToContext().getAtomicType(*UnderlyingTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitBuiltinType(const BuiltinType *T) {
+ExpectedType ASTNodeImporter::VisitBuiltinType(const BuiltinType *T) {
   switch (T->getKind()) {
 #define IMAGE_TYPE(ImgType, Id, SingletonId, Access, Suffix) \
   case BuiltinType::Id: \
     return Importer.getToContext().SingletonId;
 #include "clang/Basic/OpenCLImageTypes.def"
 #define SHARED_SINGLETON_TYPE(Expansion)
 #define BUILTIN_TYPE(Id, SingletonId) \
   case BuiltinType::Id: return Importer.getToContext().SingletonId;
case BuiltinType::WChar_U:
     // FIXME: If not in C++, shall we translate to the C equivalent of
     // wchar_t?
     return Importer.getToContext().WCharTy;
   }
 
   llvm_unreachable("Invalid BuiltinType Kind!");
 }
 
-QualType ASTNodeImporter::VisitDecayedType(const DecayedType *T) {
-  QualType OrigT = Importer.Import(T->getOriginalType());
-  if (OrigT.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitDecayedType(const DecayedType *T) {
+  ExpectedType ToOriginalTypeOrErr = import(T->getOriginalType());
+  if (!ToOriginalTypeOrErr)
+    return ToOriginalTypeOrErr.takeError();
 
-  return Importer.getToContext().getDecayedType(OrigT);
+  return Importer.getToContext().getDecayedType(*ToOriginalTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitComplexType(const ComplexType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitComplexType(const ComplexType *T) {
+  ExpectedType ToElementTypeOrErr = import(T->getElementType());
+  if (!ToElementTypeOrErr)
+    return ToElementTypeOrErr.takeError();
 
-  return Importer.getToContext().getComplexType(ToElementType);
+  return Importer.getToContext().getComplexType(*ToElementTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitPointerType(const PointerType *T) {
-  QualType ToPointeeType = Importer.Import(T->getPointeeType());
-  if (ToPointeeType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitPointerType(const PointerType *T) {
+  ExpectedType ToPointeeTypeOrErr = import(T->getPointeeType());
+  if (!ToPointeeTypeOrErr)
+    return ToPointeeTypeOrErr.takeError();
 
-  return Importer.getToContext().getPointerType(ToPointeeType);
+  return Importer.getToContext().getPointerType(*ToPointeeTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitBlockPointerType(const BlockPointerType *T) {
+ExpectedType ASTNodeImporter::VisitBlockPointerType(const BlockPointerType *T) {
   // FIXME: Check for blocks support in "to" context.
-  QualType ToPointeeType = Importer.Import(T->getPointeeType());
-  if (ToPointeeType.isNull())
-    return {};
+  ExpectedType ToPointeeTypeOrErr = import(T->getPointeeType());
+  if (!ToPointeeTypeOrErr)
+    return ToPointeeTypeOrErr.takeError();
 
-  return Importer.getToContext().getBlockPointerType(ToPointeeType);
+  return Importer.getToContext().getBlockPointerType(*ToPointeeTypeOrErr);
 }
 
-QualType
+ExpectedType
 ASTNodeImporter::VisitLValueReferenceType(const LValueReferenceType *T) {
   // FIXME: Check for C++ support in "to" context.
-  QualType ToPointeeType = Importer.Import(T->getPointeeTypeAsWritten());
-  if (ToPointeeType.isNull())
-    return {};
+  ExpectedType ToPointeeTypeOrErr = import(T->getPointeeTypeAsWritten());
+  if (!ToPointeeTypeOrErr)
+    return ToPointeeTypeOrErr.takeError();
 
-  return Importer.getToContext().getLValueReferenceType(ToPointeeType);
+  return Importer.getToContext().getLValueReferenceType(*ToPointeeTypeOrErr);
 }
 
-QualType
+ExpectedType
 ASTNodeImporter::VisitRValueReferenceType(const RValueReferenceType *T) {
   // FIXME: Check for C++0x support in "to" context.
-  QualType ToPointeeType = Importer.Import(T->getPointeeTypeAsWritten());
-  if (ToPointeeType.isNull())
-    return {};
+  ExpectedType ToPointeeTypeOrErr = import(T->getPointeeTypeAsWritten());
+  if (!ToPointeeTypeOrErr)
+    return ToPointeeTypeOrErr.takeError();
 
-  return Importer.getToContext().getRValueReferenceType(ToPointeeType);
+  return Importer.getToContext().getRValueReferenceType(*ToPointeeTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitMemberPointerType(const MemberPointerType *T) {
+ExpectedType
+ASTNodeImporter::VisitMemberPointerType(const MemberPointerType *T) {
   // FIXME: Check for C++ support in "to" context.
-  QualType ToPointeeType = Importer.Import(T->getPointeeType());
-  if (ToPointeeType.isNull())
-    return {};
+  ExpectedType ToPointeeTypeOrErr = import(T->getPointeeType());
+  if (!ToPointeeTypeOrErr)
+    return ToPointeeTypeOrErr.takeError();
 
-  QualType ClassType = Importer.Import(QualType(T->getClass(), 0));
-  return Importer.getToContext().getMemberPointerType(ToPointeeType,
-                                                      ClassType.getTypePtr());
+  ExpectedType ClassTypeOrErr = import(QualType(T->getClass(), 0));
+  if (!ClassTypeOrErr)
+    return ClassTypeOrErr.takeError();
+
+  return Importer.getToContext().getMemberPointerType(
+      *ToPointeeTypeOrErr, (*ClassTypeOrErr).getTypePtr());
 }
 
-QualType ASTNodeImporter::VisitConstantArrayType(const ConstantArrayType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
+ExpectedType
+ASTNodeImporter::VisitConstantArrayType(const ConstantArrayType *T) {
+  ExpectedType ToElementTypeOrErr = import(T->getElementType());
+  if (!ToElementTypeOrErr)
+    return ToElementTypeOrErr.takeError();
 
-  return Importer.getToContext().getConstantArrayType(ToElementType,
+  return Importer.getToContext().getConstantArrayType(*ToElementTypeOrErr,
                                                       T->getSize(),
                                                       T->getSizeModifier(),
                                                T->getIndexTypeCVRQualifiers());
 }
 
-QualType
+ExpectedType
 ASTNodeImporter::VisitIncompleteArrayType(const IncompleteArrayType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
+  ExpectedType ToElementTypeOrErr = import(T->getElementType());
+  if (!ToElementTypeOrErr)
+    return ToElementTypeOrErr.takeError();
 
-  return Importer.getToContext().getIncompleteArrayType(ToElementType,
+  return Importer.getToContext().getIncompleteArrayType(*ToElementTypeOrErr,
                                                         T->getSizeModifier(),
                                                 T->getIndexTypeCVRQualifiers());
 }
 
-QualType ASTNodeImporter::VisitVariableArrayType(const VariableArrayType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
-
-  Expr *Size = Importer.Import(T->getSizeExpr());
-  if (!Size)
-    return {};
+ExpectedType
+ASTNodeImporter::VisitVariableArrayType(const VariableArrayType *T) {
+  QualType ToElementType;
+  Expr *ToSizeExpr;
+  SourceRange ToBracketsRange;
+  if (auto Imp = importSeq(
+      T->getElementType(), T->getSizeExpr(), T->getBracketsRange()))
+    std::tie(ToElementType, ToSizeExpr, ToBracketsRange) = *Imp;
+  else
+    return Imp.takeError();
 
-  SourceRange Brackets = Importer.Import(T->getBracketsRange());
-  return Importer.getToContext().getVariableArrayType(ToElementType, Size,
-                                                      T->getSizeModifier(),
-                                                T->getIndexTypeCVRQualifiers(),
-                                                      Brackets);
+  return Importer.getToContext().getVariableArrayType(
+      ToElementType, ToSizeExpr, T->getSizeModifier(),
+      T->getIndexTypeCVRQualifiers(), ToBracketsRange);
 }
 
-QualType ASTNodeImporter::VisitDependentSizedArrayType(
+ExpectedType ASTNodeImporter::VisitDependentSizedArrayType(
     const DependentSizedArrayType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
-
+  QualType ToElementType;
+  Expr *ToSizeExpr;
+  SourceRange ToBracketsRange;
+  if (auto Imp = importSeq(
+      T->getElementType(), T->getSizeExpr(), T->getBracketsRange()))
+    std::tie(ToElementType, ToSizeExpr, ToBracketsRange) = *Imp;
+  else
+    return Imp.takeError();
   // SizeExpr may be null if size is not specified directly.
   // For example, 'int a[]'.
-  Expr *Size = Importer.Import(T->getSizeExpr());
-  if (!Size && T->getSizeExpr())
-    return {};
 
-  SourceRange Brackets = Importer.Import(T->getBracketsRange());
   return Importer.getToContext().getDependentSizedArrayType(
-      ToElementType, Size, T->getSizeModifier(), T->getIndexTypeCVRQualifiers(),
-      Brackets);
+      ToElementType, ToSizeExpr, T->getSizeModifier(),
+      T->getIndexTypeCVRQualifiers(), ToBracketsRange);
 }
 
-QualType ASTNodeImporter::VisitVectorType(const VectorType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitVectorType(const VectorType *T) {
+  ExpectedType ToElementTypeOrErr = import(T->getElementType());
+  if (!ToElementTypeOrErr)
+    return ToElementTypeOrErr.takeError();
 
-  return Importer.getToContext().getVectorType(ToElementType,
+  return Importer.getToContext().getVectorType(*ToElementTypeOrErr,
                                                T->getNumElements(),
                                                T->getVectorKind());
 }
 
-QualType ASTNodeImporter::VisitExtVectorType(const ExtVectorType *T) {
-  QualType ToElementType = Importer.Import(T->getElementType());
-  if (ToElementType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitExtVectorType(const ExtVectorType *T) {
+  ExpectedType ToElementTypeOrErr = import(T->getElementType());
+  if (!ToElementTypeOrErr)
+    return ToElementTypeOrErr.takeError();
 
-  return Importer.getToContext().getExtVectorType(ToElementType,
+  return Importer.getToContext().getExtVectorType(*ToElementTypeOrErr,
                                                   T->getNumElements());
 }
 
-QualType
+ExpectedType
 ASTNodeImporter::VisitFunctionNoProtoType(const FunctionNoProtoType *T) {
   // FIXME: What happens if we're importing a function without a prototype
   // into C++? Should we make it variadic?
-  QualType ToResultType = Importer.Import(T->getReturnType());
-  if (ToResultType.isNull())
-    return {};
+  ExpectedType ToReturnTypeOrErr = import(T->getReturnType());
+  if (!ToReturnTypeOrErr)
+    return ToReturnTypeOrErr.takeError();
 
-  return Importer.getToContext().getFunctionNoProtoType(ToResultType,
+  return Importer.getToContext().getFunctionNoProtoType(*ToReturnTypeOrErr,
                                                         T->getExtInfo());
 }
 
-QualType ASTNodeImporter::VisitFunctionProtoType(const FunctionProtoType *T) {
-  QualType ToResultType = Importer.Import(T->getReturnType());
-  if (ToResultType.isNull())
-    return {};
+ExpectedType
+ASTNodeImporter::VisitFunctionProtoType(const FunctionProtoType *T) {
+  ExpectedType ToReturnTypeOrErr = import(T->getReturnType());
+  if (!ToReturnTypeOrErr)
+    return ToReturnTypeOrErr.takeError();
 
   // Import argument types
   SmallVector<QualType, 4> ArgTypes;
   for (const auto &A : T->param_types()) {
-    QualType ArgType = Importer.Import(A);
-    if (ArgType.isNull())
-      return {};
-    ArgTypes.push_back(ArgType);
+    ExpectedType TyOrErr = import(A);
+    if (!TyOrErr)
+      return TyOrErr.takeError();
+    ArgTypes.push_back(*TyOrErr);
   }
 
   // Import exception types
   SmallVector<QualType, 4> ExceptionTypes;
   for (const auto &E : T->exceptions()) {
-    QualType ExceptionType = Importer.Import(E);
-    if (ExceptionType.isNull())
-      return {};
-    ExceptionTypes.push_back(ExceptionType);
+    ExpectedType TyOrErr = import(E);
+    if (!TyOrErr)
+      return TyOrErr.takeError();
+    ExceptionTypes.push_back(*TyOrErr);
   }
 
   FunctionProtoType::ExtProtoInfo FromEPI = T->getExtProtoInfo();
   FunctionProtoType::ExtProtoInfo ToEPI;
 
+  auto Imp = importSeq(
+      FromEPI.ExceptionSpec.NoexceptExpr,
+      FromEPI.ExceptionSpec.SourceDecl,
+      FromEPI.ExceptionSpec.SourceTemplate);
+  if (!Imp)
+    return Imp.takeError();
+
   ToEPI.ExtInfo = FromEPI.ExtInfo;
   ToEPI.Variadic = FromEPI.Variadic;
   ToEPI.HasTrailingReturn = FromEPI.HasTrailingReturn;
   ToEPI.TypeQuals = FromEPI.TypeQuals;
   ToEPI.RefQualifier = FromEPI.RefQualifier;
   ToEPI.ExceptionSpec.Type = FromEPI.ExceptionSpec.Type;
   ToEPI.ExceptionSpec.Exceptions = ExceptionTypes;
-  ToEPI.ExceptionSpec.NoexceptExpr =
-      Importer.Import(FromEPI.ExceptionSpec.NoexceptExpr);
-  ToEPI.ExceptionSpec.SourceDecl = cast_or_null<FunctionDecl>(
-      Importer.Import(FromEPI.ExceptionSpec.SourceDecl));
-  ToEPI.ExceptionSpec.SourceTemplate = cast_or_null<FunctionDecl>(
-      Importer.Import(FromEPI.ExceptionSpec.SourceTemplate));
+  std::tie(
+      ToEPI.ExceptionSpec.NoexceptExpr,
+      ToEPI.ExceptionSpec.SourceDecl,
+      ToEPI.ExceptionSpec.SourceTemplate) = *Imp;
 
-  return Importer.getToContext().getFunctionType(ToResultType, ArgTypes, ToEPI);
+  return Importer.getToContext().getFunctionType(
+      *ToReturnTypeOrErr, ArgTypes, ToEPI);
 }
 
-QualType ASTNodeImporter::VisitUnresolvedUsingType(
+ExpectedType ASTNodeImporter::VisitUnresolvedUsingType(
     const UnresolvedUsingType *T) {
-  const auto *ToD =
-      cast_or_null<UnresolvedUsingTypenameDecl>(Importer.Import(T->getDecl()));
-  if (!ToD)
-    return {};
-
-  auto *ToPrevD =
-      cast_or_null<UnresolvedUsingTypenameDecl>(
-        Importer.Import(T->getDecl()->getPreviousDecl()));
-  if (!ToPrevD && T->getDecl()->getPreviousDecl())
-    return {};
+  UnresolvedUsingTypenameDecl *ToD;
+  Decl *ToPrevD;
+  if (auto Imp = importSeq(T->getDecl(), T->getDecl()->getPreviousDecl()))
+    std::tie(ToD, ToPrevD) = *Imp;
+  else
+    return Imp.takeError();
 
-  return Importer.getToContext().getTypeDeclType(ToD, ToPrevD);
+  return Importer.getToContext().getTypeDeclType(
+      ToD, cast_or_null<TypeDecl>(ToPrevD));
 }
 
-QualType ASTNodeImporter::VisitParenType(const ParenType *T) {
-  QualType ToInnerType = Importer.Import(T->getInnerType());
-  if (ToInnerType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitParenType(const ParenType *T) {
+  ExpectedType ToInnerTypeOrErr = import(T->getInnerType());
+  if (!ToInnerTypeOrErr)
+    return ToInnerTypeOrErr.takeError();
 
-  return Importer.getToContext().getParenType(ToInnerType);
+  return Importer.getToContext().getParenType(*ToInnerTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitTypedefType(const TypedefType *T) {
-  auto *ToDecl =
-      dyn_cast_or_null<TypedefNameDecl>(Importer.Import(T->getDecl()));
-  if (!ToDecl)
-    return {};
+ExpectedType ASTNodeImporter::VisitTypedefType(const TypedefType *T) {
+  Expected<TypedefNameDecl *> ToDeclOrErr = import(T->getDecl());
+  if (!ToDeclOrErr)
+    return ToDeclOrErr.takeError();
 
-  return Importer.getToContext().getTypeDeclType(ToDecl);
+  return Importer.getToContext().getTypeDeclType(*ToDeclOrErr);
 }
 
-QualType ASTNodeImporter::VisitTypeOfExprType(const TypeOfExprType *T) {
-  Expr *ToExpr = Importer.Import(T->getUnderlyingExpr());
-  if (!ToExpr)
-    return {};
+ExpectedType ASTNodeImporter::VisitTypeOfExprType(const TypeOfExprType *T) {
+  ExpectedExpr ToExprOrErr = import(T->getUnderlyingExpr());
+  if (!ToExprOrErr)
+    return ToExprOrErr.takeError();
 
-  return Importer.getToContext().getTypeOfExprType(ToExpr);
+  return Importer.getToContext().getTypeOfExprType(*ToExprOrErr);
 }
 
-QualType ASTNodeImporter::VisitTypeOfType(const TypeOfType *T) {
-  QualType ToUnderlyingType = Importer.Import(T->getUnderlyingType());
-  if (ToUnderlyingType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitTypeOfType(const TypeOfType *T) {
+  ExpectedType ToUnderlyingTypeOrErr = import(T->getUnderlyingType());
+  if (!ToUnderlyingTypeOrErr)
+    return ToUnderlyingTypeOrErr.takeError();
 
-  return Importer.getToContext().getTypeOfType(ToUnderlyingType);
+  return Importer.getToContext().getTypeOfType(*ToUnderlyingTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitDecltypeType(const DecltypeType *T) {
+ExpectedType ASTNodeImporter::VisitDecltypeType(const DecltypeType *T) {
   // FIXME: Make sure that the "to" context supports C++0x!
-  Expr *ToExpr = Importer.Import(T->getUnderlyingExpr());
-  if (!ToExpr)
-    return {};
+  ExpectedExpr ToExprOrErr = import(T->getUnderlyingExpr());
+  if (!ToExprOrErr)
+    return ToExprOrErr.takeError();
 
-  QualType UnderlyingType = Importer.Import(T->getUnderlyingType());
-  if (UnderlyingType.isNull())
-    return {};
+  ExpectedType ToUnderlyingTypeOrErr = import(T->getUnderlyingType());
+  if (!ToUnderlyingTypeOrErr)
+    return ToUnderlyingTypeOrErr.takeError();
 
-  return Importer.getToContext().getDecltypeType(ToExpr, UnderlyingType);
+  return Importer.getToContext().getDecltypeType(
+      *ToExprOrErr, *ToUnderlyingTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitUnaryTransformType(const UnaryTransformType *T) {
-  QualType ToBaseType = Importer.Import(T->getBaseType());
-  QualType ToUnderlyingType = Importer.Import(T->getUnderlyingType());
-  if (ToBaseType.isNull() || ToUnderlyingType.isNull())
-    return {};
+ExpectedType
+ASTNodeImporter::VisitUnaryTransformType(const UnaryTransformType *T) {
+  ExpectedType ToBaseTypeOrErr = import(T->getBaseType());
+  if (!ToBaseTypeOrErr)
+    return ToBaseTypeOrErr.takeError();
+
+  ExpectedType ToUnderlyingTypeOrErr = import(T->getUnderlyingType());
+  if (!ToUnderlyingTypeOrErr)
+    return ToUnderlyingTypeOrErr.takeError();
 
-  return Importer.getToContext().getUnaryTransformType(ToBaseType,
-                                                       ToUnderlyingType,
-                                                       T->getUTTKind());
+  return Importer.getToContext().getUnaryTransformType(
+      *ToBaseTypeOrErr, *ToUnderlyingTypeOrErr, T->getUTTKind());
 }
 
-QualType ASTNodeImporter::VisitAutoType(const AutoType *T) {
+ExpectedType ASTNodeImporter::VisitAutoType(const AutoType *T) {
   // FIXME: Make sure that the "to" context supports C++11!
-  QualType FromDeduced = T->getDeducedType();
-  QualType ToDeduced;
-  if (!FromDeduced.isNull()) {
-    ToDeduced = Importer.Import(FromDeduced);
-    if (ToDeduced.isNull())
-      return {};
-  }
+  ExpectedType ToDeducedTypeOrErr = import(T->getDeducedType());
+  if (!ToDeducedTypeOrErr)
+    return ToDeducedTypeOrErr.takeError();
 
-  return Importer.getToContext().getAutoType(ToDeduced, T->getKeyword(),
+  return Importer.getToContext().getAutoType(*ToDeducedTypeOrErr,
+                                             T->getKeyword(),
                                              /*IsDependent*/false);
 }
 
-QualType ASTNodeImporter::VisitInjectedClassNameType(
+ExpectedType ASTNodeImporter::VisitInjectedClassNameType(
     const InjectedClassNameType *T) {
-  auto *D = cast_or_null<CXXRecordDecl>(Importer.Import(T->getDecl()));
-  if (!D)
-    return {};
-
-  QualType InjType = Importer.Import(T->getInjectedSpecializationType());
-  if (InjType.isNull())
-    return {};
+  Expected<CXXRecordDecl *> ToDeclOrErr = import(T->getDecl());
+  if (!ToDeclOrErr)
+    return ToDeclOrErr.takeError();
+
+  ExpectedType ToInjTypeOrErr = import(T->getInjectedSpecializationType());
+  if (!ToInjTypeOrErr)
+    return ToInjTypeOrErr.takeError();
 
   // FIXME: ASTContext::getInjectedClassNameType is not suitable for AST reading
   // See comments in InjectedClassNameType definition for details
   // return Importer.getToContext().getInjectedClassNameType(D, InjType);
   enum {
     TypeAlignmentInBits = 4,
     TypeAlignment = 1 << TypeAlignmentInBits
   };
 
   return QualType(new (Importer.getToContext(), TypeAlignment)
-                  InjectedClassNameType(D, InjType), 0);
+                  InjectedClassNameType(*ToDeclOrErr, *ToInjTypeOrErr), 0);
 }
 
-QualType ASTNodeImporter::VisitRecordType(const RecordType *T) {
-  auto *ToDecl = dyn_cast_or_null<RecordDecl>(Importer.Import(T->getDecl()));
-  if (!ToDecl)
-    return {};
+ExpectedType ASTNodeImporter::VisitRecordType(const RecordType *T) {
+  Expected<RecordDecl *> ToDeclOrErr = import(T->getDecl());
+  if (!ToDeclOrErr)
+    return ToDeclOrErr.takeError();
 
-  return Importer.getToContext().getTagDeclType(ToDecl);
+  return Importer.getToContext().getTagDeclType(*ToDeclOrErr);
 }
 
-QualType ASTNodeImporter::VisitEnumType(const EnumType *T) {
-  auto *ToDecl = dyn_cast_or_null<EnumDecl>(Importer.Import(T->getDecl()));
-  if (!ToDecl)
-    return {};
+ExpectedType ASTNodeImporter::VisitEnumType(const EnumType *T) {
+  Expected<EnumDecl *> ToDeclOrErr = import(T->getDecl());
+  if (!ToDeclOrErr)
+    return ToDeclOrErr.takeError();
 
-  return Importer.getToContext().getTagDeclType(ToDecl);
+  return Importer.getToContext().getTagDeclType(*ToDeclOrErr);
 }
 
-QualType ASTNodeImporter::VisitAttributedType(const AttributedType *T) {
-  QualType FromModifiedType = T->getModifiedType();
-  QualType FromEquivalentType = T->getEquivalentType();
-  QualType ToModifiedType;
-  QualType ToEquivalentType;
-
-  if (!FromModifiedType.isNull()) {
-    ToModifiedType = Importer.Import(FromModifiedType);
-    if (ToModifiedType.isNull())
-      return {};
-  }
-  if (!FromEquivalentType.isNull()) {
-    ToEquivalentType = Importer.Import(FromEquivalentType);
-    if (ToEquivalentType.isNull())
-      return {};
-  }
+ExpectedType ASTNodeImporter::VisitAttributedType(const AttributedType *T) {
+  ExpectedType ToModifiedTypeOrErr = import(T->getModifiedType());
+  if (!ToModifiedTypeOrErr)
+    return ToModifiedTypeOrErr.takeError();
+  ExpectedType ToEquivalentTypeOrErr = import(T->getEquivalentType());
+  if (!ToEquivalentTypeOrErr)
+    return ToEquivalentTypeOrErr.takeError();
 
   return Importer.getToContext().getAttributedType(T->getAttrKind(),
-    ToModifiedType, ToEquivalentType);
+      *ToModifiedTypeOrErr, *ToEquivalentTypeOrErr);
 }
 
-QualType ASTNodeImporter::VisitTemplateTypeParmType(
+ExpectedType ASTNodeImporter::VisitTemplateTypeParmType(
     const TemplateTypeParmType *T) {
-  auto *ParmDecl =
-      cast_or_null<TemplateTypeParmDecl>(Importer.Import(T->getDecl()));
-  if (!ParmDecl && T->getDecl())
-    return {};
+  Expected<TemplateTypeParmDecl *> ToDeclOrErr = import(T->getDecl());
+  if (!ToDeclOrErr)
+    return ToDeclOrErr.takeError();
 
   return Importer.getToContext().getTemplateTypeParmType(
-        T->getDepth(), T->getIndex(), T->isParameterPack(), ParmDecl);
+      T->getDepth(), T->getIndex(), T->isParameterPack(), *ToDeclOrErr);
 }
 
-QualType ASTNodeImporter::VisitSubstTemplateTypeParmType(
+ExpectedType ASTNodeImporter::VisitSubstTemplateTypeParmType(
     const SubstTemplateTypeParmType *T) {
-  const auto *Replaced =
-      cast_or_null<TemplateTypeParmType>(Importer.Import(
-        QualType(T->getReplacedParameter(), 0)).getTypePtr());
-  if (!Replaced)
-    return {};
-
-  QualType Replacement = Importer.Import(T->getReplacementType());
-  if (Replacement.isNull())
-    return {};
-  Replacement = Replacement.getCanonicalType();
+  ExpectedType ReplacedOrErr = import(QualType(T->getReplacedParameter(), 0));
+  if (!ReplacedOrErr)
+    return ReplacedOrErr.takeError();
+  const TemplateTypeParmType *Replaced =
+      cast<TemplateTypeParmType>((*ReplacedOrErr).getTypePtr());
+
+  ExpectedType ToReplacementTypeOrErr = import(T->getReplacementType());
+  if (!ToReplacementTypeOrErr)
+    return ToReplacementTypeOrErr.takeError();
 
   return Importer.getToContext().getSubstTemplateTypeParmType(
-        Replaced, Replacement);
+        Replaced, (*ToReplacementTypeOrErr).getCanonicalType());
 }
 
-QualType ASTNodeImporter::VisitTemplateSpecializationType(
+ExpectedType ASTNodeImporter::VisitTemplateSpecializationType(
                                        const TemplateSpecializationType *T) {
-  TemplateName ToTemplate = Importer.Import(T->getTemplateName());
-  if (ToTemplate.isNull())
-    return {};
+  auto ToTemplateOrErr = import(T->getTemplateName());
+  if (!ToTemplateOrErr)
+    return ToTemplateOrErr.takeError();
 
   SmallVector<TemplateArgument, 2> ToTemplateArgs;
-  if (ImportTemplateArguments(T->getArgs(), T->getNumArgs(), ToTemplateArgs))
-    return {};
+  if (Error Err = ImportTemplateArguments(
+      T->getArgs(), T->getNumArgs(), ToTemplateArgs))
+    return std::move(Err);
 
   QualType ToCanonType;
   if (!QualType(T, 0).isCanonical()) {
     QualType FromCanonType
       = Importer.getFromContext().getCanonicalType(QualType(T, 0));
-    ToCanonType =Importer.Import(FromCanonType);
-    if (ToCanonType.isNull())
-      return {};
+    if (ExpectedType TyOrErr = import(FromCanonType))
+      ToCanonType = *TyOrErr;
+    else
+      return TyOrErr.takeError();
   }
-  return Importer.getToContext().getTemplateSpecializationType(ToTemplate,
+  return Importer.getToContext().getTemplateSpecializationType(*ToTemplateOrErr,
                                                                ToTemplateArgs,
                                                                ToCanonType);
 }
 
-QualType ASTNodeImporter::VisitElaboratedType(const ElaboratedType *T) {
-  NestedNameSpecifier *ToQualifier = nullptr;
+ExpectedType ASTNodeImporter::VisitElaboratedType(const ElaboratedType *T) {
   // Note: the qualifier in an ElaboratedType is optional.
-  if (T->getQualifier()) {
-    ToQualifier = Importer.Import(T->getQualifier());
-    if (!ToQualifier)
-      return {};
-  }
-
-  QualType ToNamedType = Importer.Import(T->getNamedType());
-  if (ToNamedType.isNull())
-    return {};
-
-  TagDecl *OwnedTagDecl =
-      cast_or_null<TagDecl>(Importer.Import(T->getOwnedTagDecl()));
-  if (!OwnedTagDecl && T->getOwnedTagDecl())
-    return {};
+  auto ToQualifierOrErr = import(T->getQualifier());
+  if (!ToQualifierOrErr)
+    return ToQualifierOrErr.takeError();
+
+  ExpectedType ToNamedTypeOrErr = import(T->getNamedType());
+  if (!ToNamedTypeOrErr)
+    return ToNamedTypeOrErr.takeError();
+
+  Expected<TagDecl *> ToOwnedTagDeclOrErr = import(T->getOwnedTagDecl());
+  if (!ToOwnedTagDeclOrErr)
+    return ToOwnedTagDeclOrErr.takeError();
 
   return Importer.getToContext().getElaboratedType(T->getKeyword(),
-                                                   ToQualifier, ToNamedType,
-                                                   OwnedTagDecl);
+                                                   *ToQualifierOrErr,
+                                                   *ToNamedTypeOrErr,
+                                                   *ToOwnedTagDeclOrErr);
 }
 
-QualType ASTNodeImporter::VisitPackExpansionType(const PackExpansionType *T) {
-  QualType Pattern = Importer.Import(T->getPattern());
-  if (Pattern.isNull())
-    return {};
+ExpectedType
+ASTNodeImporter::VisitPackExpansionType(const PackExpansionType *T) {
+  ExpectedType ToPatternOrErr = import(T->getPattern());
+  if (!ToPatternOrErr)
+    return ToPatternOrErr.takeError();
 
-  return Importer.getToContext().getPackExpansionType(Pattern,
+  return Importer.getToContext().getPackExpansionType(*ToPatternOrErr,
                                                       T->getNumExpansions());
 }
 
-QualType ASTNodeImporter::VisitDependentTemplateSpecializationType(
+ExpectedType ASTNodeImporter::VisitDependentTemplateSpecializationType(
     const DependentTemplateSpecializationType *T) {
-  NestedNameSpecifier *Qualifier = Importer.Import(T->getQualifier());
-  if (!Qualifier && T->getQualifier())
-    return {};
+  auto ToQualifierOrErr = import(T->getQualifier());
+  if (!ToQualifierOrErr)
+    return ToQualifierOrErr.takeError();
 
-  IdentifierInfo *Name = Importer.Import(T->getIdentifier());
-  if (!Name && T->getIdentifier())
-    return {};
+  IdentifierInfo *ToName = Importer.Import(T->getIdentifier());
 
   SmallVector<TemplateArgument, 2> ToPack;
   ToPack.reserve(T->getNumArgs());
-  if (ImportTemplateArguments(T->getArgs(), T->getNumArgs(), ToPack))
-    return {};
+  if (Error Err = ImportTemplateArguments(
+      T->getArgs(), T->getNumArgs(), ToPack))
+    return std::move(Err);
 
   return Importer.getToContext().getDependentTemplateSpecializationType(
-        T->getKeyword(), Qualifier, Name, ToPack);
+      T->getKeyword(), *ToQualifierOrErr, ToName, ToPack);
 }
 
-QualType ASTNodeImporter::VisitDependentNameType(const DependentNameType *T) {
-  NestedNameSpecifier *NNS = Importer.Import(T->getQualifier());
-  if (!NNS && T->getQualifier())
-    return QualType();
+ExpectedType
+ASTNodeImporter::VisitDependentNameType(const DependentNameType *T) {
+  auto ToQualifierOrErr = import(T->getQualifier());
+  if (!ToQualifierOrErr)
+    return ToQualifierOrErr.takeError();
 
   IdentifierInfo *Name = Importer.Import(T->getIdentifier());
-  if (!Name && T->getIdentifier())
-    return QualType();
 
-  QualType Canon = (T == T->getCanonicalTypeInternal().getTypePtr())
-                       ? QualType()
-                       : Importer.Import(T->getCanonicalTypeInternal());
-  if (!Canon.isNull())
-    Canon = Canon.getCanonicalType();
+  QualType Canon;
+  if (T != T->getCanonicalTypeInternal().getTypePtr()) {
+    if (ExpectedType TyOrErr = import(T->getCanonicalTypeInternal()))
+      Canon = (*TyOrErr).getCanonicalType();
+    else
+      return TyOrErr.takeError();
+  }
 
-  return Importer.getToContext().getDependentNameType(T->getKeyword(), NNS,
+  return Importer.getToContext().getDependentNameType(T->getKeyword(),
+                                                      *ToQualifierOrErr,
                                                       Name, Canon);
 }
 
-QualType ASTNodeImporter::VisitObjCInterfaceType(const ObjCInterfaceType *T) {
-  auto *Class =
-      dyn_cast_or_null<ObjCInterfaceDecl>(Importer.Import(T->getDecl()));
-  if (!Class)
-    return {};
-
-  return Importer.getToContext().getObjCInterfaceType(Class);
+ExpectedType
+ASTNodeImporter::VisitObjCInterfaceType(const ObjCInterfaceType *T) {
+  Expected<ObjCInterfaceDecl *> ToDeclOrErr = import(T->getDecl());
+  if (!ToDeclOrErr)
+    return ToDeclOrErr.takeError();
+
+  return Importer.getToContext().getObjCInterfaceType(*ToDeclOrErr);
 }
 
-QualType ASTNodeImporter::VisitObjCObjectType(const ObjCObjectType *T) {
-  QualType ToBaseType = Importer.Import(T->getBaseType());
-  if (ToBaseType.isNull())
-    return {};
+ExpectedType ASTNodeImporter::VisitObjCObjectType(const ObjCObjectType *T) {
+  ExpectedType ToBaseTypeOrErr = import(T->getBaseType());
+  if (!ToBaseTypeOrErr)
+    return ToBaseTypeOrErr.takeError();
 
   SmallVector<QualType, 4> TypeArgs;
   for (auto TypeArg : T->getTypeArgsAsWritten()) {
-    QualType ImportedTypeArg = Importer.Import(TypeArg);
-    if (ImportedTypeArg.isNull())
-      return {};
-
-    TypeArgs.push_back(ImportedTypeArg);
+    if (ExpectedType TyOrErr = import(TypeArg))
+      TypeArgs.push_back(*TyOrErr);
+    else
+      return TyOrErr.takeError();
   }
 
   SmallVector<ObjCProtocolDecl *, 4> Protocols;
   for (auto *P : T->quals()) {
-    auto *Protocol = dyn_cast_or_null<ObjCProtocolDecl>(Importer.Import(P));
-    if (!Protocol)
-      return {};
-    Protocols.push_back(Protocol);
+    if (Expected<ObjCProtocolDecl *> ProtocolOrErr = import(P))
+      Protocols.push_back(*ProtocolOrErr);
+    else
+      return ProtocolOrErr.takeError();
+
   }
 
-  return Importer.getToContext().getObjCObjectType(ToBaseType, TypeArgs,
+  return Importer.getToContext().getObjCObjectType(*ToBaseTypeOrErr, TypeArgs,
                                                    Protocols,
                                                    T->isKindOfTypeAsWritten());
 }
 
-QualType
+ExpectedType
 ASTNodeImporter::VisitObjCObjectPointerType(const ObjCObjectPointerType *T) {
-  QualType ToPointeeType = Importer.Import(T->getPointeeType());
-  if (ToPointeeType.isNull())
-    return {};
+  ExpectedType ToPointeeTypeOrErr = import(T->getPointeeType());
+  if (!ToPointeeTypeOrErr)
+    return ToPointeeTypeOrErr.takeError();
 
-  return Importer.getToContext().getObjCObjectPointerType(ToPointeeType);
+  return Importer.getToContext().getObjCObjectPointerType(*ToPointeeTypeOrErr);
 }
 
 //----------------------------------------------------------------------------
 // Import Declarations
 //----------------------------------------------------------------------------
-bool ASTNodeImporter::ImportDeclParts(NamedDecl *D, DeclContext *&DC,
-                                      DeclContext *&LexicalDC,
-                                      DeclarationName &Name,
-                                      NamedDecl *&ToD,
-                                      SourceLocation &Loc) {
+Error ASTNodeImporter::ImportDeclParts(
+    NamedDecl *D, DeclContext *&DC, DeclContext *&LexicalDC,
+    DeclarationName &Name, NamedDecl *&ToD, SourceLocation &Loc) {
   // Check if RecordDecl is in FunctionDecl parameters to avoid infinite loop.
   // example: int struct_in_proto(struct data_t{int a;int b;} *d);
   DeclContext *OrigDC = D->getDeclContext();
   FunctionDecl *FunDecl;
   if (isa<RecordDecl>(D) && (FunDecl = dyn_cast<FunctionDecl>(OrigDC)) &&
       FunDecl->hasBody()) {
     auto getLeafPointeeType = [](const Type *T) {
       while (T->isPointerType() || T->isArrayType()) {
         T = T->getPointeeOrArrayElementType();
       }
       return T;
     };
     for (const ParmVarDecl *P : FunDecl->parameters()) {
       const Type *LeafT =
           getLeafPointeeType(P->getType().getCanonicalType().getTypePtr());
       auto *RT = dyn_cast<RecordType>(LeafT);
       if (RT && RT->getDecl() == D) {
         Importer.FromDiag(D->getLocation(), diag::err_unsupported_ast_node)
             << D->getDeclKindName();
-        return true;
+        return make_error<ImportError>(ImportError::UnsupportedConstruct);
       }
     }
   }
 
   // Import the context of this declaration.
-  DC = Importer.ImportContext(OrigDC);
-  if (!DC)
-    return true;
-
-  LexicalDC = DC;
-  if (D->getDeclContext() != D->getLexicalDeclContext()) {
-    LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
-    if (!LexicalDC)
-      return true;
-  }
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return Err;
 
   // Import the name of this declaration.
-  Name = Importer.Import(D->getDeclName());
-  if (D->getDeclName() && !Name)
-    return true;
+  if (Error Err = importInto(Name, D->getDeclName()))
+    return Err;
 
   // Import the location of this declaration.
-  Loc = Importer.Import(D->getLocation());
+  if (Error Err = importInto(Loc, D->getLocation()))
+    return Err;
+
   ToD = cast_or_null<NamedDecl>(Importer.GetAlreadyImportedOrNull(D));
-  return false;
+
+  return Error::success();
 }
 
-void ASTNodeImporter::ImportDefinitionIfNeeded(Decl *FromD, Decl *ToD) {
+Error ASTNodeImporter::ImportDefinitionIfNeeded(Decl *FromD, Decl *ToD) {
   if (!FromD)
-    return;
+    return Error::success();
 
-  if (!ToD) {
-    ToD = Importer.Import(FromD);
-    if (!ToD)
-      return;
-  }
+    if (Error Err = importInto(ToD, FromD))
+      return Err;
 
-  if (auto *FromRecord = dyn_cast<RecordDecl>(FromD)) {
-    if (auto *ToRecord = cast_or_null<RecordDecl>(ToD)) {
-      if (FromRecord->getDefinition() && FromRecord->isCompleteDefinition() && !ToRecord->getDefinition()) {
-        ImportDefinition(FromRecord, ToRecord);
+  if (RecordDecl *FromRecord = dyn_cast<RecordDecl>(FromD)) {
+    if (RecordDecl *ToRecord = cast<RecordDecl>(ToD)) {
+      if (FromRecord->getDefinition() && FromRecord->isCompleteDefinition() &&
+          !ToRecord->getDefinition()) {
+        if (Error Err = ImportDefinition(FromRecord, ToRecord))
+          return Err;
       }
     }
-    return;
+    return Error::success();
   }
 
-  if (auto *FromEnum = dyn_cast<EnumDecl>(FromD)) {
-    if (auto *ToEnum = cast_or_null<EnumDecl>(ToD)) {
+  if (EnumDecl *FromEnum = dyn_cast<EnumDecl>(FromD)) {
+    if (EnumDecl *ToEnum = cast<EnumDecl>(ToD)) {
       if (FromEnum->getDefinition() && !ToEnum->getDefinition()) {
-        ImportDefinition(FromEnum, ToEnum);
+        if (Error Err = ImportDefinition(FromEnum, ToEnum))
+          return Err;
       }
     }
-    return;
+    return Error::success();
   }
+
+  return Error::success();
 }
 
-void
-ASTNodeImporter::ImportDeclarationNameLoc(const DeclarationNameInfo &From,
-                                          DeclarationNameInfo& To) {
+Error
+ASTNodeImporter::ImportDeclarationNameLoc(
+    const DeclarationNameInfo &From, DeclarationNameInfo& To) {
   // NOTE: To.Name and To.Loc are already imported.
   // We only have to import To.LocInfo.
   switch (To.getName().getNameKind()) {
   case DeclarationName::Identifier:
   case DeclarationName::ObjCZeroArgSelector:
   case DeclarationName::ObjCOneArgSelector:
   case DeclarationName::ObjCMultiArgSelector:
   case DeclarationName::CXXUsingDirective:
   case DeclarationName::CXXDeductionGuideName:
-    return;
+    return Error::success();
 
   case DeclarationName::CXXOperatorName: {
-    SourceRange Range = From.getCXXOperatorNameRange();
-    To.setCXXOperatorNameRange(Importer.Import(Range));
-    return;
+    if (auto ToRangeOrErr = import(From.getCXXOperatorNameRange()))
+      To.setCXXOperatorNameRange(*ToRangeOrErr);
+    else
+      return ToRangeOrErr.takeError();
+    return Error::success();
   }
   case DeclarationName::CXXLiteralOperatorName: {
-    SourceLocation Loc = From.getCXXLiteralOperatorNameLoc();
-    To.setCXXLiteralOperatorNameLoc(Importer.Import(Loc));
-    return;
+    if (ExpectedSLoc LocOrErr = import(From.getCXXLiteralOperatorNameLoc()))
+      To.setCXXLiteralOperatorNameLoc(*LocOrErr);
+    else
+      return LocOrErr.takeError();
+    return Error::success();
   }
   case DeclarationName::CXXConstructorName:
   case DeclarationName::CXXDestructorName:
   case DeclarationName::CXXConversionFunctionName: {
-    TypeSourceInfo *FromTInfo = From.getNamedTypeInfo();
-    To.setNamedTypeInfo(Importer.Import(FromTInfo));
-    return;
+    if (auto ToTInfoOrErr = import(From.getNamedTypeInfo()))
+      To.setNamedTypeInfo(*ToTInfoOrErr);
+    else
+      return ToTInfoOrErr.takeError();
+    return Error::success();
   }
   }
   llvm_unreachable("Unknown name kind.");
 }
 
-void ASTNodeImporter::ImportDeclContext(DeclContext *FromDC, bool ForceImport) {
+Error
+ASTNodeImporter::ImportDeclContext(DeclContext *FromDC, bool ForceImport) {
   if (Importer.isMinimalImport() && !ForceImport) {
-    Importer.ImportContext(FromDC);
-    return;
+    auto ToDCOrErr = Importer.ImportContext(FromDC);
+    return ToDCOrErr.takeError();
+  }
+  llvm::SmallVector<Decl *, 8> ImportedDecls;
+  for (auto *From : FromDC->decls()) {
+    ExpectedDecl ImportedOrErr = import(From);
+    if (!ImportedOrErr)
+      // Ignore the error, continue with next Decl.
+      // FIXME: Handle this case somehow better.
+      consumeError(ImportedOrErr.takeError());
   }
 
-  for (auto *From : FromDC->decls())
-    Importer.Import(From);
+  return Error::success();
 }
 
-void ASTNodeImporter::ImportImplicitMethods(
+Error ASTNodeImporter::ImportDeclContext(
+    Decl *FromD, DeclContext *&ToDC, DeclContext *&ToLexicalDC) {
+  auto ToDCOrErr = Importer.ImportContext(FromD->getDeclContext());
+  if (!ToDCOrErr)
+    return ToDCOrErr.takeError();
+  ToDC = *ToDCOrErr;
+
+  if (FromD->getDeclContext() != FromD->getLexicalDeclContext()) {
+    auto ToLexicalDCOrErr = Importer.ImportContext(
+        FromD->getLexicalDeclContext());
+    if (!ToLexicalDCOrErr)
+      return ToLexicalDCOrErr.takeError();
+    ToLexicalDC = *ToLexicalDCOrErr;
+  } else
+    ToLexicalDC = ToDC;
+
+  return Error::success();
+}
+
+Error ASTNodeImporter::ImportImplicitMethods(
     const CXXRecordDecl *From, CXXRecordDecl *To) {
   assert(From->isCompleteDefinition() && To->getDefinition() == To &&
       "Import implicit methods to or from non-definition");
 
   for (CXXMethodDecl *FromM : From->methods())
-    if (FromM->isImplicit())
-      Importer.Import(FromM);
+    if (FromM->isImplicit()) {
+      Expected<CXXMethodDecl *> ToMOrErr = import(FromM);
+      if (!ToMOrErr)
+        return ToMOrErr.takeError();
+    }
+
+  return Error::success();
 }
 
-static void setTypedefNameForAnonDecl(TagDecl *From, TagDecl *To,
+static Error setTypedefNameForAnonDecl(TagDecl *From, TagDecl *To,
                                        ASTImporter &Importer) {
   if (TypedefNameDecl *FromTypedef = From->getTypedefNameForAnonDecl()) {
-    auto *ToTypedef =
-      cast_or_null<TypedefNameDecl>(Importer.Import(FromTypedef));
-    assert (ToTypedef && "Failed to import typedef of an anonymous structure");
-
-    To->setTypedefNameForAnonDecl(ToTypedef);
+    Decl *ToTypedef = Importer.Import(FromTypedef);
+    if (!ToTypedef)
+      return make_error<ImportError>();
+    To->setTypedefNameForAnonDecl(cast<TypedefNameDecl>(ToTypedef));
+    // FIXME: This should be the final code.
+    //if (Expected<Decl *> ToTypedefOrErr = Importer.Import(FromTypedef))
+    //  To->setTypedefNameForAnonDecl(cast<TypedefNameDecl>(*ToTypedefOrErr));
+    //else
+    //  return ToTypedefOrErr.takeError();
   }
+  return Error::success();
 }
 
-bool ASTNodeImporter::ImportDefinition(RecordDecl *From, RecordDecl *To,
-                                       ImportDefinitionKind Kind) {
+Error ASTNodeImporter::ImportDefinition(
+    RecordDecl *From, RecordDecl *To, ImportDefinitionKind Kind) {
   if (To->getDefinition() || To->isBeingDefined()) {
     if (Kind == IDK_Everything)
-      ImportDeclContext(From, /*ForceImport=*/true);
+      return ImportDeclContext(From, /*ForceImport=*/true);
 
-    return false;
+    return Error::success();
   }
 
   To->startDefinition();
 
-  setTypedefNameForAnonDecl(From, To, Importer);
+  if (Error Err = setTypedefNameForAnonDecl(From, To, Importer))
+    return Err;
 
   // Add base classes.
   if (auto *ToCXX = dyn_cast<CXXRecordDecl>(To)) {
     auto *FromCXX = cast<CXXRecordDecl>(From);
 
     struct CXXRecordDecl::DefinitionData &ToData = ToCXX->data();
     struct CXXRecordDecl::DefinitionData &FromData = FromCXX->data();
     ToData.UserDeclaredConstructor = FromData.UserDeclaredConstructor;
ToData.ImplicitCopyAssignmentHasConstParam
       = FromData.ImplicitCopyAssignmentHasConstParam;
     ToData.HasDeclaredCopyConstructorWithConstParam
       = FromData.HasDeclaredCopyConstructorWithConstParam;
     ToData.HasDeclaredCopyAssignmentWithConstParam
       = FromData.HasDeclaredCopyAssignmentWithConstParam;
 
     SmallVector<CXXBaseSpecifier *, 4> Bases;
     for (const auto &Base1 : FromCXX->bases()) {
-      QualType T = Importer.Import(Base1.getType());
-      if (T.isNull())
-        return true;
+      ExpectedType TyOrErr = import(Base1.getType());
+      if (!TyOrErr)
+        return TyOrErr.takeError();
 
       SourceLocation EllipsisLoc;
-      if (Base1.isPackExpansion())
-        EllipsisLoc = Importer.Import(Base1.getEllipsisLoc());
+      if (Base1.isPackExpansion()) {
+        if (ExpectedSLoc LocOrErr = import(Base1.getEllipsisLoc()))
+          EllipsisLoc = *LocOrErr;
+        else
+          return LocOrErr.takeError();
+      }
 
       // Ensure that we have a definition for the base.
-      ImportDefinitionIfNeeded(Base1.getType()->getAsCXXRecordDecl());
+      if (Error Err =
+          ImportDefinitionIfNeeded(Base1.getType()->getAsCXXRecordDecl()))
+        return Err;
+
+      auto RangeOrErr = import(Base1.getSourceRange());
+      if (!RangeOrErr)
+        return RangeOrErr.takeError();
+
+      auto TSIOrErr = import(Base1.getTypeSourceInfo());
+      if (!TSIOrErr)
+        return TSIOrErr.takeError();
 
       Bases.push_back(
-                    new (Importer.getToContext())
-                      CXXBaseSpecifier(Importer.Import(Base1.getSourceRange()),
-                                       Base1.isVirtual(),
-                                       Base1.isBaseOfClass(),
-                                       Base1.getAccessSpecifierAsWritten(),
-                                   Importer.Import(Base1.getTypeSourceInfo()),
-                                       EllipsisLoc));
+          new (Importer.getToContext()) CXXBaseSpecifier(
+              *RangeOrErr,
+              *TSIOrErr,
     }
     if (!Bases.empty())
       ToCXX->setBases(Bases.data(), Bases.size());
   }
 
   if (shouldForceImportDeclContext(Kind))
-    ImportDeclContext(From, /*ForceImport=*/true);
+    if (Error Err = ImportDeclContext(From, /*ForceImport=*/true))
+      return Err;
 
   To->completeDefinition();
-  return false;
+  return Error::success();
 }
 
-bool ASTNodeImporter::ImportInitializer(VarDecl *From, VarDecl *To) {
+Error ASTNodeImporter::ImportInitializer(VarDecl *From, VarDecl *To) {
   if (To->getAnyInitializer())
-    return false;
+    return Error::success();
 
   Expr *FromInit = From->getInit();
   if (!FromInit)
-    return false;
+    return Error::success();
 
-  Expr *ToInit = Importer.Import(const_cast<Expr *>(FromInit));
-  if (!ToInit)
-    return true;
+  ExpectedExpr ToInitOrErr = import(FromInit);
+  if (!ToInitOrErr)
+    return ToInitOrErr.takeError();
 
-  To->setInit(ToInit);
+  To->setInit(*ToInitOrErr);
   if (From->isInitKnownICE()) {
     EvaluatedStmt *Eval = To->ensureEvaluatedStmt();
     Eval->CheckedICE = true;
     Eval->IsICE = From->isInitICE();
   }
 
   // FIXME: Other bits to merge?
-  return false;
+  return Error::success();
 }
 
-bool ASTNodeImporter::ImportDefinition(EnumDecl *From, EnumDecl *To,
-                                       ImportDefinitionKind Kind) {
+Error ASTNodeImporter::ImportDefinition(
+    EnumDecl *From, EnumDecl *To, ImportDefinitionKind Kind) {
   if (To->getDefinition() || To->isBeingDefined()) {
     if (Kind == IDK_Everything)
-      ImportDeclContext(From, /*ForceImport=*/true);
-    return false;
+      return ImportDeclContext(From, /*ForceImport=*/true);
+    return Error::success();
   }
 
   To->startDefinition();
 
-  setTypedefNameForAnonDecl(From, To, Importer);
+  if (Error Err = setTypedefNameForAnonDecl(From, To, Importer))
+    return Err;
 
-  QualType T = Importer.Import(Importer.getFromContext().getTypeDeclType(From));
-  if (T.isNull())
-    return true;
-
-  QualType ToPromotionType = Importer.Import(From->getPromotionType());
-  if (ToPromotionType.isNull())
-    return true;
+  ExpectedType ToTypeOrErr =
+      import(Importer.getFromContext().getTypeDeclType(From));
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+
+  ExpectedType ToPromotionTypeOrErr = import(From->getPromotionType());
+  if (!ToPromotionTypeOrErr)
+    return ToPromotionTypeOrErr.takeError();
 
   if (shouldForceImportDeclContext(Kind))
-    ImportDeclContext(From, /*ForceImport=*/true);
+    if (Error Err = ImportDeclContext(From, /*ForceImport=*/true))
+      return Err;
 
   // FIXME: we might need to merge the number of positive or negative bits
   // if the enumerator lists don't match.
-  To->completeDefinition(T, ToPromotionType,
+  To->completeDefinition(*ToTypeOrErr, *ToPromotionTypeOrErr,
                          From->getNumPositiveBits(),
                          From->getNumNegativeBits());
-  return false;
+  return Error::success();
 }
 
-TemplateParameterList *ASTNodeImporter::ImportTemplateParameterList(
-                                                TemplateParameterList *Params) {
+// FIXME: Remove this, use `import` instead.
+Expected<TemplateParameterList *> ASTNodeImporter::ImportTemplateParameterList(
   SmallVector<NamedDecl *, 4> ToParams(Params->size());
-  if (ImportContainerChecked(*Params, ToParams))
-    return nullptr;
+  if (Error Err = ImportContainerChecked(*Params, ToParams))
+    return std::move(Err);
 
   Expr *ToRequiresClause;
   if (Expr *const R = Params->getRequiresClause()) {
-    ToRequiresClause = Importer.Import(R);
-    if (!ToRequiresClause)
-      return nullptr;
+    if (Error Err = importInto(ToRequiresClause, R))
+      return std::move(Err);
   } else {
     ToRequiresClause = nullptr;
   }
 
-  return TemplateParameterList::Create(Importer.getToContext(),
-                                       Importer.Import(Params->getTemplateLoc()),
-                                       Importer.Import(Params->getLAngleLoc()),
-                                       ToParams,
-                                       Importer.Import(Params->getRAngleLoc()),
-                                       ToRequiresClause);
+  auto ToTemplateLocOrErr = import(Params->getTemplateLoc());
+  if (!ToTemplateLocOrErr)
+    return ToTemplateLocOrErr.takeError();
+  auto ToLAngleLocOrErr = import(Params->getLAngleLoc());
+  if (!ToLAngleLocOrErr)
+    return ToLAngleLocOrErr.takeError();
+  auto ToRAngleLocOrErr = import(Params->getRAngleLoc());
+  if (!ToRAngleLocOrErr)
+    return ToRAngleLocOrErr.takeError();
+
+  return TemplateParameterList::Create(
+      Importer.getToContext(),
+      *ToTemplateLocOrErr,
+      *ToLAngleLocOrErr,
+      *ToRAngleLocOrErr,
 }
 
-TemplateArgument
-ASTNodeImporter::ImportTemplateArgument(const TemplateArgument &From) {
-  switch (From.getKind()) {
-  case TemplateArgument::Null:
-    return TemplateArgument();
-
-  case TemplateArgument::Type: {
-    QualType ToType = Importer.Import(From.getAsType());
-    if (ToType.isNull())
-      return {};
-    return TemplateArgument(ToType);
-  }
-
-  case TemplateArgument::Integral: {
-    QualType ToType = Importer.Import(From.getIntegralType());
-    if (ToType.isNull())
-      return {};
-    return TemplateArgument(From, ToType);
-  }
-
-  case TemplateArgument::Declaration: {
-    auto *To = cast_or_null<ValueDecl>(Importer.Import(From.getAsDecl()));
-    QualType ToType = Importer.Import(From.getParamTypeForDecl());
-    if (!To || ToType.isNull())
-      return {};
-    return TemplateArgument(To, ToType);
-  }
-
-  case TemplateArgument::NullPtr: {
-    QualType ToType = Importer.Import(From.getNullPtrType());
-    if (ToType.isNull())
-      return {};
-    return TemplateArgument(ToType, /*isNullPtr*/true);
-  }
-
-  case TemplateArgument::Template: {
-    TemplateName ToTemplate = Importer.Import(From.getAsTemplate());
-    if (ToTemplate.isNull())
-      return {};
-
-    return TemplateArgument(ToTemplate);
-  }
-
-  case TemplateArgument::TemplateExpansion: {
-    TemplateName ToTemplate
-      = Importer.Import(From.getAsTemplateOrTemplatePattern());
-    if (ToTemplate.isNull())
-      return {};
-
-    return TemplateArgument(ToTemplate, From.getNumTemplateExpansions());
-  }
-
-  case TemplateArgument::Expression:
-    if (Expr *ToExpr = Importer.Import(From.getAsExpr()))
-      return TemplateArgument(ToExpr);
-    return TemplateArgument();
-
-  case TemplateArgument::Pack: {
-    SmallVector<TemplateArgument, 2> ToPack;
-    ToPack.reserve(From.pack_size());
-    if (ImportTemplateArguments(From.pack_begin(), From.pack_size(), ToPack))
-      return {};
-
-    return TemplateArgument(
-        llvm::makeArrayRef(ToPack).copy(Importer.getToContext()));
-  }
-  }
-
-  llvm_unreachable("Invalid template argument kind");
-}
-
-Optional<TemplateArgumentLoc>
-ASTNodeImporter::ImportTemplateArgumentLoc(const TemplateArgumentLoc &TALoc) {
-  TemplateArgument Arg = ImportTemplateArgument(TALoc.getArgument());
-  TemplateArgumentLocInfo FromInfo = TALoc.getLocInfo();
-  TemplateArgumentLocInfo ToInfo;
-  if (Arg.getKind() == TemplateArgument::Expression) {
-    Expr *E = Importer.Import(FromInfo.getAsExpr());
-    ToInfo = TemplateArgumentLocInfo(E);
-    if (!E)
-      return None;
-  } else if (Arg.getKind() == TemplateArgument::Type) {
-    if (TypeSourceInfo *TSI = Importer.Import(FromInfo.getAsTypeSourceInfo()))
-      ToInfo = TemplateArgumentLocInfo(TSI);
-    else
-      return None;
-  } else {
-    ToInfo = TemplateArgumentLocInfo(
-          Importer.Import(FromInfo.getTemplateQualifierLoc()),
-          Importer.Import(FromInfo.getTemplateNameLoc()),
-          Importer.Import(FromInfo.getTemplateEllipsisLoc()));
-  }
-  return TemplateArgumentLoc(Arg, ToInfo);
-}
-
-bool ASTNodeImporter::ImportTemplateArguments(const TemplateArgument *FromArgs,
-                                              unsigned NumFromArgs,
-                              SmallVectorImpl<TemplateArgument> &ToArgs) {
+Error ASTNodeImporter::ImportTemplateArguments(
+    const TemplateArgument *FromArgs, unsigned NumFromArgs,
   for (unsigned I = 0; I != NumFromArgs; ++I) {
-    TemplateArgument To = ImportTemplateArgument(FromArgs[I]);
-    if (To.isNull() && !FromArgs[I].isNull())
-      return true;
+    if (auto ToOrErr = import(FromArgs[I]))
+      ToArgs.push_back(*ToOrErr);
+    else
+      return ToOrErr.takeError();
+  }
 
-    ToArgs.push_back(To);
-  }
+  return Error::success();
 
-  return false;
+// FIXME: Do not forget to remove this and use only 'import'.
+Expected<TemplateArgument>
+ASTNodeImporter::ImportTemplateArgument(const TemplateArgument &From) {
+  return import(From);
 }
 
-// We cannot use Optional<> pattern here and below because
-// TemplateArgumentListInfo's operator new is declared as deleted so it cannot
-// be stored in Optional.
 template <typename InContainerTy>
-bool ASTNodeImporter::ImportTemplateArgumentListInfo(
+Error ASTNodeImporter::ImportTemplateArgumentListInfo(
     const InContainerTy &Container, TemplateArgumentListInfo &ToTAInfo) {
   for (const auto &FromLoc : Container) {
-    if (auto ToLoc = ImportTemplateArgumentLoc(FromLoc))
-      ToTAInfo.addArgument(*ToLoc);
+    if (auto ToLocOrErr = import(FromLoc))
+      ToTAInfo.addArgument(*ToLocOrErr);
     else
-      return true;
+      return ToLocOrErr.takeError();
   }
-  return false;
+  return Error::success();
 }
 
 static StructuralEquivalenceKind
 getStructuralEquivalenceKind(const ASTImporter &Importer) {
   return Importer.isMinimalImport() ? StructuralEquivalenceKind::Minimal
                                     : StructuralEquivalenceKind::Default;
 }
 
bool ASTNodeImporter::IsStructuralMatch(VarTemplateDecl *From,
                                         VarTemplateDecl *To) {
   StructuralEquivalenceContext Ctx(Importer.getFromContext(),
                                    Importer.getToContext(),
                                    Importer.getNonEquivalentDecls(),
                                    getStructuralEquivalenceKind(Importer));
   return Ctx.IsEquivalent(From, To);
 }
 
-Decl *ASTNodeImporter::VisitDecl(Decl *D) {
+ExpectedDecl ASTNodeImporter::VisitDecl(Decl *D) {
   Importer.FromDiag(D->getLocation(), diag::err_unsupported_ast_node)
     << D->getDeclKindName();
-  return nullptr;
+  return make_error<ImportError>(ImportError::UnsupportedConstruct);
 }
 
-Decl *ASTNodeImporter::VisitEmptyDecl(EmptyDecl *D) {
-  // Import the context of this declaration.
-  DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-  if (!DC)
-    return nullptr;
-
-  DeclContext *LexicalDC = DC;
-  if (D->getDeclContext() != D->getLexicalDeclContext()) {
-    LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
-    if (!LexicalDC)
-      return nullptr;
-  }
+ExpectedDecl ASTNodeImporter::VisitImportDecl(ImportDecl *D) {
+  Importer.FromDiag(D->getLocation(), diag::err_unsupported_ast_node)
+      << D->getDeclKindName();
+  return make_error<ImportError>(ImportError::UnsupportedConstruct);
 
+ExpectedDecl ASTNodeImporter::VisitEmptyDecl(EmptyDecl *D) {
+  // Import the context of this declaration.
+  DeclContext *DC, *LexicalDC;
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return std::move(Err);
+
   // Import the location of this declaration.
-  SourceLocation Loc = Importer.Import(D->getLocation());
+  ExpectedSLoc LocOrErr = import(D->getLocation());
+  if (!LocOrErr)
+    return LocOrErr.takeError();
 
   EmptyDecl *ToD;
-  if (GetImportedOrCreateDecl(ToD, D, Importer.getToContext(), DC, Loc))
+  if (GetImportedOrCreateDecl(ToD, D, Importer.getToContext(), DC, *LocOrErr))
     return ToD;
 
   ToD->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToD);
   return ToD;
 }
 
-Decl *ASTNodeImporter::VisitTranslationUnitDecl(TranslationUnitDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitTranslationUnitDecl(TranslationUnitDecl *D) {
   TranslationUnitDecl *ToD =
     Importer.getToContext().getTranslationUnitDecl();
 
   Importer.MapImported(D, ToD);
 
   return ToD;
 }
 
-Decl *ASTNodeImporter::VisitAccessSpecDecl(AccessSpecDecl *D) {
-  SourceLocation Loc = Importer.Import(D->getLocation());
-  SourceLocation ColonLoc = Importer.Import(D->getColonLoc());
+ExpectedDecl ASTNodeImporter::VisitAccessSpecDecl(AccessSpecDecl *D) {
+  ExpectedSLoc LocOrErr = import(D->getLocation());
+  if (!LocOrErr)
+    return LocOrErr.takeError();
+  auto ColonLocOrErr = import(D->getColonLoc());
+  if (!ColonLocOrErr)
+    return ColonLocOrErr.takeError();
 
   // Import the context of this declaration.
-  DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-  if (!DC)
-    return nullptr;
+  auto DCOrErr = Importer.ImportContext(D->getDeclContext());
+  if (!DCOrErr)
+    return DCOrErr.takeError();
+  DeclContext *DC = *DCOrErr;
 
   AccessSpecDecl *ToD;
   if (GetImportedOrCreateDecl(ToD, D, Importer.getToContext(), D->getAccess(),
-                              DC, Loc, ColonLoc))
+                              DC, *LocOrErr, *ColonLocOrErr))
     return ToD;
 
   // Lexical DeclContext and Semantic DeclContext
   // is always the same for the accessSpec.
   ToD->setLexicalDeclContext(DC);
   DC->addDeclInternal(ToD);
 
   return ToD;
 }
 
-Decl *ASTNodeImporter::VisitStaticAssertDecl(StaticAssertDecl *D) {
-  DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-  if (!DC)
-    return nullptr;
-
+ExpectedDecl ASTNodeImporter::VisitStaticAssertDecl(StaticAssertDecl *D) {
+  auto DCOrErr = Importer.ImportContext(D->getDeclContext());
+  if (!DCOrErr)
+    return DCOrErr.takeError();
+  DeclContext *DC = *DCOrErr;
   DeclContext *LexicalDC = DC;
 
-  // Import the location of this declaration.
-  SourceLocation Loc = Importer.Import(D->getLocation());
-
-  Expr *AssertExpr = Importer.Import(D->getAssertExpr());
-  if (!AssertExpr)
-    return nullptr;
-
-  StringLiteral *FromMsg = D->getMessage();
-  auto *ToMsg = cast_or_null<StringLiteral>(Importer.Import(FromMsg));
-  if (!ToMsg && FromMsg)
-    return nullptr;
+  SourceLocation ToLocation, ToRParenLoc;
+  Expr *ToAssertExpr;
+  StringLiteral *ToMessage;
+  if (auto Imp = importSeq(
+      D->getLocation(), D->getAssertExpr(), D->getMessage(), D->getRParenLoc()))
+    std::tie(ToLocation, ToAssertExpr, ToMessage, ToRParenLoc) = *Imp;
+  else
+    return Imp.takeError();
 
   StaticAssertDecl *ToD;
   if (GetImportedOrCreateDecl(
-          ToD, D, Importer.getToContext(), DC, Loc, AssertExpr, ToMsg,
-          Importer.Import(D->getRParenLoc()), D->isFailed()))
+      ToD, D, Importer.getToContext(), DC, ToLocation, ToAssertExpr, ToMessage,
+      ToRParenLoc, D->isFailed()))
     return ToD;
 
   ToD->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToD);
   return ToD;
 }
 
-Decl *ASTNodeImporter::VisitNamespaceDecl(NamespaceDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitNamespaceDecl(NamespaceDecl *D) {
   // Import the major distinguishing characteristics of this namespace.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   NamespaceDecl *MergeWithNamespace = nullptr;
   if (!Name) {
     // This is an anonymous namespace. Adopt an existing anonymous
     // namespace if we can.
     // FIXME: Not testable.
for (auto *FoundDecl : FoundDecls) {
 
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, Decl::IDNS_Namespace,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
+      if (!Name)
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
+  ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
+  if (!BeginLocOrErr)
+    return BeginLocOrErr.takeError();
+
   // Create the "to" namespace, if needed.
   NamespaceDecl *ToNamespace = MergeWithNamespace;
   if (!ToNamespace) {
     if (GetImportedOrCreateDecl(
             ToNamespace, D, Importer.getToContext(), DC, D->isInline(),
-            Importer.Import(D->getBeginLoc()), Loc, Name.getAsIdentifierInfo(),
+            *BeginLocOrErr, Loc, Name.getAsIdentifierInfo(),
             /*PrevDecl=*/nullptr))
       return ToNamespace;
     ToNamespace->setLexicalDeclContext(LexicalDC);
     LexicalDC->addDeclInternal(ToNamespace);
 
     // If this is an anonymous namespace, register it as the anonymous
     // namespace within its context.
     if (!Name) {
       if (auto *TU = dyn_cast<TranslationUnitDecl>(DC))
         TU->setAnonymousNamespace(ToNamespace);
       else
         cast<NamespaceDecl>(DC)->setAnonymousNamespace(ToNamespace);
     }
   }
   Importer.MapImported(D, ToNamespace);
 
-  ImportDeclContext(D);
+  if (Error Err = ImportDeclContext(D))
+    return std::move(Err);
 
   return ToNamespace;
 }
 
-Decl *ASTNodeImporter::VisitNamespaceAliasDecl(NamespaceAliasDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitNamespaceAliasDecl(NamespaceAliasDecl *D) {
   // Import the major distinguishing characteristics of this namespace.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *LookupD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, LookupD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, LookupD, Loc))
+    return std::move(Err);
   if (LookupD)
     return LookupD;
 
   // NOTE: No conflict resolution is done for namespace aliases now.
 
-  auto *TargetDecl = cast_or_null<NamespaceDecl>(
-      Importer.Import(D->getNamespace()));
-  if (!TargetDecl)
-    return nullptr;
-
-  IdentifierInfo *ToII = Importer.Import(D->getIdentifier());
-  if (!ToII)
-    return nullptr;
-
-  NestedNameSpecifierLoc ToQLoc = Importer.Import(D->getQualifierLoc());
-  if (D->getQualifierLoc() && !ToQLoc)
-    return nullptr;
+  SourceLocation ToNamespaceLoc, ToAliasLoc, ToTargetNameLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  NamespaceDecl *ToNamespace;
+  if (auto Imp = importSeq(
+      D->getNamespaceLoc(), D->getAliasLoc(), D->getQualifierLoc(),
+      D->getTargetNameLoc(), D->getNamespace()))
+    std::tie(
+        ToNamespaceLoc, ToAliasLoc, ToQualifierLoc, ToTargetNameLoc,
+        ToNamespace) = *Imp;
+  else
+    return Imp.takeError();
+  IdentifierInfo *ToIdentifier = Importer.Import(D->getIdentifier());
 
   NamespaceAliasDecl *ToD;
-  if (GetImportedOrCreateDecl(ToD, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getNamespaceLoc()),
-                              Importer.Import(D->getAliasLoc()), ToII, ToQLoc,
-                              Importer.Import(D->getTargetNameLoc()),
-                              TargetDecl))
+  if (GetImportedOrCreateDecl(
+      ToD, D, Importer.getToContext(), DC, ToNamespaceLoc, ToAliasLoc,
+      ToIdentifier, ToQualifierLoc, ToTargetNameLoc, ToNamespace))
     return ToD;
 
   ToD->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToD);
 
   return ToD;
 }
 
-Decl *ASTNodeImporter::VisitTypedefNameDecl(TypedefNameDecl *D, bool IsAlias) {
+ExpectedDecl
+ASTNodeImporter::VisitTypedefNameDecl(TypedefNameDecl *D, bool IsAlias) {
   // Import the major distinguishing characteristics of this typedef.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // If this typedef is not in block scope, determine whether we've
   // seen a typedef with the same name (that we can merge with) or any
   // other entity by that name (which name lookup could conflict with).
   if (!DC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     unsigned IDNS = Decl::IDNS_Ordinary;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
     for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
       if (auto *FoundTypedef = dyn_cast<TypedefNameDecl>(FoundDecl)) {
-        if (Importer.IsStructurallyEquivalent(D->getUnderlyingType(),
-                                            FoundTypedef->getUnderlyingType()))
+        if (Importer.IsStructurallyEquivalent(
+                D->getUnderlyingType(), FoundTypedef->getUnderlyingType()))
             return Importer.MapImported(D, FoundTypedef);
       }
 
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
       if (!Name)
-        return nullptr;
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
-  // Import the underlying type of this typedef;
-  QualType T = Importer.Import(D->getUnderlyingType());
-  if (T.isNull())
-    return nullptr;
+  QualType ToUnderlyingType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  SourceLocation ToBeginLoc;
+  if (auto Imp = importSeq(
+      D->getUnderlyingType(), D->getTypeSourceInfo(), D->getBeginLoc()))
+    std::tie(ToUnderlyingType, ToTypeSourceInfo, ToBeginLoc) = *Imp;
+  else
+    return Imp.takeError();
 
   // Create the new typedef node.
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
-  SourceLocation StartL = Importer.Import(D->getBeginLoc());
-
+  // FIXME: ToUnderlyingType is not used.
   TypedefNameDecl *ToTypedef;
   if (IsAlias) {
     if (GetImportedOrCreateDecl<TypeAliasDecl>(
-            ToTypedef, D, Importer.getToContext(), DC, StartL, Loc,
-            Name.getAsIdentifierInfo(), TInfo))
+        ToTypedef, D, Importer.getToContext(), DC, ToBeginLoc, Loc,
+        Name.getAsIdentifierInfo(), ToTypeSourceInfo))
       return ToTypedef;
   } else if (GetImportedOrCreateDecl<TypedefDecl>(
-                 ToTypedef, D, Importer.getToContext(), DC, StartL, Loc,
-                 Name.getAsIdentifierInfo(), TInfo))
+      ToTypedef, D, Importer.getToContext(), DC, ToBeginLoc, Loc,
+      Name.getAsIdentifierInfo(), ToTypeSourceInfo))
     return ToTypedef;
 
   ToTypedef->setAccess(D->getAccess());
   ToTypedef->setLexicalDeclContext(LexicalDC);
 
   // Templated declarations should not appear in DeclContext.
   TypeAliasDecl *FromAlias = IsAlias ? cast<TypeAliasDecl>(D) : nullptr;
   if (!FromAlias || !FromAlias->getDescribedAliasTemplate())
     LexicalDC->addDeclInternal(ToTypedef);
 
   return ToTypedef;
 }
 
-Decl *ASTNodeImporter::VisitTypedefDecl(TypedefDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitTypedefDecl(TypedefDecl *D) {
   return VisitTypedefNameDecl(D, /*IsAlias=*/false);
 }
 
-Decl *ASTNodeImporter::VisitTypeAliasDecl(TypeAliasDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitTypeAliasDecl(TypeAliasDecl *D) {
   return VisitTypedefNameDecl(D, /*IsAlias=*/true);
 }
 
-Decl *ASTNodeImporter::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
+ExpectedDecl
+ASTNodeImporter::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
   // Import the major distinguishing characteristics of this typedef.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *FoundD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, FoundD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, FoundD, Loc))
+    return std::move(Err);
   if (FoundD)
     return FoundD;
 
   // If this typedef is not in block scope, determine whether we've
   // seen a typedef with the same name (that we can merge with) or any
   // other entity by that name (which name lookup could conflict with).
   if (!DC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     unsigned IDNS = Decl::IDNS_Ordinary;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
     for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
       if (auto *FoundAlias = dyn_cast<TypeAliasTemplateDecl>(FoundDecl))
         return Importer.MapImported(D, FoundAlias);
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
       if (!Name)
-        return nullptr;
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
-  TemplateParameterList *Params = ImportTemplateParameterList(
-        D->getTemplateParameters());
-  if (!Params)
-    return nullptr;
-
-  auto *TemplDecl = cast_or_null<TypeAliasDecl>(
-        Importer.Import(D->getTemplatedDecl()));
-  if (!TemplDecl)
-    return nullptr;
+  TemplateParameterList *ToTemplateParameters;
+  TypeAliasDecl *ToTemplatedDecl;
+  if (auto Imp = importSeq(D->getTemplateParameters(), D->getTemplatedDecl()))
+    std::tie(ToTemplateParameters, ToTemplatedDecl) = *Imp;
+  else
+    return Imp.takeError();
 
   TypeAliasTemplateDecl *ToAlias;
   if (GetImportedOrCreateDecl(ToAlias, D, Importer.getToContext(), DC, Loc,
-                              Name, Params, TemplDecl))
+                              Name, ToTemplateParameters, ToTemplatedDecl))
     return ToAlias;
 
-  TemplDecl->setDescribedAliasTemplate(ToAlias);
+  ToTemplatedDecl->setDescribedAliasTemplate(ToAlias);
 
   ToAlias->setAccess(D->getAccess());
   ToAlias->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToAlias);
   return ToAlias;
 }
 
-Decl *ASTNodeImporter::VisitLabelDecl(LabelDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitLabelDecl(LabelDecl *D) {
   // Import the major distinguishing characteristics of this label.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   assert(LexicalDC->isFunctionOrMethod());
 
   LabelDecl *ToLabel;
-  if (D->isGnuLocal()
-          ? GetImportedOrCreateDecl(ToLabel, D, Importer.getToContext(), DC,
-                                    Importer.Import(D->getLocation()),
-                                    Name.getAsIdentifierInfo(),
-                                    Importer.Import(D->getBeginLoc()))
-          : GetImportedOrCreateDecl(ToLabel, D, Importer.getToContext(), DC,
-                                    Importer.Import(D->getLocation()),
-                                    Name.getAsIdentifierInfo()))
+  if (D->isGnuLocal()) {
+    ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
+    if (!BeginLocOrErr)
+      return BeginLocOrErr.takeError();
+    if (GetImportedOrCreateDecl(ToLabel, D, Importer.getToContext(), DC, Loc,
+                                Name.getAsIdentifierInfo(), *BeginLocOrErr))
+      return ToLabel;
+
+  } else {
+    if (GetImportedOrCreateDecl(ToLabel, D, Importer.getToContext(), DC, Loc,
       return ToLabel;
 
-  auto *Label = cast_or_null<LabelStmt>(Importer.Import(D->getStmt()));
-  if (!Label)
-    return nullptr;
+  }
+
+  Expected<LabelStmt *> ToStmtOrErr = import(D->getStmt());
+  if (!ToStmtOrErr)
+    return ToStmtOrErr.takeError();
 
-  ToLabel->setStmt(Label);
+  ToLabel->setStmt(*ToStmtOrErr);
   ToLabel->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToLabel);
   return ToLabel;
 }
 
-Decl *ASTNodeImporter::VisitEnumDecl(EnumDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitEnumDecl(EnumDecl *D) {
   // Import the major distinguishing characteristics of this enum.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Figure out what enum name we're looking for.
   unsigned IDNS = Decl::IDNS_Tag;
   DeclarationName SearchName = Name;
   if (!SearchName && D->getTypedefNameForAnonDecl()) {
-    SearchName = Importer.Import(D->getTypedefNameForAnonDecl()->getDeclName());
+    if (Error Err = importInto(
+        SearchName, D->getTypedefNameForAnonDecl()->getDeclName()))
+      return std::move(Err);
     IDNS = Decl::IDNS_Ordinary;
   } else if (Importer.getToContext().getLangOpts().CPlusPlus)
     IDNS |= Decl::IDNS_Ordinary;
 
   // We may already have an enum of the same name; try to find and match it.
   if (!DC->isFunctionOrMethod() && SearchName) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(SearchName, FoundDecls);
     for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
 
-      Decl *Found = FoundDecl;
-      if (auto *Typedef = dyn_cast<TypedefNameDecl>(Found)) {
+      if (auto *Typedef = dyn_cast<TypedefNameDecl>(FoundDecl)) {
         if (const auto *Tag = Typedef->getUnderlyingType()->getAs<TagType>())
-          Found = Tag->getDecl();
+          FoundDecl = Tag->getDecl();
       }
 
-      if (auto *FoundEnum = dyn_cast<EnumDecl>(Found)) {
+      if (auto *FoundEnum = dyn_cast<EnumDecl>(FoundDecl)) {
         if (IsStructuralMatch(D, FoundEnum))
           return Importer.MapImported(D, FoundEnum);
       }
 
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
+      if (!Name)
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
+  SourceLocation ToBeginLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  QualType ToIntegerType;
+  if (auto Imp = importSeq(
+      D->getBeginLoc(), D->getQualifierLoc(), D->getIntegerType()))
+    std::tie(ToBeginLoc, ToQualifierLoc, ToIntegerType) = *Imp;
+  else
+    return Imp.takeError();
+
   // Create the enum declaration.
   EnumDecl *D2;
   if (GetImportedOrCreateDecl(
-          D2, D, Importer.getToContext(), DC, Importer.Import(D->getBeginLoc()),
+          D2, D, Importer.getToContext(), DC, ToBeginLoc,
           Loc, Name.getAsIdentifierInfo(), nullptr, D->isScoped(),
           D->isScopedUsingClassTag(), D->isFixed()))
     return D2;
 
-  // Import the qualifier, if any.
-  D2->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
+  D2->setQualifierInfo(ToQualifierLoc);
+  D2->setIntegerType(ToIntegerType);
   D2->setAccess(D->getAccess());
   D2->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(D2);
 
-  // Import the integer type.
-  QualType ToIntegerType = Importer.Import(D->getIntegerType());
-  if (ToIntegerType.isNull())
-    return nullptr;
-  D2->setIntegerType(ToIntegerType);
-
   // Import the definition
-  if (D->isCompleteDefinition() && ImportDefinition(D, D2))
-    return nullptr;
+  if (D->isCompleteDefinition())
+    if (Error Err = ImportDefinition(D, D2))
+      return std::move(Err);
 
   return D2;
 }
 
-Decl *ASTNodeImporter::VisitRecordDecl(RecordDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitRecordDecl(RecordDecl *D) {
   bool IsFriendTemplate = false;
   if (auto *DCXX = dyn_cast<CXXRecordDecl>(D)) {
     IsFriendTemplate =
         DCXX->getDescribedClassTemplate() &&
         DCXX->getDescribedClassTemplate()->getFriendObjectKind() !=
             Decl::FOK_None;
   }
 
   // If this record has a definition in the translation unit we're coming from,
   // but this particular declaration is not that definition, import the
   // definition and map to that.
   TagDecl *Definition = D->getDefinition();
   if (Definition && Definition != D &&
       // Friend template declaration must be imported on its own.
       !IsFriendTemplate &&
       // In contrast to a normal CXXRecordDecl, the implicit
       // CXXRecordDecl of ClassTemplateSpecializationDecl is its redeclaration.
       // The definition of the implicit CXXRecordDecl in this case is the
       // ClassTemplateSpecializationDecl itself. Thus, we start with an extra
       // condition in order to be able to import the implict Decl.
       !D->isImplicit()) {
-    Decl *ImportedDef = Importer.Import(Definition);
-    if (!ImportedDef)
-      return nullptr;
+    ExpectedDecl ImportedDefOrErr = import(Definition);
+    if (!ImportedDefOrErr)
+      return ImportedDefOrErr.takeError();
 
-    return Importer.MapImported(D, ImportedDef);
+    return Importer.MapImported(D, *ImportedDefOrErr);
   }
 
   // Import the major distinguishing characteristics of this record.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Figure out what structure name we're looking for.
   unsigned IDNS = Decl::IDNS_Tag;
   DeclarationName SearchName = Name;
   if (!SearchName && D->getTypedefNameForAnonDecl()) {
-    SearchName = Importer.Import(D->getTypedefNameForAnonDecl()->getDeclName());
+    if (Error Err = importInto(
+        SearchName, D->getTypedefNameForAnonDecl()->getDeclName()))
+      return std::move(Err);
     IDNS = Decl::IDNS_Ordinary;
   } else if (Importer.getToContext().getLangOpts().CPlusPlus)
     IDNS |= Decl::IDNS_Ordinary;
 
   // We may already have a record of the same name; try to find and match it.
   RecordDecl *AdoptDecl = nullptr;
   RecordDecl *PrevDecl = nullptr;
   if (!DC->isFunctionOrMethod()) {
for (auto *FoundDecl : FoundDecls) {
 
       Decl *Found = FoundDecl;
       if (auto *Typedef = dyn_cast<TypedefNameDecl>(Found)) {
         if (const auto *Tag = Typedef->getUnderlyingType()->getAs<TagType>())
           Found = Tag->getDecl();
       }
 
       if (D->getDescribedTemplate()) {
-        if (auto *Template = dyn_cast<ClassTemplateDecl>(Found))
+        if (auto *Template = dyn_cast<ClassTemplateDecl>(Found)) {
           Found = Template->getTemplatedDecl();
-        else
+        } else {
+          ConflictingDecls.push_back(FoundDecl);
           continue;
         }
+      }
 
       if (auto *FoundRecord = dyn_cast<RecordDecl>(Found)) {
         if (!SearchName) {
           if (!IsStructuralMatch(D, FoundRecord, false))
             continue;
+        } else {
+          if (!IsStructuralMatch(D, FoundRecord)) {
+            ConflictingDecls.push_back(FoundDecl);
+            continue;
+          }
         }
 
         PrevDecl = FoundRecord;
 
         if (RecordDecl *FoundDef = FoundRecord->getDefinition()) {
           if ((SearchName && !D->isCompleteDefinition() && !IsFriendTemplate)
               || (D->isCompleteDefinition() &&
                   D->isAnonymousStructOrUnion()
-                    == FoundDef->isAnonymousStructOrUnion() &&
-                  IsStructuralMatch(D, FoundDef))) {
+                    == FoundDef->isAnonymousStructOrUnion())) {
             // The record types structurally match, or the "from" translation
             // unit only had a forward declaration anyway; call it the same
             // function.
             // FIXME: Structural equivalence check should check for same
             // user-defined methods.
             Importer.MapImported(D, FoundDef);
             if (const auto *DCXX = dyn_cast<CXXRecordDecl>(D)) {
               auto *FoundCXX = dyn_cast<CXXRecordDecl>(FoundDef);
               assert(FoundCXX && "Record type mismatch");
 
               if (D->isCompleteDefinition() && !Importer.isMinimalImport())
                 // FoundDef may not have every implicit method that D has
                 // because implicit methods are created only if they are used.
-                ImportImplicitMethods(DCXX, FoundCXX);
+                if (Error Err = ImportImplicitMethods(DCXX, FoundCXX))
+                  return std::move(Err);
             }
             return FoundDef;
           }
+          if (IsFriendTemplate)
+            continue;
         } else if (!D->isCompleteDefinition()) {
           // We have a forward declaration of this type, so adopt that forward
           // declaration rather than building a new one.
 
           // If one or both can be completed from external storage then try one
           // last time to complete and compare them before doing this.
 
           if (FoundRecord->hasExternalLexicalStorage() &&
               !FoundRecord->isCompleteDefinition())
             FoundRecord->getASTContext().getExternalSource()->CompleteType(FoundRecord);
           if (D->hasExternalLexicalStorage())
             D->getASTContext().getExternalSource()->CompleteType(D);
 
           if (FoundRecord->isCompleteDefinition() &&
               D->isCompleteDefinition() &&
-              !IsStructuralMatch(D, FoundRecord))
+              !IsStructuralMatch(D, FoundRecord)) {
+            ConflictingDecls.push_back(FoundDecl);
             continue;
+          }
 
-          if (IsFriendTemplate)
-            continue;
+          AdoptDecl = FoundRecord;
+        }
 
-          AdoptDecl = FoundRecord;
-          continue;
-        } else if (!SearchName) {
-          continue;
-        }
-      }
+      } else if (isa<ValueDecl>(Found))
 
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty() && SearchName) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
+      if (!Name)
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
+  ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
+  if (!BeginLocOrErr)
+    return BeginLocOrErr.takeError();
+
   // Create the record declaration.
   RecordDecl *D2 = AdoptDecl;
-  SourceLocation StartLoc = Importer.Import(D->getBeginLoc());
   if (!D2) {
     CXXRecordDecl *D2CXX = nullptr;
     if (auto *DCXX = dyn_cast<CXXRecordDecl>(D)) {
       if (DCXX->isLambda()) {
-        TypeSourceInfo *TInfo = Importer.Import(DCXX->getLambdaTypeInfo());
+        auto TInfoOrErr = import(DCXX->getLambdaTypeInfo());
+        if (!TInfoOrErr)
+          return TInfoOrErr.takeError();
         if (GetImportedOrCreateSpecialDecl(
                 D2CXX, CXXRecordDecl::CreateLambda, D, Importer.getToContext(),
-                DC, TInfo, Loc, DCXX->isDependentLambda(),
+                DC, *TInfoOrErr, Loc, DCXX->isDependentLambda(),
                 DCXX->isGenericLambda(), DCXX->getLambdaCaptureDefault()))
           return D2CXX;
-        Decl *CDecl = Importer.Import(DCXX->getLambdaContextDecl());
-        if (DCXX->getLambdaContextDecl() && !CDecl)
-          return nullptr;
-        D2CXX->setLambdaMangling(DCXX->getLambdaManglingNumber(), CDecl);
+        ExpectedDecl CDeclOrErr = import(DCXX->getLambdaContextDecl());
+        if (!CDeclOrErr)
+          return CDeclOrErr.takeError();
+        D2CXX->setLambdaMangling(DCXX->getLambdaManglingNumber(), *CDeclOrErr);
       } else if (DCXX->isInjectedClassName()) {
         // We have to be careful to do a similar dance to the one in
         // Sema::ActOnStartCXXMemberDeclarations
         CXXRecordDecl *const PrevDecl = nullptr;
         const bool DelayTypeCreation = true;
         if (GetImportedOrCreateDecl(D2CXX, D, Importer.getToContext(),
-                                    D->getTagKind(), DC, StartLoc, Loc,
+                                    D->getTagKind(), DC, *BeginLocOrErr, Loc,
                                     Name.getAsIdentifierInfo(), PrevDecl,
                                     DelayTypeCreation))
           return D2CXX;
         Importer.getToContext().getTypeDeclType(
             D2CXX, dyn_cast<CXXRecordDecl>(DC));
       } else {
         if (GetImportedOrCreateDecl(D2CXX, D, Importer.getToContext(),
-                                    D->getTagKind(), DC, StartLoc, Loc,
+                                    D->getTagKind(), DC, *BeginLocOrErr, Loc,
                                     Name.getAsIdentifierInfo(),
                                     cast_or_null<CXXRecordDecl>(PrevDecl)))
           return D2CXX;
       }
 
       D2 = D2CXX;
       D2->setAccess(D->getAccess());
       D2->setLexicalDeclContext(LexicalDC);
       if (!DCXX->getDescribedClassTemplate() || DCXX->isImplicit())
         LexicalDC->addDeclInternal(D2);
 
       if (ClassTemplateDecl *FromDescribed =
           DCXX->getDescribedClassTemplate()) {
-        auto *ToDescribed = cast_or_null<ClassTemplateDecl>(
-            Importer.Import(FromDescribed));
-        if (!ToDescribed)
-          return nullptr;
+        ClassTemplateDecl *ToDescribed;
+        if (Error Err = importInto(ToDescribed, FromDescribed))
+          return std::move(Err);
         D2CXX->setDescribedClassTemplate(ToDescribed);
         if (!DCXX->isInjectedClassName() && !IsFriendTemplate) {
           // In a record describing a template the type should be an
           // InjectedClassNameType (see Sema::CheckClassTemplate). Update the
           // previously set type to the correct value here (ToDescribed is not
           // available at record create).
           // FIXME: The previous type is cleared but not removed from
           // ASTContext's internal storage.
if (auto *DCXX = dyn_cast<CXXRecordDecl>(D)) {
             Importer.getToContext().getTypeDeclType(Injected, D2CXX);
           }
         }
       } else if (MemberSpecializationInfo *MemberInfo =
                    DCXX->getMemberSpecializationInfo()) {
         TemplateSpecializationKind SK =
             MemberInfo->getTemplateSpecializationKind();
         CXXRecordDecl *FromInst = DCXX->getInstantiatedFromMemberClass();
-        auto *ToInst =
-            cast_or_null<CXXRecordDecl>(Importer.Import(FromInst));
-        if (FromInst && !ToInst)
-          return nullptr;
-        D2CXX->setInstantiationOfMemberClass(ToInst, SK);
+
+        if (Expected<CXXRecordDecl *> ToInstOrErr = import(FromInst))
+          D2CXX->setInstantiationOfMemberClass(*ToInstOrErr, SK);
+        else
+          return ToInstOrErr.takeError();
+
+        if (ExpectedSLoc POIOrErr =
+            import(MemberInfo->getPointOfInstantiation()))
           D2CXX->getMemberSpecializationInfo()->setPointOfInstantiation(
-              Importer.Import(MemberInfo->getPointOfInstantiation()));
+            *POIOrErr);
+        else
+          return POIOrErr.takeError();
       }
+
     } else {
       if (GetImportedOrCreateDecl(D2, D, Importer.getToContext(),
-                                  D->getTagKind(), DC, StartLoc, Loc,
+                                  D->getTagKind(), DC, *BeginLocOrErr, Loc,
                                   Name.getAsIdentifierInfo(), PrevDecl))
         return D2;
       D2->setLexicalDeclContext(LexicalDC);
       LexicalDC->addDeclInternal(D2);
     }
 
-    D2->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
+    if (auto QualifierLocOrErr = import(D->getQualifierLoc()))
+      D2->setQualifierInfo(*QualifierLocOrErr);
+    else
+      return QualifierLocOrErr.takeError();
+
     if (D->isAnonymousStructOrUnion())
       D2->setAnonymousStructOrUnion(true);
   }
 
   Importer.MapImported(D, D2);
 
-  if (D->isCompleteDefinition() && ImportDefinition(D, D2, IDK_Default))
-    return nullptr;
+  if (D->isCompleteDefinition())
+    if (Error Err = ImportDefinition(D, D2, IDK_Default))
+      return std::move(Err);
 
   return D2;
 }
 
-Decl *ASTNodeImporter::VisitEnumConstantDecl(EnumConstantDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitEnumConstantDecl(EnumConstantDecl *D) {
   // Import the major distinguishing characteristics of this enumerator.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
-
   // Determine whether there are any other declarations with the same name and
   // in the same context.
   if (!LexicalDC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     unsigned IDNS = Decl::IDNS_Ordinary;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
     for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
 
       if (auto *FoundEnumConstant = dyn_cast<EnumConstantDecl>(FoundDecl)) {
         if (IsStructuralMatch(D, FoundEnumConstant))
           return Importer.MapImported(D, FoundEnumConstant);
       }
 
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
       if (!Name)
-        return nullptr;
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
-  Expr *Init = Importer.Import(D->getInitExpr());
-  if (D->getInitExpr() && !Init)
-    return nullptr;
+  ExpectedType TypeOrErr = import(D->getType());
+  if (!TypeOrErr)
+    return TypeOrErr.takeError();
+
+  ExpectedExpr InitOrErr = import(D->getInitExpr());
+  if (!InitOrErr)
+    return InitOrErr.takeError();
 
   EnumConstantDecl *ToEnumerator;
   if (GetImportedOrCreateDecl(
           ToEnumerator, D, Importer.getToContext(), cast<EnumDecl>(DC), Loc,
-          Name.getAsIdentifierInfo(), T, Init, D->getInitVal()))
+          Name.getAsIdentifierInfo(), *TypeOrErr, *InitOrErr, D->getInitVal()))
     return ToEnumerator;
 
   ToEnumerator->setAccess(D->getAccess());
   ToEnumerator->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToEnumerator);
   return ToEnumerator;
 }
 
-bool ASTNodeImporter::ImportTemplateInformation(FunctionDecl *FromFD,
-                                                FunctionDecl *ToFD) {
+Error ASTNodeImporter::ImportTemplateInformation(
+    FunctionDecl *FromFD, FunctionDecl *ToFD) {
   switch (FromFD->getTemplatedKind()) {
   case FunctionDecl::TK_NonTemplate:
   case FunctionDecl::TK_FunctionTemplate:
-    return false;
+    return Error::success();
 
   case FunctionDecl::TK_MemberSpecialization: {
-    auto *InstFD = cast_or_null<FunctionDecl>(
-          Importer.Import(FromFD->getInstantiatedFromMemberFunction()));
-    if (!InstFD)
-      return true;
-
     TemplateSpecializationKind TSK = FromFD->getTemplateSpecializationKind();
-    SourceLocation POI = Importer.Import(
-          FromFD->getMemberSpecializationInfo()->getPointOfInstantiation());
-    ToFD->setInstantiationOfMemberFunction(InstFD, TSK);
-    ToFD->getMemberSpecializationInfo()->setPointOfInstantiation(POI);
-    return false;
+
+    if (Expected<FunctionDecl *> InstFDOrErr =
+        import(FromFD->getInstantiatedFromMemberFunction()))
+      ToFD->setInstantiationOfMemberFunction(*InstFDOrErr, TSK);
+    else
+      return InstFDOrErr.takeError();
+
+    if (ExpectedSLoc POIOrErr = import(
+        FromFD->getMemberSpecializationInfo()->getPointOfInstantiation()))
+      ToFD->getMemberSpecializationInfo()->setPointOfInstantiation(*POIOrErr);
+    else
+      return POIOrErr.takeError();
+
+    return Error::success();
   }
 
   case FunctionDecl::TK_FunctionTemplateSpecialization: {
-    FunctionTemplateDecl* Template;
-    OptionalTemplateArgsTy ToTemplArgs;
-    std::tie(Template, ToTemplArgs) =
+    auto FunctionAndArgsOrErr =
         ImportFunctionTemplateWithTemplateArgsFromSpecialization(FromFD);
-    if (!Template || !ToTemplArgs)
-      return true;
+    if (!FunctionAndArgsOrErr)
+      return FunctionAndArgsOrErr.takeError();
 
     TemplateArgumentList *ToTAList = TemplateArgumentList::CreateCopy(
-          Importer.getToContext(), *ToTemplArgs);
+          Importer.getToContext(), std::get<1>(*FunctionAndArgsOrErr));
 
     auto *FTSInfo = FromFD->getTemplateSpecializationInfo();
     TemplateArgumentListInfo ToTAInfo;
     const auto *FromTAArgsAsWritten = FTSInfo->TemplateArgumentsAsWritten;
     if (FromTAArgsAsWritten)
-      if (ImportTemplateArgumentListInfo(*FromTAArgsAsWritten, ToTAInfo))
-        return true;
-
-    SourceLocation POI = Importer.Import(FTSInfo->getPointOfInstantiation());
+      if (Error Err = ImportTemplateArgumentListInfo(
+          *FromTAArgsAsWritten, ToTAInfo))
+        return Err;
+
+    ExpectedSLoc POIOrErr = import(FTSInfo->getPointOfInstantiation());
+    if (!POIOrErr)
+      return POIOrErr.takeError();
 
     TemplateSpecializationKind TSK = FTSInfo->getTemplateSpecializationKind();
     ToFD->setFunctionTemplateSpecialization(
-        Template, ToTAList, /* InsertPos= */ nullptr,
-        TSK, FromTAArgsAsWritten ? &ToTAInfo : nullptr, POI);
-    return false;
+        std::get<0>(*FunctionAndArgsOrErr), ToTAList, /* InsertPos= */ nullptr,
+        TSK, FromTAArgsAsWritten ? &ToTAInfo : nullptr, *POIOrErr);
+    return Error::success();
   }
 
   case FunctionDecl::TK_DependentFunctionTemplateSpecialization: {
     auto *FromInfo = FromFD->getDependentSpecializationInfo();
     UnresolvedSet<8> TemplDecls;
     unsigned NumTemplates = FromInfo->getNumTemplates();
     for (unsigned I = 0; I < NumTemplates; I++) {
-      if (auto *ToFTD = cast_or_null<FunctionTemplateDecl>(
-              Importer.Import(FromInfo->getTemplate(I))))
-        TemplDecls.addDecl(ToFTD);
+      if (Expected<FunctionTemplateDecl *> ToFTDOrErr =
+          import(FromInfo->getTemplate(I)))
+        TemplDecls.addDecl(*ToFTDOrErr);
       else
-        return true;
+        return ToFTDOrErr.takeError();
     }
 
     // Import TemplateArgumentListInfo.
     TemplateArgumentListInfo ToTAInfo;
-    if (ImportTemplateArgumentListInfo(
-            FromInfo->getLAngleLoc(), FromInfo->getRAngleLoc(),
-            llvm::makeArrayRef(FromInfo->getTemplateArgs(),
-                               FromInfo->getNumTemplateArgs()),
-            ToTAInfo))
-      return true;
+    if (Error Err = ImportTemplateArgumentListInfo(
+        llvm::makeArrayRef(
+            FromInfo->getTemplateArgs(), FromInfo->getNumTemplateArgs()),
+      return Err;
 
     ToFD->setDependentTemplateSpecialization(Importer.getToContext(),
                                              TemplDecls, ToTAInfo);
-    return false;
+    return Error::success();
   }
   }
   llvm_unreachable("All cases should be covered!");
 }
 
-FunctionDecl *
+Expected<FunctionDecl *>
 ASTNodeImporter::FindFunctionTemplateSpecialization(FunctionDecl *FromFD) {
-  FunctionTemplateDecl* Template;
-  OptionalTemplateArgsTy ToTemplArgs;
-  std::tie(Template, ToTemplArgs) =
+  auto FunctionAndArgsOrErr =
       ImportFunctionTemplateWithTemplateArgsFromSpecialization(FromFD);
-  if (!Template || !ToTemplArgs)
-    return nullptr;
+  if (!FunctionAndArgsOrErr)
+    return FunctionAndArgsOrErr.takeError();
 
+  FunctionTemplateDecl *Template;
+  TemplateArgsTy ToTemplArgs;
+  std::tie(Template, ToTemplArgs) = *FunctionAndArgsOrErr;
   void *InsertPos = nullptr;
-  auto *FoundSpec = Template->findSpecialization(*ToTemplArgs, InsertPos);
+  auto *FoundSpec = Template->findSpecialization(ToTemplArgs, InsertPos);
   return FoundSpec;
 }
 
-Decl *ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
 
-  SmallVector<Decl*, 2> Redecls = getCanonicalForwardRedeclChain(D);
+  SmallVector<Decl *, 2> Redecls = getCanonicalForwardRedeclChain(D);
   auto RedeclIt = Redecls.begin();
   // Import the first part of the decl chain. I.e. import all previous
   // declarations starting from the canonical decl.
-  for (; RedeclIt != Redecls.end() && *RedeclIt != D; ++RedeclIt)
-    if (!Importer.Import(*RedeclIt))
-      return nullptr;
+  for (; RedeclIt != Redecls.end() && *RedeclIt != D; ++RedeclIt) {
+    ExpectedDecl ToRedeclOrErr = import(*RedeclIt);
+    if (!ToRedeclOrErr)
+      return ToRedeclOrErr.takeError();
+  }
   assert(*RedeclIt == D);
 
   // Import the major distinguishing characteristics of this function.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   const FunctionDecl *FoundByLookup = nullptr;
   FunctionTemplateDecl *FromFT = D->getDescribedFunctionTemplate();
 
   // If this is a function template specialization, then try to find the same
   // existing specialization in the "to" context. The localUncachedLookup
   // below will not find any specialization, but would find the primary
   // template; thus, we have to skip normal lookup in case of specializations.
   // FIXME handle member function templates (TK_MemberSpecialization) similarly?
   if (D->getTemplatedKind() ==
       FunctionDecl::TK_FunctionTemplateSpecialization) {
-    if (FunctionDecl *FoundFunction = FindFunctionTemplateSpecialization(D)) {
-      if (D->doesThisDeclarationHaveABody() &&
-          FoundFunction->hasBody())
-        return Importer.Imported(D, FoundFunction);
+    auto FoundFunctionOrErr = FindFunctionTemplateSpecialization(D);
+    if (!FoundFunctionOrErr)
+      return FoundFunctionOrErr.takeError();
+    if (FunctionDecl *FoundFunction = *FoundFunctionOrErr) {
+      if (D->doesThisDeclarationHaveABody() && FoundFunction->hasBody())
+        return Importer.MapImported(D, FoundFunction);
       FoundByLookup = FoundFunction;
     }
   }
   // Try to find a function in our own ("to") context with the same name, same
   // type, and in the same context as the function we're importing.
   else if (!LexicalDC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     unsigned IDNS = Decl::IDNS_Ordinary | Decl::IDNS_OrdinaryFriend;
for (auto *FoundDecl : FoundDecls) {
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
       if (!Name)
-        return nullptr;
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
   DeclarationNameInfo NameInfo(Name, Loc);
   // Import additional name location/type info.
-  ImportDeclarationNameLoc(D->getNameInfo(), NameInfo);
+  if (Error Err = ImportDeclarationNameLoc(D->getNameInfo(), NameInfo))
+    return std::move(Err);
 
   QualType FromTy = D->getType();
   bool usedDifferentExceptionSpec = false;
 
   if (const auto *FromFPT = D->getType()->getAs<FunctionProtoType>()) {
     FunctionProtoType::ExtProtoInfo FromEPI = FromFPT->getExtProtoInfo();
     // FunctionProtoType::ExtProtoInfo's ExceptionSpecDecl can point to the
     // FunctionDecl that we are importing the FunctionProtoType for.
     // To avoid an infinite recursion when importing, create the FunctionDecl
     // with a simplified function type and update it afterwards.
     if (FromEPI.ExceptionSpec.SourceDecl ||
         FromEPI.ExceptionSpec.SourceTemplate ||
         FromEPI.ExceptionSpec.NoexceptExpr) {
       FunctionProtoType::ExtProtoInfo DefaultEPI;
       FromTy = Importer.getFromContext().getFunctionType(
           FromFPT->getReturnType(), FromFPT->getParamTypes(), DefaultEPI);
       usedDifferentExceptionSpec = true;
     }
   }
 
-  // Import the type.
-  QualType T = Importer.Import(FromTy);
-  if (T.isNull())
-    return nullptr;
+  QualType T;
+  TypeSourceInfo *TInfo;
+  SourceLocation ToInnerLocStart, ToEndLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  if (auto Imp = importSeq(
+      FromTy, D->getTypeSourceInfo(), D->getInnerLocStart(),
+      D->getQualifierLoc(), D->getEndLoc()))
+    std::tie(T, TInfo, ToInnerLocStart, ToQualifierLoc, ToEndLoc) = *Imp;
+  else
+    return Imp.takeError();
 
   // Import the function parameters.
   SmallVector<ParmVarDecl *, 8> Parameters;
   for (auto P : D->parameters()) {
-    auto *ToP = cast_or_null<ParmVarDecl>(Importer.Import(P));
-    if (!ToP)
-      return nullptr;
-
-    Parameters.push_back(ToP);
+    if (Expected<ParmVarDecl *> ToPOrErr = import(P))
+      Parameters.push_back(*ToPOrErr);
+    else
+      return ToPOrErr.takeError();
   }
 
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
-  if (D->getTypeSourceInfo() && !TInfo)
-    return nullptr;
-
   // Create the imported function.
   FunctionDecl *ToFunction = nullptr;
-  SourceLocation InnerLocStart = Importer.Import(D->getInnerLocStart());
   if (auto *FromConstructor = dyn_cast<CXXConstructorDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXConstructorDecl>(
-            ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
-            InnerLocStart, NameInfo, T, TInfo, FromConstructor->isExplicit(),
-            D->isInlineSpecified(), D->isImplicit(), D->isConstexpr()))
+        ToInnerLocStart, NameInfo, T, TInfo,
+        FromConstructor->isExplicit(),
       return ToFunction;
     if (unsigned NumInitializers = FromConstructor->getNumCtorInitializers()) {
-      SmallVector<CXXCtorInitializer *, 4> CtorInitializers;
-      for (auto *I : FromConstructor->inits()) {
-        auto *ToI = cast_or_null<CXXCtorInitializer>(Importer.Import(I));
-        if (!ToI && I)
-          return nullptr;
-        CtorInitializers.push_back(ToI);
-      }
+      SmallVector<CXXCtorInitializer *, 4> CtorInitializers(NumInitializers);
+      // Import first, then allocate memory and copy if there was no error.
+      if (Error Err = ImportContainerChecked(
+          FromConstructor->inits(), CtorInitializers))
+        return std::move(Err);
       auto **Memory =
           new (Importer.getToContext()) CXXCtorInitializer *[NumInitializers];
       std::copy(CtorInitializers.begin(), CtorInitializers.end(), Memory);
       auto *ToCtor = cast<CXXConstructorDecl>(ToFunction);
       ToCtor->setCtorInitializers(Memory);
       ToCtor->setNumCtorInitializers(NumInitializers);
     }
   } else if (isa<CXXDestructorDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXDestructorDecl>(
-            ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
-            InnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
-            D->isImplicit()))
+        ToInnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
       return ToFunction;
   } else if (CXXConversionDecl *FromConversion =
                  dyn_cast<CXXConversionDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXConversionDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
-            InnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
+            ToInnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
             FromConversion->isExplicit(), D->isConstexpr(), SourceLocation()))
       return ToFunction;
   } else if (auto *Method = dyn_cast<CXXMethodDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXMethodDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
-            InnerLocStart, NameInfo, T, TInfo, Method->getStorageClass(),
+            ToInnerLocStart, NameInfo, T, TInfo, Method->getStorageClass(),
             Method->isInlineSpecified(), D->isConstexpr(), SourceLocation()))
       return ToFunction;
   } else {
     if (GetImportedOrCreateDecl(ToFunction, D, Importer.getToContext(), DC,
-                                InnerLocStart, NameInfo, T, TInfo,
+                                ToInnerLocStart, NameInfo, T, TInfo,
                                 D->getStorageClass(), D->isInlineSpecified(),
                                 D->hasWrittenPrototype(), D->isConstexpr()))
       return ToFunction;
   }
 
-  // Import the qualifier, if any.
-  ToFunction->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
+  ToFunction->setQualifierInfo(ToQualifierLoc);
   ToFunction->setAccess(D->getAccess());
   ToFunction->setLexicalDeclContext(LexicalDC);
   ToFunction->setVirtualAsWritten(D->isVirtualAsWritten());
   ToFunction->setTrivial(D->isTrivial());
   ToFunction->setPure(D->isPure());
-  ToFunction->setRangeEnd(Importer.Import(D->getEndLoc()));
+  ToFunction->setRangeEnd(ToEndLoc);
 
   // Set the parameters.
   for (auto *Param : Parameters) {
     Param->setOwningFunction(ToFunction);
     ToFunction->addDeclInternal(Param);
   }
   ToFunction->setParams(Parameters);
 
if (auto ProtoLoc =
         TInfo->getTypeLoc().IgnoreParens().getAs<FunctionProtoTypeLoc>()) {
       for (unsigned I = 0, N = Parameters.size(); I != N; ++I)
         ProtoLoc.setParam(I, Parameters[I]);
     }
   }
 
   if (usedDifferentExceptionSpec) {
     // Update FunctionProtoType::ExtProtoInfo.
-    QualType T = Importer.Import(D->getType());
-    if (T.isNull())
-      return nullptr;
-    ToFunction->setType(T);
+    if (ExpectedType TyOrErr = import(D->getType()))
+      ToFunction->setType(*TyOrErr);
+    else
+      return TyOrErr.takeError();
   }
 
   // Import the describing template function, if any.
-  if (FromFT)
-    if (!Importer.Import(FromFT))
-      return nullptr;
+  if (FromFT) {
+    auto ToFTOrErr = import(FromFT);
+    if (!ToFTOrErr)
+      return ToFTOrErr.takeError();
+  }
 
   if (D->doesThisDeclarationHaveABody()) {
     if (Stmt *FromBody = D->getBody()) {
-      if (Stmt *ToBody = Importer.Import(FromBody)) {
-        ToFunction->setBody(ToBody);
-      }
+      if (ExpectedStmt ToBodyOrErr = import(FromBody))
+        ToFunction->setBody(*ToBodyOrErr);
+      else
+        return ToBodyOrErr.takeError();
     }
   }
 
   // FIXME: Other bits to merge?
 
   // If it is a template, import all related things.
-  if (ImportTemplateInformation(D, ToFunction))
-    return nullptr;
+  if (Error Err = ImportTemplateInformation(D, ToFunction))
+    return std::move(Err);
 
   bool IsFriend = D->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend);
 
   // TODO Can we generalize this approach to other AST nodes as well?
   if (D->getDeclContext()->containsDeclAndLoad(D))
     DC->addDeclInternal(ToFunction);
   if (DC != LexicalDC && D->getLexicalDeclContext()->containsDeclAndLoad(D))
     LexicalDC->addDeclInternal(ToFunction);
 
   // Friend declaration's lexical context is the befriending class, but the
   // semantic context is the enclosing scope of the befriending class.
   // We want the friend functions to be found in the semantic context by lookup.
   // FIXME should we handle this generically in VisitFriendDecl?
   // In Other cases when LexicalDC != DC we don't want it to be added,
   // e.g out-of-class definitions like void B::f() {} .
   if (LexicalDC != DC && IsFriend) {
     DC->makeDeclVisibleInContext(ToFunction);
   }
 
   // Import the rest of the chain. I.e. import all subsequent declarations.
-  for (++RedeclIt; RedeclIt != Redecls.end(); ++RedeclIt)
-    if (!Importer.Import(*RedeclIt))
-      return nullptr;
+  for (++RedeclIt; RedeclIt != Redecls.end(); ++RedeclIt) {
+    ExpectedDecl ToRedeclOrErr = import(*RedeclIt);
+    if (!ToRedeclOrErr)
+      return ToRedeclOrErr.takeError();
+  }
 
   if (auto *FromCXXMethod = dyn_cast<CXXMethodDecl>(D))
     ImportOverrides(cast<CXXMethodDecl>(ToFunction), FromCXXMethod);
 
   return ToFunction;
 }
 
-Decl *ASTNodeImporter::VisitCXXMethodDecl(CXXMethodDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitCXXMethodDecl(CXXMethodDecl *D) {
   return VisitFunctionDecl(D);
 }
 
-Decl *ASTNodeImporter::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
   return VisitCXXMethodDecl(D);
 }
 
-Decl *ASTNodeImporter::VisitCXXDestructorDecl(CXXDestructorDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitCXXDestructorDecl(CXXDestructorDecl *D) {
   return VisitCXXMethodDecl(D);
 }
 
-Decl *ASTNodeImporter::VisitCXXConversionDecl(CXXConversionDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitCXXConversionDecl(CXXConversionDecl *D) {
   return VisitCXXMethodDecl(D);
 }
 
-Decl *ASTNodeImporter::VisitFieldDecl(FieldDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitFieldDecl(FieldDecl *D) {
   // Import the major distinguishing characteristics of a variable.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Determine whether we've already imported this field.
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
-    if (auto *FoundField = dyn_cast<FieldDecl>(FoundDecl)) {
+    if (FieldDecl *FoundField = dyn_cast<FieldDecl>(FoundDecl)) {
       // For anonymous fields, match up by index.
       if (!Name &&
           ASTImporter::getFieldIndex(D) !=
           ASTImporter::getFieldIndex(FoundField))
         continue;
 
       if (Importer.IsStructurallyEquivalent(D->getType(),
                                             FoundField->getType())) {
         Importer.MapImported(D, FoundField);
         // In case of a FieldDecl of a ClassTemplateSpecializationDecl, the
         // initializer of a FieldDecl might not had been instantiated in the
         // "To" context.  However, the "From" context might instantiated that,
         // thus we have to merge that.
         if (Expr *FromInitializer = D->getInClassInitializer()) {
           // We don't have yet the initializer set.
           if (FoundField->hasInClassInitializer() &&
               !FoundField->getInClassInitializer()) {
-            Expr *ToInitializer = Importer.Import(FromInitializer);
-            if (!ToInitializer)
-              // We can't return a nullptr here,
+            if (ExpectedExpr ToInitializerOrErr = import(FromInitializer))
+              FoundField->setInClassInitializer(*ToInitializerOrErr);
+            else {
+              // We can't return error here,
               // since we already mapped D as imported.
+              // FIXME: warning message?
+              consumeError(ToInitializerOrErr.takeError());
               return FoundField;
-            FoundField->setInClassInitializer(ToInitializer);
+            }
           }
         }
         return FoundField;
       }
 
+      // FIXME: Why is this case not handled with calling HandleNameConflict?
       Importer.ToDiag(Loc, diag::err_odr_field_type_inconsistent)
         << Name << D->getType() << FoundField->getType();
       Importer.ToDiag(FoundField->getLocation(), diag::note_odr_value_here)
         << FoundField->getType();
-      return nullptr;
+
+      return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
-  // Import the type.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
-
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
-  Expr *BitWidth = Importer.Import(D->getBitWidth());
-  if (!BitWidth && D->getBitWidth())
-    return nullptr;
+  QualType ToType;
+  TypeSourceInfo *ToTInfo;
+  Expr *ToBitWidth;
+  SourceLocation ToInnerLocStart;
+  Expr *ToInitializer;
+  if (auto Imp = importSeq(
+      D->getType(), D->getTypeSourceInfo(), D->getBitWidth(),
+      D->getInnerLocStart(), D->getInClassInitializer()))
+    std::tie(
+        ToType, ToTInfo, ToBitWidth, ToInnerLocStart, ToInitializer) = *Imp;
+  else
+    return Imp.takeError();
 
   FieldDecl *ToField;
   if (GetImportedOrCreateDecl(ToField, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getInnerLocStart()), Loc,
-                              Name.getAsIdentifierInfo(), T, TInfo, BitWidth,
-                              D->isMutable(), D->getInClassInitStyle()))
+                              ToInnerLocStart, Loc, Name.getAsIdentifierInfo(),
+                              ToType, ToTInfo, ToBitWidth, D->isMutable(),
+                              D->getInClassInitStyle()))
     return ToField;
 
   ToField->setAccess(D->getAccess());
   ToField->setLexicalDeclContext(LexicalDC);
-  if (Expr *FromInitializer = D->getInClassInitializer()) {
-    Expr *ToInitializer = Importer.Import(FromInitializer);
-    if (ToInitializer)
-      ToField->setInClassInitializer(ToInitializer);
-    else
-      return nullptr;
-  }
   ToField->setImplicit(D->isImplicit());
   LexicalDC->addDeclInternal(ToField);
   return ToField;
 }
 
-Decl *ASTNodeImporter::VisitIndirectFieldDecl(IndirectFieldDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitIndirectFieldDecl(IndirectFieldDecl *D) {
   // Import the major distinguishing characteristics of a variable.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Determine whether we've already imported this field.
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (unsigned I = 0, N = FoundDecls.size(); I != N; ++I) {
     if (auto *FoundField = dyn_cast<IndirectFieldDecl>(FoundDecls[I])) {
if (auto *FoundField = dyn_cast<IndirectFieldDecl>(FoundDecls[I])) {
         Importer.MapImported(D, FoundField);
         return FoundField;
       }
 
       // If there are more anonymous fields to check, continue.
       if (!Name && I < N-1)
         continue;
 
+      // FIXME: Why is this case not handled with calling HandleNameConflict?
       Importer.ToDiag(Loc, diag::err_odr_field_type_inconsistent)
         << Name << D->getType() << FoundField->getType();
       Importer.ToDiag(FoundField->getLocation(), diag::note_odr_value_here)
         << FoundField->getType();
-      return nullptr;
+
+      return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
   // Import the type.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
+  auto TypeOrErr = import(D->getType());
+  if (!TypeOrErr)
+    return TypeOrErr.takeError();
 
   auto **NamedChain =
     new (Importer.getToContext()) NamedDecl*[D->getChainingSize()];
 
   unsigned i = 0;
-  for (auto *PI : D->chain()) {
-    Decl *D = Importer.Import(PI);
-    if (!D)
-      return nullptr;
-    NamedChain[i++] = cast<NamedDecl>(D);
-  }
+  for (auto *PI : D->chain())
+    if (Expected<NamedDecl *> ToD = import(PI))
+      NamedChain[i++] = *ToD;
+    else
+      return ToD.takeError();
 
   llvm::MutableArrayRef<NamedDecl *> CH = {NamedChain, D->getChainingSize()};
   IndirectFieldDecl *ToIndirectField;
   if (GetImportedOrCreateDecl(ToIndirectField, D, Importer.getToContext(), DC,
-                              Loc, Name.getAsIdentifierInfo(), T, CH))
+                              Loc, Name.getAsIdentifierInfo(), *TypeOrErr, CH))
     // FIXME here we leak `NamedChain` which is allocated before
     return ToIndirectField;
 
-  for (const auto *A : D->attrs())
-    ToIndirectField->addAttr(Importer.Import(A));
+  for (const auto *Attr : D->attrs())
+    ToIndirectField->addAttr(Importer.Import(Attr));
 
   ToIndirectField->setAccess(D->getAccess());
   ToIndirectField->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToIndirectField);
   return ToIndirectField;
 }
 
-Decl *ASTNodeImporter::VisitFriendDecl(FriendDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitFriendDecl(FriendDecl *D) {
   // Import the major distinguishing characteristics of a declaration.
-  DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-  DeclContext *LexicalDC = D->getDeclContext() == D->getLexicalDeclContext()
-      ? DC : Importer.ImportContext(D->getLexicalDeclContext());
-  if (!DC || !LexicalDC)
-    return nullptr;
+  DeclContext *DC, *LexicalDC;
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return std::move(Err);
 
   // Determine whether we've already imported this decl.
   // FriendDecl is not a NamedDecl so we cannot use localUncachedLookup.
   auto *RD = cast<CXXRecordDecl>(DC);
   FriendDecl *ImportedFriend = RD->getFirstFriend();
 
   while (ImportedFriend) {
     if (D->getFriendDecl() && ImportedFriend->getFriendDecl()) {
       if (IsStructuralMatch(D->getFriendDecl(), ImportedFriend->getFriendDecl(),
                             /*Complain=*/false))
         return Importer.MapImported(D, ImportedFriend);
 
     } else if (D->getFriendType() && ImportedFriend->getFriendType()) {
       if (Importer.IsStructurallyEquivalent(
             D->getFriendType()->getType(),
             ImportedFriend->getFriendType()->getType(), true))
         return Importer.MapImported(D, ImportedFriend);
     }
     ImportedFriend = ImportedFriend->getNextFriend();
   }
 
   // Not found. Create it.
   FriendDecl::FriendUnion ToFU;
   if (NamedDecl *FriendD = D->getFriendDecl()) {
-    auto *ToFriendD = cast_or_null<NamedDecl>(Importer.Import(FriendD));
-    if (ToFriendD && FriendD->getFriendObjectKind() != Decl::FOK_None &&
+    NamedDecl *ToFriendD;
+    if (Error Err = importInto(ToFriendD, FriendD))
+      return std::move(Err);
+
+    if (FriendD->getFriendObjectKind() != Decl::FOK_None &&
         !(FriendD->isInIdentifierNamespace(Decl::IDNS_NonMemberOperator)))
       ToFriendD->setObjectOfFriendDecl(false);
 
     ToFU = ToFriendD;
-  }  else // The friend is a type, not a decl.
-    ToFU = Importer.Import(D->getFriendType());
-  if (!ToFU)
-    return nullptr;
+  } else { // The friend is a type, not a decl.
+    if (auto TSIOrErr = import(D->getFriendType()))
+      ToFU = *TSIOrErr;
+    else
+      return TSIOrErr.takeError();
+  }
 
   SmallVector<TemplateParameterList *, 1> ToTPLists(D->NumTPLists);
   auto **FromTPLists = D->getTrailingObjects<TemplateParameterList *>();
   for (unsigned I = 0; I < D->NumTPLists; I++) {
-    TemplateParameterList *List = ImportTemplateParameterList(FromTPLists[I]);
-    if (!List)
-      return nullptr;
-    ToTPLists[I] = List;
+    if (auto ListOrErr = ImportTemplateParameterList(FromTPLists[I]))
+      ToTPLists[I] = *ListOrErr;
+    else
+      return ListOrErr.takeError();
   }
 
+  auto LocationOrErr = import(D->getLocation());
+  if (!LocationOrErr)
+    return LocationOrErr.takeError();
+  auto FriendLocOrErr = import(D->getFriendLoc());
+  if (!FriendLocOrErr)
+    return FriendLocOrErr.takeError();
+
   FriendDecl *FrD;
   if (GetImportedOrCreateDecl(FrD, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getLocation()), ToFU,
-                              Importer.Import(D->getFriendLoc()), ToTPLists))
+                              *LocationOrErr, ToFU,
+                              *FriendLocOrErr, ToTPLists))
     return FrD;
 
   FrD->setAccess(D->getAccess());
   FrD->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(FrD);
   return FrD;
 }
 
-Decl *ASTNodeImporter::VisitObjCIvarDecl(ObjCIvarDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCIvarDecl(ObjCIvarDecl *D) {
   // Import the major distinguishing characteristics of an ivar.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Determine whether we've already imported this ivar
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
-    if (auto *FoundIvar = dyn_cast<ObjCIvarDecl>(FoundDecl)) {
+    if (ObjCIvarDecl *FoundIvar = dyn_cast<ObjCIvarDecl>(FoundDecl)) {
       if (Importer.IsStructurallyEquivalent(D->getType(),
                                             FoundIvar->getType())) {
         Importer.MapImported(D, FoundIvar);
         return FoundIvar;
       }
 
       Importer.ToDiag(Loc, diag::err_odr_ivar_type_inconsistent)
         << Name << D->getType() << FoundIvar->getType();
       Importer.ToDiag(FoundIvar->getLocation(), diag::note_odr_value_here)
         << FoundIvar->getType();
-      return nullptr;
+
+      return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
-  // Import the type.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
-
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
-  Expr *BitWidth = Importer.Import(D->getBitWidth());
-  if (!BitWidth && D->getBitWidth())
-    return nullptr;
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  Expr *ToBitWidth;
+  SourceLocation ToInnerLocStart;
+  if (auto Imp = importSeq(
+      D->getType(), D->getTypeSourceInfo(), D->getBitWidth(), D->getInnerLocStart()))
+    std::tie(ToType, ToTypeSourceInfo, ToBitWidth, ToInnerLocStart) = *Imp;
+  else
+    return Imp.takeError();
 
   ObjCIvarDecl *ToIvar;
   if (GetImportedOrCreateDecl(
           ToIvar, D, Importer.getToContext(), cast<ObjCContainerDecl>(DC),
-          Importer.Import(D->getInnerLocStart()), Loc,
-          Name.getAsIdentifierInfo(), T, TInfo, D->getAccessControl(), BitWidth,
-          D->getSynthesize()))
+          ToInnerLocStart, Loc, Name.getAsIdentifierInfo(),
+          ToType, ToTypeSourceInfo,
+          D->getAccessControl(),ToBitWidth, D->getSynthesize()))
     return ToIvar;
 
   ToIvar->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToIvar);
   return ToIvar;
 }
 
-Decl *ASTNodeImporter::VisitVarDecl(VarDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitVarDecl(VarDecl *D) {
 
   SmallVector<Decl*, 2> Redecls = getCanonicalForwardRedeclChain(D);
   auto RedeclIt = Redecls.begin();
   // Import the first part of the decl chain. I.e. import all previous
   // declarations starting from the canonical decl.
-  for (; RedeclIt != Redecls.end() && *RedeclIt != D; ++RedeclIt)
-    if (!Importer.Import(*RedeclIt))
-      return nullptr;
+  for (; RedeclIt != Redecls.end() && *RedeclIt != D; ++RedeclIt) {
+    ExpectedDecl RedeclOrErr = import(*RedeclIt);
+    if (!RedeclOrErr)
+      return RedeclOrErr.takeError();
+  }
   assert(*RedeclIt == D);
 
   // Import the major distinguishing characteristics of a variable.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Try to find a variable in our own ("to") context with the same name and
   // in the same context as the variable we're importing.
   VarDecl *FoundByLookup = nullptr;
   if (D->isFileVarDecl()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
for (auto *FoundDecl : FoundDecls) {
           const ArrayType *FoundArray
             = Importer.getToContext().getAsArrayType(FoundVar->getType());
           const ArrayType *TArray
             = Importer.getToContext().getAsArrayType(D->getType());
           if (FoundArray && TArray) {
             if (isa<IncompleteArrayType>(FoundArray) &&
                 isa<ConstantArrayType>(TArray)) {
               // Import the type.
-              QualType T = Importer.Import(D->getType());
-              if (T.isNull())
-                return nullptr;
+              if (auto TyOrErr = import(D->getType()))
+                FoundVar->setType(*TyOrErr);
+              else
+                return TyOrErr.takeError();
 
-              FoundVar->setType(T);
               FoundByLookup = FoundVar;
               break;
             } else if (isa<IncompleteArrayType>(TArray) &&
                        isa<ConstantArrayType>(FoundArray)) {
               FoundByLookup = FoundVar;
               break;
             }
           }
 
           Importer.ToDiag(Loc, diag::err_odr_variable_type_inconsistent)
             << Name << D->getType() << FoundVar->getType();
           Importer.ToDiag(FoundVar->getLocation(), diag::note_odr_value_here)
             << FoundVar->getType();
         }
       }
 
       ConflictingDecls.push_back(FoundDecl);
     }
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, IDNS,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
       if (!Name)
-        return nullptr;
+        return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
-  // Import the type.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  SourceLocation ToInnerLocStart;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  if (auto Imp = importSeq(
+      D->getType(), D->getTypeSourceInfo(), D->getInnerLocStart(),
+      D->getQualifierLoc()))
+    std::tie(ToType, ToTypeSourceInfo, ToInnerLocStart, ToQualifierLoc) = *Imp;
+  else
+    return Imp.takeError();
 
   // Create the imported variable.
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
   VarDecl *ToVar;
   if (GetImportedOrCreateDecl(ToVar, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getInnerLocStart()), Loc,
-                              Name.getAsIdentifierInfo(), T, TInfo,
+                              ToInnerLocStart, Loc,
+                              Name.getAsIdentifierInfo(),
+                              ToType, ToTypeSourceInfo,
                               D->getStorageClass()))
     return ToVar;
 
-  ToVar->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
+  ToVar->setQualifierInfo(ToQualifierLoc);
   ToVar->setAccess(D->getAccess());
   ToVar->setLexicalDeclContext(LexicalDC);
 
   if (FoundByLookup) {
     auto *Recent = const_cast<VarDecl *>(FoundByLookup->getMostRecentDecl());
     ToVar->setPreviousDecl(Recent);
   }
 
-  if (ImportInitializer(D, ToVar))
-    return nullptr;
+  if (Error Err = ImportInitializer(D, ToVar))
+    return std::move(Err);
 
   if (D->isConstexpr())
     ToVar->setConstexpr(true);
 
   if (D->getDeclContext()->containsDeclAndLoad(D))
     DC->addDeclInternal(ToVar);
   if (DC != LexicalDC && D->getLexicalDeclContext()->containsDeclAndLoad(D))
     LexicalDC->addDeclInternal(ToVar);
 
   // Import the rest of the chain. I.e. import all subsequent declarations.
-  for (++RedeclIt; RedeclIt != Redecls.end(); ++RedeclIt)
-    if (!Importer.Import(*RedeclIt))
-      return nullptr;
+  for (++RedeclIt; RedeclIt != Redecls.end(); ++RedeclIt) {
+    ExpectedDecl RedeclOrErr = import(*RedeclIt);
+    if (!RedeclOrErr)
+      return RedeclOrErr.takeError();
+  }
 
   return ToVar;
 }
 
-Decl *ASTNodeImporter::VisitImplicitParamDecl(ImplicitParamDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitImplicitParamDecl(ImplicitParamDecl *D) {
   // Parameters are created in the translation unit's context, then moved
   // into the function declaration's context afterward.
   DeclContext *DC = Importer.getToContext().getTranslationUnitDecl();
 
-  // Import the name of this declaration.
-  DeclarationName Name = Importer.Import(D->getDeclName());
-  if (D->getDeclName() && !Name)
-    return nullptr;
-
-  // Import the location of this declaration.
-  SourceLocation Loc = Importer.Import(D->getLocation());
-
-  // Import the parameter's type.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
+  DeclarationName ToDeclName;
+  SourceLocation ToLocation;
+  QualType ToType;
+  if (auto Imp = importSeq(D->getDeclName(), D->getLocation(), D->getType()))
+    std::tie(ToDeclName, ToLocation, ToType) = *Imp;
+  else
+    return Imp.takeError();
 
   // Create the imported parameter.
   ImplicitParamDecl *ToParm = nullptr;
-  if (GetImportedOrCreateDecl(ToParm, D, Importer.getToContext(), DC, Loc,
-                              Name.getAsIdentifierInfo(), T,
-                              D->getParameterKind()))
+  if (GetImportedOrCreateDecl(ToParm, D, Importer.getToContext(), DC,
+                              ToLocation, ToDeclName.getAsIdentifierInfo(),
+                              ToType, D->getParameterKind()))
     return ToParm;
   return ToParm;
 }
 
-Decl *ASTNodeImporter::VisitParmVarDecl(ParmVarDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitParmVarDecl(ParmVarDecl *D) {
   // Parameters are created in the translation unit's context, then moved
   // into the function declaration's context afterward.
   DeclContext *DC = Importer.getToContext().getTranslationUnitDecl();
 
-  // Import the name of this declaration.
-  DeclarationName Name = Importer.Import(D->getDeclName());
-  if (D->getDeclName() && !Name)
-    return nullptr;
-
-  // Import the location of this declaration.
-  SourceLocation Loc = Importer.Import(D->getLocation());
-
-  // Import the parameter's type.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
+  DeclarationName ToDeclName;
+  SourceLocation ToLocation, ToInnerLocStart;
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  if (auto Imp = importSeq(
+      D->getDeclName(), D->getLocation(), D->getType(), D->getInnerLocStart(),
+      D->getTypeSourceInfo()))
+    std::tie(
+        ToDeclName, ToLocation, ToType, ToInnerLocStart,
+        ToTypeSourceInfo) = *Imp;
+  else
+    return Imp.takeError();
 
-  // Create the imported parameter.
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
   ParmVarDecl *ToParm;
   if (GetImportedOrCreateDecl(ToParm, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getInnerLocStart()), Loc,
-                              Name.getAsIdentifierInfo(), T, TInfo,
-                              D->getStorageClass(),
+                              ToInnerLocStart, ToLocation,
+                              ToDeclName.getAsIdentifierInfo(), ToType,
+                              ToTypeSourceInfo, D->getStorageClass(),
                               /*DefaultArg*/ nullptr))
     return ToParm;
 
   // Set the default argument.
   ToParm->setHasInheritedDefaultArg(D->hasInheritedDefaultArg());
   ToParm->setKNRPromoted(D->isKNRPromoted());
 
-  Expr *ToDefArg = nullptr;
-  Expr *FromDefArg = nullptr;
   if (D->hasUninstantiatedDefaultArg()) {
-    FromDefArg = D->getUninstantiatedDefaultArg();
-    ToDefArg = Importer.Import(FromDefArg);
-    ToParm->setUninstantiatedDefaultArg(ToDefArg);
+    if (auto ToDefArgOrErr = import(D->getUninstantiatedDefaultArg()))
+      ToParm->setUninstantiatedDefaultArg(*ToDefArgOrErr);
+    else
+      return ToDefArgOrErr.takeError();
   } else if (D->hasUnparsedDefaultArg()) {
     ToParm->setUnparsedDefaultArg();
   } else if (D->hasDefaultArg()) {
-    FromDefArg = D->getDefaultArg();
-    ToDefArg = Importer.Import(FromDefArg);
-    ToParm->setDefaultArg(ToDefArg);
+    if (auto ToDefArgOrErr = import(D->getDefaultArg()))
+      ToParm->setDefaultArg(*ToDefArgOrErr);
+    else
+      return ToDefArgOrErr.takeError();
   }
-  if (FromDefArg && !ToDefArg)
-    return nullptr;
 
   if (D->isObjCMethodParameter()) {
     ToParm->setObjCMethodScopeInfo(D->getFunctionScopeIndex());
     ToParm->setObjCDeclQualifier(D->getObjCDeclQualifier());
   } else {
     ToParm->setScopeInfo(D->getFunctionScopeDepth(),
                          D->getFunctionScopeIndex());
   }
 
   return ToParm;
 }
 
-Decl *ASTNodeImporter::VisitObjCMethodDecl(ObjCMethodDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCMethodDecl(ObjCMethodDecl *D) {
   // Import the major distinguishing characteristics of a method.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
     if (auto *FoundMethod = dyn_cast<ObjCMethodDecl>(FoundDecl)) {
       if (FoundMethod->isInstanceMethod() != D->isInstanceMethod())
         continue;
 
       // Check return types.
       if (!Importer.IsStructurallyEquivalent(D->getReturnType(),
                                              FoundMethod->getReturnType())) {
         Importer.ToDiag(Loc, diag::err_odr_objc_method_result_type_inconsistent)
             << D->isInstanceMethod() << Name << D->getReturnType()
             << FoundMethod->getReturnType();
         Importer.ToDiag(FoundMethod->getLocation(),
                         diag::note_odr_objc_method_here)
           << D->isInstanceMethod() << Name;
-        return nullptr;
+
+        return make_error<ImportError>(ImportError::NameConflict);
       }
 
       // Check the number of parameters.
       if (D->param_size() != FoundMethod->param_size()) {
         Importer.ToDiag(Loc, diag::err_odr_objc_method_num_params_inconsistent)
           << D->isInstanceMethod() << Name
           << D->param_size() << FoundMethod->param_size();
         Importer.ToDiag(FoundMethod->getLocation(),
                         diag::note_odr_objc_method_here)
           << D->isInstanceMethod() << Name;
-        return nullptr;
+
+        return make_error<ImportError>(ImportError::NameConflict);
       }
 
       // Check parameter types.
       for (ObjCMethodDecl::param_iterator P = D->param_begin(),
              PEnd = D->param_end(), FoundP = FoundMethod->param_begin();
            P != PEnd; ++P, ++FoundP) {
         if (!Importer.IsStructurallyEquivalent((*P)->getType(),
                                                (*FoundP)->getType())) {
           Importer.FromDiag((*P)->getLocation(),
                             diag::err_odr_objc_method_param_type_inconsistent)
             << D->isInstanceMethod() << Name
             << (*P)->getType() << (*FoundP)->getType();
           Importer.ToDiag((*FoundP)->getLocation(), diag::note_odr_value_here)
             << (*FoundP)->getType();
-          return nullptr;
+
+          return make_error<ImportError>(ImportError::NameConflict);
         }
       }
 
       // Check variadic/non-variadic.
       // Check the number of parameters.
       if (D->isVariadic() != FoundMethod->isVariadic()) {
         Importer.ToDiag(Loc, diag::err_odr_objc_method_variadic_inconsistent)
           << D->isInstanceMethod() << Name;
         Importer.ToDiag(FoundMethod->getLocation(),
                         diag::note_odr_objc_method_here)
           << D->isInstanceMethod() << Name;
-        return nullptr;
+
+        return make_error<ImportError>(ImportError::NameConflict);
       }
 
       // FIXME: Any other bits we need to merge?
       return Importer.MapImported(D, FoundMethod);
     }
   }
 
-  // Import the result type.
-  QualType ResultTy = Importer.Import(D->getReturnType());
-  if (ResultTy.isNull())
-    return nullptr;
-
-  TypeSourceInfo *ReturnTInfo = Importer.Import(D->getReturnTypeSourceInfo());
+  SourceLocation ToEndLoc;
+  QualType ToReturnType;
+  TypeSourceInfo *ToReturnTypeSourceInfo;
+  if (auto Imp = importSeq(
+      D->getEndLoc(), D->getReturnType(), D->getReturnTypeSourceInfo()))
+    std::tie(ToEndLoc, ToReturnType, ToReturnTypeSourceInfo) = *Imp;
+  else
+    return Imp.takeError();
 
   ObjCMethodDecl *ToMethod;
   if (GetImportedOrCreateDecl(
           ToMethod, D, Importer.getToContext(), Loc,
-          Importer.Import(D->getEndLoc()), Name.getObjCSelector(), ResultTy,
-          ReturnTInfo, DC, D->isInstanceMethod(), D->isVariadic(),
+          ToEndLoc, Name.getObjCSelector(), ToReturnType,
+          ToReturnTypeSourceInfo, DC, D->isInstanceMethod(), D->isVariadic(),
           D->isPropertyAccessor(), D->isImplicit(), D->isDefined(),
           D->getImplementationControl(), D->hasRelatedResultType()))
     return ToMethod;
 
   // FIXME: When we decide to merge method definitions, we'll need to
   // deal with implicit parameters.
 
   // Import the parameters
   SmallVector<ParmVarDecl *, 5> ToParams;
   for (auto *FromP : D->parameters()) {
-    auto *ToP = cast_or_null<ParmVarDecl>(Importer.Import(FromP));
-    if (!ToP)
-      return nullptr;
-
-    ToParams.push_back(ToP);
+    if (Expected<ParmVarDecl *> ToPOrErr = import(FromP))
+      ToParams.push_back(*ToPOrErr);
+    else
+      return ToPOrErr.takeError();
   }
 
   // Set the parameters.
   for (auto *ToParam : ToParams) {
     ToParam->setOwningFunction(ToMethod);
     ToMethod->addDeclInternal(ToParam);
   }
 
-  SmallVector<SourceLocation, 12> SelLocs;
-  D->getSelectorLocs(SelLocs);
-  for (auto &Loc : SelLocs)
-    Loc = Importer.Import(Loc);
+  SmallVector<SourceLocation, 12> FromSelLocs;
+  D->getSelectorLocs(FromSelLocs);
+  SmallVector<SourceLocation, 12> ToSelLocs(FromSelLocs.size());
+  if (Error Err = ImportContainerChecked(FromSelLocs, ToSelLocs))
+    return std::move(Err);
 
-  ToMethod->setMethodParams(Importer.getToContext(), ToParams, SelLocs);
+  ToMethod->setMethodParams(Importer.getToContext(), ToParams, ToSelLocs);
 
   ToMethod->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToMethod);
   return ToMethod;
 }
 
-Decl *ASTNodeImporter::VisitObjCTypeParamDecl(ObjCTypeParamDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCTypeParamDecl(ObjCTypeParamDecl *D) {
   // Import the major distinguishing characteristics of a category.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  TypeSourceInfo *BoundInfo = Importer.Import(D->getTypeSourceInfo());
-  if (!BoundInfo)
-    return nullptr;
+  SourceLocation ToVarianceLoc, ToLocation, ToColonLoc;
+  TypeSourceInfo *ToTypeSourceInfo;
+  if (auto Imp = importSeq(
+      D->getVarianceLoc(), D->getLocation(), D->getColonLoc(),
+      D->getTypeSourceInfo()))
+    std::tie(ToVarianceLoc, ToLocation, ToColonLoc, ToTypeSourceInfo) = *Imp;
+  else
+    return Imp.takeError();
 
   ObjCTypeParamDecl *Result;
   if (GetImportedOrCreateDecl(
           Result, D, Importer.getToContext(), DC, D->getVariance(),
-          Importer.Import(D->getVarianceLoc()), D->getIndex(),
-          Importer.Import(D->getLocation()), Name.getAsIdentifierInfo(),
-          Importer.Import(D->getColonLoc()), BoundInfo))
+          ToVarianceLoc, D->getIndex(),
+          ToLocation, Name.getAsIdentifierInfo(),
+          ToColonLoc, ToTypeSourceInfo))
     return Result;
 
   Result->setLexicalDeclContext(LexicalDC);
   return Result;
 }
 
-Decl *ASTNodeImporter::VisitObjCCategoryDecl(ObjCCategoryDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCCategoryDecl(ObjCCategoryDecl *D) {
   // Import the major distinguishing characteristics of a category.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  auto *ToInterface =
-      cast_or_null<ObjCInterfaceDecl>(Importer.Import(D->getClassInterface()));
-  if (!ToInterface)
-    return nullptr;
+  ObjCInterfaceDecl *ToInterface;
+  if (Error Err = importInto(ToInterface, D->getClassInterface()))
+    return std::move(Err);
 
   // Determine if we've already encountered this category.
   ObjCCategoryDecl *MergeWithCategory
     = ToInterface->FindCategoryDeclaration(Name.getAsIdentifierInfo());
   ObjCCategoryDecl *ToCategory = MergeWithCategory;
   if (!ToCategory) {
+    SourceLocation ToAtStartLoc, ToCategoryNameLoc;
+    SourceLocation ToIvarLBraceLoc, ToIvarRBraceLoc;
+    if (auto Imp = importSeq(
+        D->getAtStartLoc(), D->getCategoryNameLoc(),
+        D->getIvarLBraceLoc(), D->getIvarRBraceLoc()))
+      std::tie(
+          ToAtStartLoc, ToCategoryNameLoc,
+          ToIvarLBraceLoc, ToIvarRBraceLoc) = *Imp;
+    else
+      return Imp.takeError();
 
     if (GetImportedOrCreateDecl(ToCategory, D, Importer.getToContext(), DC,
-                                Importer.Import(D->getAtStartLoc()), Loc,
-                                Importer.Import(D->getCategoryNameLoc()),
+                                ToAtStartLoc, Loc,
+                                ToCategoryNameLoc,
                                 Name.getAsIdentifierInfo(), ToInterface,
                                 /*TypeParamList=*/nullptr,
-                                Importer.Import(D->getIvarLBraceLoc()),
-                                Importer.Import(D->getIvarRBraceLoc())))
+                                ToIvarLBraceLoc,
+                                ToIvarRBraceLoc))
       return ToCategory;
 
     ToCategory->setLexicalDeclContext(LexicalDC);
     LexicalDC->addDeclInternal(ToCategory);
-    // Import the type parameter list after calling Imported, to avoid
+    // Import the type parameter list after MapImported, to avoid
     // loops when bringing in their DeclContext.
-    ToCategory->setTypeParamList(ImportObjCTypeParamList(
-                                   D->getTypeParamList()));
+    if (auto PListOrErr = ImportObjCTypeParamList(D->getTypeParamList()))
+      ToCategory->setTypeParamList(*PListOrErr);
+    else
+      return PListOrErr.takeError();
 
     // Import protocols
     SmallVector<ObjCProtocolDecl *, 4> Protocols;
     SmallVector<SourceLocation, 4> ProtocolLocs;
     ObjCCategoryDecl::protocol_loc_iterator FromProtoLoc
       = D->protocol_loc_begin();
     for (ObjCCategoryDecl::protocol_iterator FromProto = D->protocol_begin(),
                                           FromProtoEnd = D->protocol_end();
          FromProto != FromProtoEnd;
          ++FromProto, ++FromProtoLoc) {
-      auto *ToProto =
-          cast_or_null<ObjCProtocolDecl>(Importer.Import(*FromProto));
-      if (!ToProto)
-        return nullptr;
-      Protocols.push_back(ToProto);
-      ProtocolLocs.push_back(Importer.Import(*FromProtoLoc));
+      if (Expected<ObjCProtocolDecl *> ToProtoOrErr = import(*FromProto))
+        Protocols.push_back(*ToProtoOrErr);
+      else
+        return ToProtoOrErr.takeError();
+
+      if (ExpectedSLoc ToProtoLocOrErr = import(*FromProtoLoc))
+        ProtocolLocs.push_back(*ToProtoLocOrErr);
+      else
+        return ToProtoLocOrErr.takeError();
     }
 
     // FIXME: If we're merging, make sure that the protocol list is the same.
     ToCategory->setProtocolList(Protocols.data(), Protocols.size(),
                                 ProtocolLocs.data(), Importer.getToContext());
+
   } else {
     Importer.MapImported(D, ToCategory);
   }
 
   // Import all of the members of this category.
-  ImportDeclContext(D);
+  if (Error Err = ImportDeclContext(D))
+    return std::move(Err);
 
   // If we have an implementation, import it as well.
   if (D->getImplementation()) {
-    auto *Impl =
-        cast_or_null<ObjCCategoryImplDecl>(
-                                       Importer.Import(D->getImplementation()));
-    if (!Impl)
-      return nullptr;
-
-    ToCategory->setImplementation(Impl);
+    if (Expected<ObjCCategoryImplDecl *> ToImplOrErr =
+        import(D->getImplementation()))
+      ToCategory->setImplementation(*ToImplOrErr);
+    else
+      return ToImplOrErr.takeError();
   }
 
   return ToCategory;
 }
 
-bool ASTNodeImporter::ImportDefinition(ObjCProtocolDecl *From,
-                                       ObjCProtocolDecl *To,
-                                       ImportDefinitionKind Kind) {
+Error ASTNodeImporter::ImportDefinition(
+    ObjCProtocolDecl *From, ObjCProtocolDecl *To, ImportDefinitionKind Kind) {
   if (To->getDefinition()) {
     if (shouldForceImportDeclContext(Kind))
-      ImportDeclContext(From);
-    return false;
+      if (Error Err = ImportDeclContext(From))
+        return Err;
+    return Error::success();
   }
 
   // Start the protocol definition
   To->startDefinition();
 
   // Import protocols
   SmallVector<ObjCProtocolDecl *, 4> Protocols;
   SmallVector<SourceLocation, 4> ProtocolLocs;
-  ObjCProtocolDecl::protocol_loc_iterator
-  FromProtoLoc = From->protocol_loc_begin();
+  ObjCProtocolDecl::protocol_loc_iterator FromProtoLoc =
+      From->protocol_loc_begin();
   for (ObjCProtocolDecl::protocol_iterator FromProto = From->protocol_begin(),
                                         FromProtoEnd = From->protocol_end();
        FromProto != FromProtoEnd;
        ++FromProto, ++FromProtoLoc) {
-    auto *ToProto = cast_or_null<ObjCProtocolDecl>(Importer.Import(*FromProto));
-    if (!ToProto)
-      return true;
-    Protocols.push_back(ToProto);
-    ProtocolLocs.push_back(Importer.Import(*FromProtoLoc));
+    if (Expected<ObjCProtocolDecl *> ToProtoOrErr = import(*FromProto))
+      Protocols.push_back(*ToProtoOrErr);
+    else
+      return ToProtoOrErr.takeError();
+
+    if (ExpectedSLoc ToProtoLocOrErr = import(*FromProtoLoc))
+      ProtocolLocs.push_back(*ToProtoLocOrErr);
+    else
+      return ToProtoLocOrErr.takeError();
+
   }
 
   // FIXME: If we're merging, make sure that the protocol list is the same.
   To->setProtocolList(Protocols.data(), Protocols.size(),
                       ProtocolLocs.data(), Importer.getToContext());
 
   if (shouldForceImportDeclContext(Kind)) {
     // Import all of the members of this protocol.
-    ImportDeclContext(From, /*ForceImport=*/true);
+    if (Error Err = ImportDeclContext(From, /*ForceImport=*/true))
+      return Err;
   }
-  return false;
+  return Error::success();
 }
 
-Decl *ASTNodeImporter::VisitObjCProtocolDecl(ObjCProtocolDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCProtocolDecl(ObjCProtocolDecl *D) {
   // If this protocol has a definition in the translation unit we're coming
   // from, but this particular declaration is not that definition, import the
   // definition and map to that.
   ObjCProtocolDecl *Definition = D->getDefinition();
   if (Definition && Definition != D) {
-    Decl *ImportedDef = Importer.Import(Definition);
-    if (!ImportedDef)
-      return nullptr;
-
-    return Importer.MapImported(D, ImportedDef);
+    if (ExpectedDecl ImportedDefOrErr = import(Definition))
+      return Importer.MapImported(D, *ImportedDefOrErr);
+    else
+      return ImportedDefOrErr.takeError();
   }
 
   // Import the major distinguishing characteristics of a protocol.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   ObjCProtocolDecl *MergeWithProtocol = nullptr;
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
     if (!FoundDecl->isInIdentifierNamespace(Decl::IDNS_ObjCProtocol))
       continue;
 
     if ((MergeWithProtocol = dyn_cast<ObjCProtocolDecl>(FoundDecl)))
       break;
   }
 
   ObjCProtocolDecl *ToProto = MergeWithProtocol;
   if (!ToProto) {
+    auto ToAtBeginLocOrErr = import(D->getAtStartLoc());
+    if (!ToAtBeginLocOrErr)
+      return ToAtBeginLocOrErr.takeError();
+
     if (GetImportedOrCreateDecl(ToProto, D, Importer.getToContext(), DC,
                                 Name.getAsIdentifierInfo(), Loc,
-                                Importer.Import(D->getAtStartLoc()),
+                                *ToAtBeginLocOrErr,
                                 /*PrevDecl=*/nullptr))
       return ToProto;
     ToProto->setLexicalDeclContext(LexicalDC);
     LexicalDC->addDeclInternal(ToProto);
   }
 
   Importer.MapImported(D, ToProto);
 
-  if (D->isThisDeclarationADefinition() && ImportDefinition(D, ToProto))
-    return nullptr;
+  if (D->isThisDeclarationADefinition())
+    if (Error Err = ImportDefinition(D, ToProto))
+      return std::move(Err);
 
   return ToProto;
 }
 
-Decl *ASTNodeImporter::VisitLinkageSpecDecl(LinkageSpecDecl *D) {
-  DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-  DeclContext *LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
+ExpectedDecl ASTNodeImporter::VisitLinkageSpecDecl(LinkageSpecDecl *D) {
+  DeclContext *DC, *LexicalDC;
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return std::move(Err);
 
-  SourceLocation ExternLoc = Importer.Import(D->getExternLoc());
-  SourceLocation LangLoc = Importer.Import(D->getLocation());
+  ExpectedSLoc ExternLocOrErr = import(D->getExternLoc());
+  if (!ExternLocOrErr)
+    return ExternLocOrErr.takeError();
+
+  ExpectedSLoc LangLocOrErr = import(D->getLocation());
+  if (!LangLocOrErr)
+    return LangLocOrErr.takeError();
 
   bool HasBraces = D->hasBraces();
 
   LinkageSpecDecl *ToLinkageSpec;
   if (GetImportedOrCreateDecl(ToLinkageSpec, D, Importer.getToContext(), DC,
-                              ExternLoc, LangLoc, D->getLanguage(), HasBraces))
+                              *ExternLocOrErr, *LangLocOrErr,
+                              D->getLanguage(), HasBraces))
     return ToLinkageSpec;
 
   if (HasBraces) {
-    SourceLocation RBraceLoc = Importer.Import(D->getRBraceLoc());
-    ToLinkageSpec->setRBraceLoc(RBraceLoc);
+    ExpectedSLoc RBraceLocOrErr = import(D->getRBraceLoc());
+    if (!RBraceLocOrErr)
+      return RBraceLocOrErr.takeError();
+    ToLinkageSpec->setRBraceLoc(*RBraceLocOrErr);
   }
 
   ToLinkageSpec->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToLinkageSpec);
 
   return ToLinkageSpec;
 }
 
-Decl *ASTNodeImporter::VisitUsingDecl(UsingDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitUsingDecl(UsingDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD = nullptr;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  DeclarationNameInfo NameInfo(Name,
-                               Importer.Import(D->getNameInfo().getLoc()));
-  ImportDeclarationNameLoc(D->getNameInfo(), NameInfo);
+  SourceLocation ToLoc, ToUsingLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  if (auto Imp = importSeq(
+      D->getNameInfo().getLoc(), D->getUsingLoc(), D->getQualifierLoc()))
+    std::tie(ToLoc, ToUsingLoc, ToQualifierLoc) = *Imp;
+  else
+    return Imp.takeError();
+
+  DeclarationNameInfo NameInfo(Name, ToLoc);
+  if (Error Err = ImportDeclarationNameLoc(D->getNameInfo(), NameInfo))
+    return std::move(Err);
 
   UsingDecl *ToUsing;
   if (GetImportedOrCreateDecl(ToUsing, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getUsingLoc()),
-                              Importer.Import(D->getQualifierLoc()), NameInfo,
+                              ToUsingLoc, ToQualifierLoc, NameInfo,
                               D->hasTypename()))
     return ToUsing;
 
   ToUsing->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToUsing);
 
   if (NamedDecl *FromPattern =
       Importer.getFromContext().getInstantiatedFromUsingDecl(D)) {
-    if (auto *ToPattern =
-            dyn_cast_or_null<NamedDecl>(Importer.Import(FromPattern)))
-      Importer.getToContext().setInstantiatedFromUsingDecl(ToUsing, ToPattern);
+    if (Expected<NamedDecl *> ToPatternOrErr = import(FromPattern))
+      Importer.getToContext().setInstantiatedFromUsingDecl(
+          ToUsing, *ToPatternOrErr);
     else
-      return nullptr;
+      return ToPatternOrErr.takeError();
   }
 
-  for (auto *FromShadow : D->shadows()) {
-    if (auto *ToShadow =
-            dyn_cast_or_null<UsingShadowDecl>(Importer.Import(FromShadow)))
-      ToUsing->addShadowDecl(ToShadow);
+  for (UsingShadowDecl *FromShadow : D->shadows()) {
+    if (Expected<UsingShadowDecl *> ToShadowOrErr = import(FromShadow))
+      ToUsing->addShadowDecl(*ToShadowOrErr);
     else
-      // FIXME: We return a nullptr here but the definition is already created
+      // FIXME: We return error here but the definition is already created
       // and available with lookups. How to fix this?..
-      return nullptr;
+      return ToShadowOrErr.takeError();
   }
   return ToUsing;
 }
 
-Decl *ASTNodeImporter::VisitUsingShadowDecl(UsingShadowDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitUsingShadowDecl(UsingShadowDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD = nullptr;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  auto *ToUsing = dyn_cast_or_null<UsingDecl>(
-      Importer.Import(D->getUsingDecl()));
-  if (!ToUsing)
-    return nullptr;
-
-  auto *ToTarget = dyn_cast_or_null<NamedDecl>(
-      Importer.Import(D->getTargetDecl()));
-  if (!ToTarget)
-    return nullptr;
+  Expected<UsingDecl *> ToUsingOrErr = import(D->getUsingDecl());
+  if (!ToUsingOrErr)
+    return ToUsingOrErr.takeError();
+
+  Expected<NamedDecl *> ToTargetOrErr = import(D->getTargetDecl());
+  if (!ToTargetOrErr)
+    return ToTargetOrErr.takeError();
 
   UsingShadowDecl *ToShadow;
   if (GetImportedOrCreateDecl(ToShadow, D, Importer.getToContext(), DC, Loc,
-                              ToUsing, ToTarget))
+                              *ToUsingOrErr, *ToTargetOrErr))
     return ToShadow;
 
   ToShadow->setLexicalDeclContext(LexicalDC);
   ToShadow->setAccess(D->getAccess());
 
   if (UsingShadowDecl *FromPattern =
       Importer.getFromContext().getInstantiatedFromUsingShadowDecl(D)) {
-    if (auto *ToPattern =
-            dyn_cast_or_null<UsingShadowDecl>(Importer.Import(FromPattern)))
-      Importer.getToContext().setInstantiatedFromUsingShadowDecl(ToShadow,
-                                                                 ToPattern);
+    if (Expected<UsingShadowDecl *> ToPatternOrErr = import(FromPattern))
+      Importer.getToContext().setInstantiatedFromUsingShadowDecl(
+          ToShadow, *ToPatternOrErr);
     else
-      // FIXME: We return a nullptr here but the definition is already created
+      // FIXME: We return error here but the definition is already created
       // and available with lookups. How to fix this?..
-      return nullptr;
+      return ToPatternOrErr.takeError();
   }
 
   LexicalDC->addDeclInternal(ToShadow);
 
   return ToShadow;
 }
 
-Decl *ASTNodeImporter::VisitUsingDirectiveDecl(UsingDirectiveDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitUsingDirectiveDecl(UsingDirectiveDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD = nullptr;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  DeclContext *ToComAncestor = Importer.ImportContext(D->getCommonAncestor());
-  if (!ToComAncestor)
-    return nullptr;
-
-  auto *ToNominated = cast_or_null<NamespaceDecl>(
-      Importer.Import(D->getNominatedNamespace()));
-  if (!ToNominated)
-    return nullptr;
+  auto ToComAncestorOrErr = Importer.ImportContext(D->getCommonAncestor());
+  if (!ToComAncestorOrErr)
+    return ToComAncestorOrErr.takeError();
+
+  NamespaceDecl *ToNominatedNamespace;
+  SourceLocation ToUsingLoc, ToNamespaceKeyLocation, ToIdentLocation;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  if (auto Imp = importSeq(
+      D->getNominatedNamespace(), D->getUsingLoc(),
+      D->getNamespaceKeyLocation(), D->getQualifierLoc(),
+      D->getIdentLocation()))
+    std::tie(
+        ToNominatedNamespace, ToUsingLoc, ToNamespaceKeyLocation,
+        ToQualifierLoc, ToIdentLocation) = *Imp;
+  else
+    return Imp.takeError();
 
   UsingDirectiveDecl *ToUsingDir;
   if (GetImportedOrCreateDecl(ToUsingDir, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getUsingLoc()),
-                              Importer.Import(D->getNamespaceKeyLocation()),
-                              Importer.Import(D->getQualifierLoc()),
-                              Importer.Import(D->getIdentLocation()),
-                              ToNominated, ToComAncestor))
+                              ToUsingLoc,
+                              ToNamespaceKeyLocation,
+                              ToQualifierLoc,
+                              ToIdentLocation,
+                              ToNominatedNamespace, *ToComAncestorOrErr))
     return ToUsingDir;
 
   ToUsingDir->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToUsingDir);
 
   return ToUsingDir;
 }
 
-Decl *ASTNodeImporter::VisitUnresolvedUsingValueDecl(
+ExpectedDecl ASTNodeImporter::VisitUnresolvedUsingValueDecl(
     UnresolvedUsingValueDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD = nullptr;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
-  DeclarationNameInfo NameInfo(Name, Importer.Import(D->getNameInfo().getLoc()));
-  ImportDeclarationNameLoc(D->getNameInfo(), NameInfo);
+  SourceLocation ToLoc, ToUsingLoc, ToEllipsisLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  if (auto Imp = importSeq(
+      D->getNameInfo().getLoc(), D->getUsingLoc(), D->getQualifierLoc(),
+      D->getEllipsisLoc()))
+    std::tie(ToLoc, ToUsingLoc, ToQualifierLoc, ToEllipsisLoc) = *Imp;
+  else
+    return Imp.takeError();
+
+  DeclarationNameInfo NameInfo(Name, ToLoc);
+  if (Error Err = ImportDeclarationNameLoc(D->getNameInfo(), NameInfo))
+    return std::move(Err);
 
   UnresolvedUsingValueDecl *ToUsingValue;
   if (GetImportedOrCreateDecl(ToUsingValue, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getUsingLoc()),
-                              Importer.Import(D->getQualifierLoc()), NameInfo,
-                              Importer.Import(D->getEllipsisLoc())))
+                              ToUsingLoc, ToQualifierLoc, NameInfo,
+                              ToEllipsisLoc))
     return ToUsingValue;
 
   ToUsingValue->setAccess(D->getAccess());
   ToUsingValue->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToUsingValue);
 
   return ToUsingValue;
 }
 
-Decl *ASTNodeImporter::VisitUnresolvedUsingTypenameDecl(
+ExpectedDecl ASTNodeImporter::VisitUnresolvedUsingTypenameDecl(
     UnresolvedUsingTypenameDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD = nullptr;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
+  SourceLocation ToUsingLoc, ToTypenameLoc, ToEllipsisLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  if (auto Imp = importSeq(
+      D->getUsingLoc(), D->getTypenameLoc(), D->getQualifierLoc(),
+      D->getEllipsisLoc()))
+    std::tie(ToUsingLoc, ToTypenameLoc, ToQualifierLoc, ToEllipsisLoc) = *Imp;
+  else
+    return Imp.takeError();
+
   UnresolvedUsingTypenameDecl *ToUsing;
   if (GetImportedOrCreateDecl(ToUsing, D, Importer.getToContext(), DC,
-                              Importer.Import(D->getUsingLoc()),
-                              Importer.Import(D->getTypenameLoc()),
-                              Importer.Import(D->getQualifierLoc()), Loc, Name,
-                              Importer.Import(D->getEllipsisLoc())))
+                              ToUsingLoc, ToTypenameLoc,
+                              ToQualifierLoc, Loc, Name, ToEllipsisLoc))
     return ToUsing;
 
   ToUsing->setAccess(D->getAccess());
   ToUsing->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToUsing);
 
   return ToUsing;
 }
 
-bool ASTNodeImporter::ImportDefinition(ObjCInterfaceDecl *From,
-                                       ObjCInterfaceDecl *To,
-                                       ImportDefinitionKind Kind) {
+
+Error ASTNodeImporter::ImportDefinition(
+    ObjCInterfaceDecl *From, ObjCInterfaceDecl *To, ImportDefinitionKind Kind) {
   if (To->getDefinition()) {
     // Check consistency of superclass.
     ObjCInterfaceDecl *FromSuper = From->getSuperClass();
     if (FromSuper) {
-      FromSuper = cast_or_null<ObjCInterfaceDecl>(Importer.Import(FromSuper));
-      if (!FromSuper)
-        return true;
+      if (auto FromSuperOrErr = import(FromSuper))
+        FromSuper = *FromSuperOrErr;
+      else
+        return FromSuperOrErr.takeError();
     }
 
     ObjCInterfaceDecl *ToSuper = To->getSuperClass();
     if ((bool)FromSuper != (bool)ToSuper ||
         (FromSuper && !declaresSameEntity(FromSuper, ToSuper))) {
       Importer.ToDiag(To->getLocation(),
                       diag::err_odr_objc_superclass_inconsistent)
         << To->getDeclName();
       if (ToSuper)
         Importer.ToDiag(To->getSuperClassLoc(), diag::note_odr_objc_superclass)
           << To->getSuperClass()->getDeclName();
       else
         Importer.ToDiag(To->getLocation(),
                         diag::note_odr_objc_missing_superclass);
       if (From->getSuperClass())
         Importer.FromDiag(From->getSuperClassLoc(),
                           diag::note_odr_objc_superclass)
         << From->getSuperClass()->getDeclName();
       else
         Importer.FromDiag(From->getLocation(),
                           diag::note_odr_objc_missing_superclass);
     }
 
     if (shouldForceImportDeclContext(Kind))
-      ImportDeclContext(From);
-    return false;
+      if (Error Err = ImportDeclContext(From))
+        return Err;
+    return Error::success();
   }
 
   // Start the definition.
   To->startDefinition();
 
   // If this class has a superclass, import it.
   if (From->getSuperClass()) {
-    TypeSourceInfo *SuperTInfo = Importer.Import(From->getSuperClassTInfo());
-    if (!SuperTInfo)
-      return true;
-
-    To->setSuperClass(SuperTInfo);
+    if (auto SuperTInfoOrErr = import(From->getSuperClassTInfo()))
+      To->setSuperClass(*SuperTInfoOrErr);
+    else
+      return SuperTInfoOrErr.takeError();
   }
 
   // Import protocols
   SmallVector<ObjCProtocolDecl *, 4> Protocols;
   SmallVector<SourceLocation, 4> ProtocolLocs;
-  ObjCInterfaceDecl::protocol_loc_iterator
-  FromProtoLoc = From->protocol_loc_begin();
+  ObjCInterfaceDecl::protocol_loc_iterator FromProtoLoc =
+      From->protocol_loc_begin();
 
   for (ObjCInterfaceDecl::protocol_iterator FromProto = From->protocol_begin(),
                                          FromProtoEnd = From->protocol_end();
        FromProto != FromProtoEnd;
        ++FromProto, ++FromProtoLoc) {
-    auto *ToProto = cast_or_null<ObjCProtocolDecl>(Importer.Import(*FromProto));
-    if (!ToProto)
-      return true;
-    Protocols.push_back(ToProto);
-    ProtocolLocs.push_back(Importer.Import(*FromProtoLoc));
+    if (Expected<ObjCProtocolDecl *> ToProtoOrErr = import(*FromProto))
+      Protocols.push_back(*ToProtoOrErr);
+    else
+      return ToProtoOrErr.takeError();
+
+    if (ExpectedSLoc ToProtoLocOrErr = import(*FromProtoLoc))
+      ProtocolLocs.push_back(*ToProtoLocOrErr);
+    else
+      return ToProtoLocOrErr.takeError();
+
   }
 
   // FIXME: If we're merging, make sure that the protocol list is the same.
   To->setProtocolList(Protocols.data(), Protocols.size(),
                       ProtocolLocs.data(), Importer.getToContext());
 
   // Import categories. When the categories themselves are imported, they'll
   // hook themselves into this interface.
-  for (auto *Cat : From->known_categories())
-    Importer.Import(Cat);
+  for (auto *Cat : From->known_categories()) {
+    auto ToCatOrErr = import(Cat);
+    if (!ToCatOrErr)
+      return ToCatOrErr.takeError();
+  }
 
   // If we have an @implementation, import it as well.
   if (From->getImplementation()) {
-    auto *Impl = cast_or_null<ObjCImplementationDecl>(
-        Importer.Import(From->getImplementation()));
-    if (!Impl)
-      return true;
-
-    To->setImplementation(Impl);
+    if (Expected<ObjCImplementationDecl *> ToImplOrErr =
+        import(From->getImplementation()))
+      To->setImplementation(*ToImplOrErr);
+    else
+      return ToImplOrErr.takeError();
   }
 
   if (shouldForceImportDeclContext(Kind)) {
     // Import all of the members of this class.
-    ImportDeclContext(From, /*ForceImport=*/true);
+    if (Error Err = ImportDeclContext(From, /*ForceImport=*/true))
+      return Err;
   }
-  return false;
+  return Error::success();
 }
 
-ObjCTypeParamList *
+Expected<ObjCTypeParamList *>
 ASTNodeImporter::ImportObjCTypeParamList(ObjCTypeParamList *list) {
   if (!list)
     return nullptr;
 
   SmallVector<ObjCTypeParamDecl *, 4> toTypeParams;
-  for (auto fromTypeParam : *list) {
-    auto *toTypeParam = cast_or_null<ObjCTypeParamDecl>(
-        Importer.Import(fromTypeParam));
-    if (!toTypeParam)
-      return nullptr;
-
-    toTypeParams.push_back(toTypeParam);
+  for (auto *fromTypeParam : *list) {
+    if (auto toTypeParamOrErr = import(fromTypeParam))
+      toTypeParams.push_back(*toTypeParamOrErr);
+    else
+      return toTypeParamOrErr.takeError();
   }
 
+  auto LAngleLocOrErr = import(list->getLAngleLoc());
+  if (!LAngleLocOrErr)
+    return LAngleLocOrErr.takeError();
+
+  auto RAngleLocOrErr = import(list->getRAngleLoc());
+  if (!RAngleLocOrErr)
+    return RAngleLocOrErr.takeError();
+
   return ObjCTypeParamList::create(Importer.getToContext(),
-                                   Importer.Import(list->getLAngleLoc()),
+                                   *LAngleLocOrErr,
                                    toTypeParams,
-                                   Importer.Import(list->getRAngleLoc()));
+                                   *RAngleLocOrErr);
 }
 
-Decl *ASTNodeImporter::VisitObjCInterfaceDecl(ObjCInterfaceDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCInterfaceDecl(ObjCInterfaceDecl *D) {
   // If this class has a definition in the translation unit we're coming from,
   // but this particular declaration is not that definition, import the
   // definition and map to that.
   ObjCInterfaceDecl *Definition = D->getDefinition();
   if (Definition && Definition != D) {
-    Decl *ImportedDef = Importer.Import(Definition);
-    if (!ImportedDef)
-      return nullptr;
-
-    return Importer.MapImported(D, ImportedDef);
+    if (ExpectedDecl ImportedDefOrErr = import(Definition))
+      return Importer.MapImported(D, *ImportedDefOrErr);
+    else
+      return ImportedDefOrErr.takeError();
   }
 
   // Import the major distinguishing characteristics of an @interface.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Look for an existing interface with the same name.
   ObjCInterfaceDecl *MergeWithIface = nullptr;
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
     if (!FoundDecl->isInIdentifierNamespace(Decl::IDNS_Ordinary))
       continue;
 
     if ((MergeWithIface = dyn_cast<ObjCInterfaceDecl>(FoundDecl)))
       break;
   }
 
   // Create an interface declaration, if one does not already exist.
   ObjCInterfaceDecl *ToIface = MergeWithIface;
   if (!ToIface) {
+    ExpectedSLoc AtBeginLocOrErr = import(D->getAtStartLoc());
+    if (!AtBeginLocOrErr)
+      return AtBeginLocOrErr.takeError();
+
     if (GetImportedOrCreateDecl(
             ToIface, D, Importer.getToContext(), DC,
-            Importer.Import(D->getAtStartLoc()), Name.getAsIdentifierInfo(),
+            *AtBeginLocOrErr, Name.getAsIdentifierInfo(),
             /*TypeParamList=*/nullptr,
             /*PrevDecl=*/nullptr, Loc, D->isImplicitInterfaceDecl()))
       return ToIface;
     ToIface->setLexicalDeclContext(LexicalDC);
     LexicalDC->addDeclInternal(ToIface);
   }
   Importer.MapImported(D, ToIface);
-  // Import the type parameter list after calling Imported, to avoid
+  // Import the type parameter list after MapImported, to avoid
   // loops when bringing in their DeclContext.
-  ToIface->setTypeParamList(ImportObjCTypeParamList(
-                              D->getTypeParamListAsWritten()));
+  if (auto ToPListOrErr =
+      ImportObjCTypeParamList(D->getTypeParamListAsWritten()))
+    ToIface->setTypeParamList(*ToPListOrErr);
+  else
+    return ToPListOrErr.takeError();
 
-  if (D->isThisDeclarationADefinition() && ImportDefinition(D, ToIface))
-    return nullptr;
+  if (D->isThisDeclarationADefinition())
+    if (Error Err = ImportDefinition(D, ToIface))
+      return std::move(Err);
 
   return ToIface;
 }
 
-Decl *ASTNodeImporter::VisitObjCCategoryImplDecl(ObjCCategoryImplDecl *D) {
-  auto *Category = cast_or_null<ObjCCategoryDecl>(
-      Importer.Import(D->getCategoryDecl()));
-  if (!Category)
-    return nullptr;
+ExpectedDecl
+ASTNodeImporter::VisitObjCCategoryImplDecl(ObjCCategoryImplDecl *D) {
+  ObjCCategoryDecl *Category;
+  if (Error Err = importInto(Category, D->getCategoryDecl()))
+    return std::move(Err);
 
   ObjCCategoryImplDecl *ToImpl = Category->getImplementation();
   if (!ToImpl) {
-    DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-    if (!DC)
-      return nullptr;
+    DeclContext *DC, *LexicalDC;
+    if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+      return std::move(Err);
+
+    SourceLocation ToLocation, ToAtStartLoc, ToCategoryNameLoc;
+    if (auto Imp = importSeq(
+        D->getLocation(), D->getAtStartLoc(), D->getCategoryNameLoc()))
+      std::tie(ToLocation, ToAtStartLoc, ToCategoryNameLoc) = *Imp;
+    else
+      return Imp.takeError();
 
-    SourceLocation CategoryNameLoc = Importer.Import(D->getCategoryNameLoc());
     if (GetImportedOrCreateDecl(
             ToImpl, D, Importer.getToContext(), DC,
             Importer.Import(D->getIdentifier()), Category->getClassInterface(),
-            Importer.Import(D->getLocation()),
-            Importer.Import(D->getAtStartLoc()), CategoryNameLoc))
+            ToLocation, ToAtStartLoc, ToCategoryNameLoc))
       return ToImpl;
 
-    DeclContext *LexicalDC = DC;
-    if (D->getDeclContext() != D->getLexicalDeclContext()) {
-      LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
-      if (!LexicalDC)
-        return nullptr;
-
-      ToImpl->setLexicalDeclContext(LexicalDC);
-    }
-
     LexicalDC->addDeclInternal(ToImpl);
     Category->setImplementation(ToImpl);
   }
 
   Importer.MapImported(D, ToImpl);
-  ImportDeclContext(D);
+  if (Error Err = ImportDeclContext(D))
+    return std::move(Err);
+
   return ToImpl;
 }
 
-Decl *ASTNodeImporter::VisitObjCImplementationDecl(ObjCImplementationDecl *D) {
+ExpectedDecl
+ASTNodeImporter::VisitObjCImplementationDecl(ObjCImplementationDecl *D) {
   // Find the corresponding interface.
-  auto *Iface = cast_or_null<ObjCInterfaceDecl>(
-      Importer.Import(D->getClassInterface()));
-  if (!Iface)
-    return nullptr;
+  ObjCInterfaceDecl *Iface;
+  if (Error Err = importInto(Iface, D->getClassInterface()))
+    return std::move(Err);
 
   // Import the superclass, if any.
-  ObjCInterfaceDecl *Super = nullptr;
-  if (D->getSuperClass()) {
-    Super = cast_or_null<ObjCInterfaceDecl>(
-                                          Importer.Import(D->getSuperClass()));
-    if (!Super)
-      return nullptr;
-  }
+  ObjCInterfaceDecl *Super;
+  if (Error Err = importInto(Super, D->getSuperClass()))
+    return std::move(Err);
 
   ObjCImplementationDecl *Impl = Iface->getImplementation();
   if (!Impl) {
     // We haven't imported an implementation yet. Create a new @implementation
     // now.
+    DeclContext *DC, *LexicalDC;
+    if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+      return std::move(Err);
+
+    SourceLocation ToLocation, ToAtStartLoc, ToSuperClassLoc;
+    SourceLocation ToIvarLBraceLoc, ToIvarRBraceLoc;
+    if (auto Imp = importSeq(
+        D->getLocation(), D->getAtStartLoc(), D->getSuperClassLoc(),
+        D->getIvarLBraceLoc(), D->getIvarRBraceLoc()))
+      std::tie(
+          ToLocation, ToAtStartLoc, ToSuperClassLoc,
+          ToIvarLBraceLoc, ToIvarRBraceLoc) = *Imp;
+    else
+      return Imp.takeError();
+
     if (GetImportedOrCreateDecl(Impl, D, Importer.getToContext(),
-                                Importer.ImportContext(D->getDeclContext()),
-                                Iface, Super, Importer.Import(D->getLocation()),
-                                Importer.Import(D->getAtStartLoc()),
-                                Importer.Import(D->getSuperClassLoc()),
-                                Importer.Import(D->getIvarLBraceLoc()),
-                                Importer.Import(D->getIvarRBraceLoc())))
+                                DC, Iface, Super,
+                                ToLocation,
+                                ToAtStartLoc,
+                                ToSuperClassLoc,
+                                ToIvarLBraceLoc,
+                                ToIvarRBraceLoc))
       return Impl;
 
-    if (D->getDeclContext() != D->getLexicalDeclContext()) {
-      DeclContext *LexicalDC
-        = Importer.ImportContext(D->getLexicalDeclContext());
-      if (!LexicalDC)
-        return nullptr;
-      Impl->setLexicalDeclContext(LexicalDC);
-    }
 
     // Associate the implementation with the class it implements.
     Iface->setImplementation(Impl);
     Importer.MapImported(D, Iface->getImplementation());
   } else {
     Importer.MapImported(D, Iface->getImplementation());
 
     // Verify that the existing @implementation has the same superclass.
if ((Super && !Impl->getSuperClass()) ||
                         diag::note_odr_objc_missing_superclass);
       if (D->getSuperClass())
         Importer.FromDiag(D->getLocation(),
                           diag::note_odr_objc_superclass)
         << D->getSuperClass()->getDeclName();
       else
         Importer.FromDiag(D->getLocation(),
                           diag::note_odr_objc_missing_superclass);
-      return nullptr;
+
+      return make_error<ImportError>(ImportError::NameConflict);
     }
   }
 
   // Import all of the members of this @implementation.
-  ImportDeclContext(D);
+  if (Error Err = ImportDeclContext(D))
+    return std::move(Err);
 
   return Impl;
 }
 
-Decl *ASTNodeImporter::VisitObjCPropertyDecl(ObjCPropertyDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitObjCPropertyDecl(ObjCPropertyDecl *D) {
   // Import the major distinguishing characteristics of an @property.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // Check whether we have already imported this property.
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
     if (auto *FoundProp = dyn_cast<ObjCPropertyDecl>(FoundDecl)) {
       // Check property types.
       if (!Importer.IsStructurallyEquivalent(D->getType(),
                                              FoundProp->getType())) {
         Importer.ToDiag(Loc, diag::err_odr_objc_property_type_inconsistent)
           << Name << D->getType() << FoundProp->getType();
         Importer.ToDiag(FoundProp->getLocation(), diag::note_odr_value_here)
           << FoundProp->getType();
-        return nullptr;
+
+        return make_error<ImportError>(ImportError::NameConflict);
       }
 
       // FIXME: Check property attributes, getters, setters, etc.?
 
       // Consider these properties to be equivalent.
       Importer.MapImported(D, FoundProp);
       return FoundProp;
     }
   }
 
-  // Import the type.
-  TypeSourceInfo *TSI = Importer.Import(D->getTypeSourceInfo());
-  if (!TSI)
-    return nullptr;
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  SourceLocation ToAtLoc, ToLParenLoc;
+  if (auto Imp = importSeq(
+      D->getType(), D->getTypeSourceInfo(), D->getAtLoc(), D->getLParenLoc()))
+    std::tie(ToType, ToTypeSourceInfo, ToAtLoc, ToLParenLoc) = *Imp;
+  else
+    return Imp.takeError();
 
   // Create the new property.
   ObjCPropertyDecl *ToProperty;
   if (GetImportedOrCreateDecl(
           ToProperty, D, Importer.getToContext(), DC, Loc,
-          Name.getAsIdentifierInfo(), Importer.Import(D->getAtLoc()),
-          Importer.Import(D->getLParenLoc()), Importer.Import(D->getType()),
-          TSI, D->getPropertyImplementation()))
+          Name.getAsIdentifierInfo(), ToAtLoc,
+          ToLParenLoc, ToType,
+          ToTypeSourceInfo, D->getPropertyImplementation()))
     return ToProperty;
 
+  Selector ToGetterName, ToSetterName;
+  SourceLocation ToGetterNameLoc, ToSetterNameLoc;
+  ObjCMethodDecl *ToGetterMethodDecl, *ToSetterMethodDecl;
+  ObjCIvarDecl *ToPropertyIvarDecl;
+  if (auto Imp = importSeq(
+      D->getGetterName(), D->getSetterName(),
+      D->getGetterNameLoc(), D->getSetterNameLoc(),
+      D->getGetterMethodDecl(), D->getSetterMethodDecl(),
+      D->getPropertyIvarDecl()))
+    std::tie(
+        ToGetterName, ToSetterName,
+        ToGetterNameLoc, ToSetterNameLoc,
+        ToGetterMethodDecl, ToSetterMethodDecl,
+        ToPropertyIvarDecl) = *Imp;
+  else
+    return Imp.takeError();
+
   ToProperty->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToProperty);
 
   ToProperty->setPropertyAttributes(D->getPropertyAttributes());
   ToProperty->setPropertyAttributesAsWritten(
                                       D->getPropertyAttributesAsWritten());
-  ToProperty->setGetterName(Importer.Import(D->getGetterName()),
-                            Importer.Import(D->getGetterNameLoc()));
-  ToProperty->setSetterName(Importer.Import(D->getSetterName()),
-                            Importer.Import(D->getSetterNameLoc()));
-  ToProperty->setGetterMethodDecl(
-     cast_or_null<ObjCMethodDecl>(Importer.Import(D->getGetterMethodDecl())));
-  ToProperty->setSetterMethodDecl(
-     cast_or_null<ObjCMethodDecl>(Importer.Import(D->getSetterMethodDecl())));
-  ToProperty->setPropertyIvarDecl(
-       cast_or_null<ObjCIvarDecl>(Importer.Import(D->getPropertyIvarDecl())));
+  ToProperty->setGetterName(ToGetterName, ToGetterNameLoc);
+  ToProperty->setSetterName(ToSetterName, ToSetterNameLoc);
+  ToProperty->setGetterMethodDecl(ToGetterMethodDecl);
+  ToProperty->setSetterMethodDecl(ToSetterMethodDecl);
+  ToProperty->setPropertyIvarDecl(ToPropertyIvarDecl);
   return ToProperty;
 }
 
-Decl *ASTNodeImporter::VisitObjCPropertyImplDecl(ObjCPropertyImplDecl *D) {
-  auto *Property = cast_or_null<ObjCPropertyDecl>(
-      Importer.Import(D->getPropertyDecl()));
-  if (!Property)
-    return nullptr;
+ExpectedDecl
+ASTNodeImporter::VisitObjCPropertyImplDecl(ObjCPropertyImplDecl *D) {
+  ObjCPropertyDecl *Property;
+  if (Error Err = importInto(Property, D->getPropertyDecl()))
+    return std::move(Err);
 
-  DeclContext *DC = Importer.ImportContext(D->getDeclContext());
-  if (!DC)
-    return nullptr;
+  DeclContext *DC, *LexicalDC;
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return std::move(Err);
 
-  // Import the lexical declaration context.
-  DeclContext *LexicalDC = DC;
-  if (D->getDeclContext() != D->getLexicalDeclContext()) {
-    LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
-    if (!LexicalDC)
-      return nullptr;
-  }
-
-  auto *InImpl = dyn_cast<ObjCImplDecl>(LexicalDC);
-  if (!InImpl)
-    return nullptr;
+  auto *InImpl = cast<ObjCImplDecl>(LexicalDC);
 
   // Import the ivar (for an @synthesize).
   ObjCIvarDecl *Ivar = nullptr;
-  if (D->getPropertyIvarDecl()) {
-    Ivar = cast_or_null<ObjCIvarDecl>(
-                                    Importer.Import(D->getPropertyIvarDecl()));
-    if (!Ivar)
-      return nullptr;
-  }
+  if (Error Err = importInto(Ivar, D->getPropertyIvarDecl()))
+    return std::move(Err);
 
   ObjCPropertyImplDecl *ToImpl
     = InImpl->FindPropertyImplDecl(Property->getIdentifier(),
                                    Property->getQueryKind());
   if (!ToImpl) {
+    SourceLocation ToBeginLoc, ToLocation, ToPropertyIvarDeclLoc;
+    if (auto Imp = importSeq(
+        D->getBeginLoc(), D->getLocation(), D->getPropertyIvarDeclLoc()))
+      std::tie(ToBeginLoc, ToLocation, ToPropertyIvarDeclLoc) = *Imp;
+    else
+      return Imp.takeError();
+
     if (GetImportedOrCreateDecl(ToImpl, D, Importer.getToContext(), DC,
-                                Importer.Import(D->getBeginLoc()),
-                                Importer.Import(D->getLocation()), Property,
+                                ToBeginLoc,
+                                ToLocation, Property,
                                 D->getPropertyImplementation(), Ivar,
-                                Importer.Import(D->getPropertyIvarDeclLoc())))
+                                ToPropertyIvarDeclLoc))
       return ToImpl;
 
     ToImpl->setLexicalDeclContext(LexicalDC);
     LexicalDC->addDeclInternal(ToImpl);
   } else {
     // Check that we have the same kind of property implementation (@synthesize
     // vs. @dynamic).
     if (D->getPropertyImplementation() != ToImpl->getPropertyImplementation()) {
       Importer.ToDiag(ToImpl->getLocation(),
                       diag::err_odr_objc_property_impl_kind_inconsistent)
         << Property->getDeclName()
         << (ToImpl->getPropertyImplementation()
                                               == ObjCPropertyImplDecl::Dynamic);
       Importer.FromDiag(D->getLocation(),
                         diag::note_odr_objc_property_impl_kind)
         << D->getPropertyDecl()->getDeclName()
         << (D->getPropertyImplementation() == ObjCPropertyImplDecl::Dynamic);
-      return nullptr;
+
+      return make_error<ImportError>(ImportError::NameConflict);
     }
 
     // For @synthesize, check that we have the same
     if (D->getPropertyImplementation() == ObjCPropertyImplDecl::Synthesize &&
         Ivar != ToImpl->getPropertyIvarDecl()) {
       Importer.ToDiag(ToImpl->getPropertyIvarDeclLoc(),
                       diag::err_odr_objc_synthesize_ivar_inconsistent)
         << Property->getDeclName()
         << ToImpl->getPropertyIvarDecl()->getDeclName()
         << Ivar->getDeclName();
       Importer.FromDiag(D->getPropertyIvarDeclLoc(),
                         diag::note_odr_objc_synthesize_ivar_here)
         << D->getPropertyIvarDecl()->getDeclName();
-      return nullptr;
+
+      return make_error<ImportError>(ImportError::NameConflict);
     }
 
     // Merge the existing implementation with the new implementation.
     Importer.MapImported(D, ToImpl);
   }
 
   return ToImpl;
 }
 
-Decl *ASTNodeImporter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
+ExpectedDecl
+ASTNodeImporter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   // For template arguments, we adopt the translation unit as our declaration
   // context. This context will be fixed when the actual template declaration
   // is created.
 
   // FIXME: Import default argument.
+
+  ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
+  if (!BeginLocOrErr)
+    return BeginLocOrErr.takeError();
+
+  ExpectedSLoc LocationOrErr = import(D->getLocation());
+  if (!LocationOrErr)
+    return LocationOrErr.takeError();
+
   TemplateTypeParmDecl *ToD = nullptr;
   (void)GetImportedOrCreateDecl(
       ToD, D, Importer.getToContext(),
       Importer.getToContext().getTranslationUnitDecl(),
-      Importer.Import(D->getBeginLoc()), Importer.Import(D->getLocation()),
+      *BeginLocOrErr, *LocationOrErr,
       D->getDepth(), D->getIndex(), Importer.Import(D->getIdentifier()),
       D->wasDeclaredWithTypename(), D->isParameterPack());
   return ToD;
 }
 
-Decl *
+ExpectedDecl
 ASTNodeImporter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
-  // Import the name of this declaration.
-  DeclarationName Name = Importer.Import(D->getDeclName());
-  if (D->getDeclName() && !Name)
-    return nullptr;
-
-  // Import the location of this declaration.
-  SourceLocation Loc = Importer.Import(D->getLocation());
-
-  // Import the type of this declaration.
-  QualType T = Importer.Import(D->getType());
-  if (T.isNull())
-    return nullptr;
-
-  // Import type-source information.
-  TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
-  if (D->getTypeSourceInfo() && !TInfo)
-    return nullptr;
+  DeclarationName ToDeclName;
+  SourceLocation ToLocation, ToInnerLocStart;
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  if (auto Imp = importSeq(
+      D->getDeclName(), D->getLocation(), D->getType(), D->getTypeSourceInfo(),
+      D->getInnerLocStart()))
+    std::tie(
+        ToDeclName, ToLocation, ToType, ToTypeSourceInfo,
+        ToInnerLocStart) = *Imp;
+  else
+    return Imp.takeError();
 
   // FIXME: Import default argument.
 
   NonTypeTemplateParmDecl *ToD = nullptr;
   (void)GetImportedOrCreateDecl(
       ToD, D, Importer.getToContext(),
       Importer.getToContext().getTranslationUnitDecl(),
-      Importer.Import(D->getInnerLocStart()), Loc, D->getDepth(),
-      D->getPosition(), Name.getAsIdentifierInfo(), T, D->isParameterPack(),
-      TInfo);
+      ToInnerLocStart, ToLocation, D->getDepth(),
+      D->getPosition(), ToDeclName.getAsIdentifierInfo(), ToType,
+      D->isParameterPack(), ToTypeSourceInfo);
   return ToD;
 }
 
-Decl *
+ExpectedDecl
 ASTNodeImporter::VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D) {
   // Import the name of this declaration.
-  DeclarationName Name = Importer.Import(D->getDeclName());
-  if (D->getDeclName() && !Name)
-    return nullptr;
+  auto NameOrErr = import(D->getDeclName());
+  if (!NameOrErr)
+    return NameOrErr.takeError();
 
   // Import the location of this declaration.
-  SourceLocation Loc = Importer.Import(D->getLocation());
+  ExpectedSLoc LocationOrErr = import(D->getLocation());
+  if (!LocationOrErr)
+    return LocationOrErr.takeError();
 
   // Import template parameters.
-  TemplateParameterList *TemplateParams
-    = ImportTemplateParameterList(D->getTemplateParameters());
-  if (!TemplateParams)
-    return nullptr;
+  auto TemplateParamsOrErr = ImportTemplateParameterList(
+      D->getTemplateParameters());
+  if (!TemplateParamsOrErr)
+    return TemplateParamsOrErr.takeError();
 
   // FIXME: Import default argument.
 
   TemplateTemplateParmDecl *ToD = nullptr;
   (void)GetImportedOrCreateDecl(
       ToD, D, Importer.getToContext(),
-      Importer.getToContext().getTranslationUnitDecl(), Loc, D->getDepth(),
-      D->getPosition(), D->isParameterPack(), Name.getAsIdentifierInfo(),
-      TemplateParams);
+      Importer.getToContext().getTranslationUnitDecl(), *LocationOrErr,
+      D->getDepth(), D->getPosition(), D->isParameterPack(),
+      (*NameOrErr).getAsIdentifierInfo(),
+      *TemplateParamsOrErr);
   return ToD;
 }
 
 // Returns the definition for a (forward) declaration of a ClassTemplateDecl, if
 // it has any definition in the redecl chain.
 static ClassTemplateDecl *getDefinition(ClassTemplateDecl *D) {
   CXXRecordDecl *ToTemplatedDef = D->getTemplatedDecl()->getDefinition();
   if (!ToTemplatedDef)
     return nullptr;
   ClassTemplateDecl *TemplateWithDef =
       ToTemplatedDef->getDescribedClassTemplate();
   return TemplateWithDef;
 }
 
-Decl *ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
   bool IsFriend = D->getFriendObjectKind() != Decl::FOK_None;
 
   // If this record has a definition in the translation unit we're coming from,
   // but this particular declaration is not that definition, import the
   // definition and map to that.
   auto *Definition =
       cast_or_null<CXXRecordDecl>(D->getTemplatedDecl()->getDefinition());
   if (Definition && Definition != D->getTemplatedDecl() && !IsFriend) {
-    Decl *ImportedDef
-      = Importer.Import(Definition->getDescribedClassTemplate());
-    if (!ImportedDef)
-      return nullptr;
-
-    return Importer.MapImported(D, ImportedDef);
+    if (ExpectedDecl ImportedDefOrErr = import(
+        Definition->getDescribedClassTemplate()))
+      return Importer.MapImported(D, *ImportedDefOrErr);
+    else
+      return ImportedDefOrErr.takeError();
   }
 
   // Import the major distinguishing characteristics of this class template.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // We may already have a template of the same name; try to find and match it.
   if (!DC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
if (!DC->isFunctionOrMethod()) {
 
     if (!ConflictingDecls.empty()) {
       Name = Importer.HandleNameConflict(Name, DC, Decl::IDNS_Ordinary,
                                          ConflictingDecls.data(),
                                          ConflictingDecls.size());
     }
 
     if (!Name)
-      return nullptr;
+      return make_error<ImportError>(ImportError::NameConflict);
   }
 
   CXXRecordDecl *FromTemplated = D->getTemplatedDecl();
 
   // Create the declaration that is being templated.
-  auto *ToTemplated = cast_or_null<CXXRecordDecl>(
-      Importer.Import(FromTemplated));
-  if (!ToTemplated)
-    return nullptr;
+  CXXRecordDecl *ToTemplated;
+  if (Error Err = importInto(ToTemplated, FromTemplated))
+    return std::move(Err);
 
   // Create the class template declaration itself.
-  TemplateParameterList *TemplateParams =
-      ImportTemplateParameterList(D->getTemplateParameters());
-  if (!TemplateParams)
-    return nullptr;
+  auto TemplateParamsOrErr = ImportTemplateParameterList(
+      D->getTemplateParameters());
+  if (!TemplateParamsOrErr)
+    return TemplateParamsOrErr.takeError();
 
   ClassTemplateDecl *D2;
   if (GetImportedOrCreateDecl(D2, D, Importer.getToContext(), DC, Loc, Name,
-                              TemplateParams, ToTemplated))
+                              *TemplateParamsOrErr, ToTemplated))
     return D2;
 
   ToTemplated->setDescribedClassTemplate(D2);
 
   if (ToTemplated->getPreviousDecl()) {
     assert(
         ToTemplated->getPreviousDecl()->getDescribedClassTemplate() &&
         "Missing described template");
     D2->setPreviousDecl(
         ToTemplated->getPreviousDecl()->getDescribedClassTemplate());
   }
   D2->setAccess(D->getAccess());
   D2->setLexicalDeclContext(LexicalDC);
   if (!IsFriend)
     LexicalDC->addDeclInternal(D2);
 
   if (FromTemplated->isCompleteDefinition() &&
       !ToTemplated->isCompleteDefinition()) {
     // FIXME: Import definition!
   }
 
   return D2;
 }
 
-Decl *ASTNodeImporter::VisitClassTemplateSpecializationDecl(
+ExpectedDecl ASTNodeImporter::VisitClassTemplateSpecializationDecl(
                                           ClassTemplateSpecializationDecl *D) {
   // If this record has a definition in the translation unit we're coming from,
   // but this particular declaration is not that definition, import the
   // definition and map to that.
   TagDecl *Definition = D->getDefinition();
   if (Definition && Definition != D) {
-    Decl *ImportedDef = Importer.Import(Definition);
-    if (!ImportedDef)
-      return nullptr;
-
-    return Importer.MapImported(D, ImportedDef);
+    if (ExpectedDecl ImportedDefOrErr = import(Definition))
+      return Importer.MapImported(D, *ImportedDefOrErr);
+    else
+      return ImportedDefOrErr.takeError();
   }
 
-  auto *ClassTemplate =
-      cast_or_null<ClassTemplateDecl>(Importer.Import(
-                                                 D->getSpecializedTemplate()));
-  if (!ClassTemplate)
-    return nullptr;
+  ClassTemplateDecl *ClassTemplate;
+  if (Error Err = importInto(ClassTemplate, D->getSpecializedTemplate()))
+    return std::move(Err);
 
   // Import the context of this declaration.
-  DeclContext *DC = ClassTemplate->getDeclContext();
-  if (!DC)
-    return nullptr;
-
-  DeclContext *LexicalDC = DC;
-  if (D->getDeclContext() != D->getLexicalDeclContext()) {
-    LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
-    if (!LexicalDC)
-      return nullptr;
-  }
-
-  // Import the location of this declaration.
-  SourceLocation StartLoc = Importer.Import(D->getBeginLoc());
-  SourceLocation IdLoc = Importer.Import(D->getLocation());
+  DeclContext *DC, *LexicalDC;
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return std::move(Err);
 
   // Import template arguments.
   SmallVector<TemplateArgument, 2> TemplateArgs;
-  if (ImportTemplateArguments(D->getTemplateArgs().data(),
-                              D->getTemplateArgs().size(),
-                              TemplateArgs))
-    return nullptr;
+  if (Error Err = ImportTemplateArguments(
+      D->getTemplateArgs().data(), D->getTemplateArgs().size(), TemplateArgs))
+    return std::move(Err);
 
   // Try to find an existing specialization with these template arguments.
   void *InsertPos = nullptr;
   ClassTemplateSpecializationDecl *D2 = nullptr;
   ClassTemplatePartialSpecializationDecl *PartialSpec =
             dyn_cast<ClassTemplatePartialSpecializationDecl>(D);
   if (PartialSpec)
     D2 = ClassTemplate->findPartialSpecialization(TemplateArgs, InsertPos);
if (FoundDef) {
     }
 
     if (IsStructuralMatch(D, FoundDef)) {
 
       Importer.MapImported(D, FoundDef);
 
       // Import those those default field initializers which have been
       // instantiated in the "From" context, but not in the "To" context.
-      for (auto *FromField : D->fields())
-        Importer.Import(FromField);
+      for (auto *FromField : D->fields()) {
+        auto ToOrErr = import(FromField);
+        if (!ToOrErr)
+          // FIXME: return the error?
+          consumeError(ToOrErr.takeError());
+      }
 
       // Import those methods which have been instantiated in the
       // "From" context, but not in the "To" context.
-      for (CXXMethodDecl *FromM : D->methods())
-        Importer.Import(FromM);
+      for (CXXMethodDecl *FromM : D->methods()) {
+        auto ToOrErr = import(FromM);
+        if (!ToOrErr)
+          // FIXME: return the error?
+          consumeError(ToOrErr.takeError());
+      }
 
       // TODO Import instantiated default arguments.
       // TODO Import instantiated exception specifications.
       //
       // Generally, ASTCommon.h/DeclUpdateKind enum gives a very good hint what
       // else could be fused during an AST merge.
 
       return FoundDef;
     }
   } else { // We either couldn't find any previous specialization in the "To"
            // context,  or we found one but without definition.  Let's create a
            // new specialization and register that at the class template.
+
+    // Import the location of this declaration.
+    ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
+    if (!BeginLocOrErr)
+      return BeginLocOrErr.takeError();
+    ExpectedSLoc IdLocOrErr = import(D->getLocation());
+    if (!IdLocOrErr)
+      return IdLocOrErr.takeError();
+
     if (PartialSpec) {
       // Import TemplateArgumentListInfo.
       TemplateArgumentListInfo ToTAInfo;
       const auto &ASTTemplateArgs = *PartialSpec->getTemplateArgsAsWritten();
-      if (ImportTemplateArgumentListInfo(ASTTemplateArgs, ToTAInfo))
-        return nullptr;
+      if (Error Err = ImportTemplateArgumentListInfo(ASTTemplateArgs, ToTAInfo))
+        return std::move(Err);
 
-      QualType CanonInjType = Importer.Import(
-            PartialSpec->getInjectedSpecializationType());
-      if (CanonInjType.isNull())
-        return nullptr;
+      QualType CanonInjType;
+      if (Error Err = importInto(
+          CanonInjType, PartialSpec->getInjectedSpecializationType()))
+        return std::move(Err);
       CanonInjType = CanonInjType.getCanonicalType();
 
-      TemplateParameterList *ToTPList = ImportTemplateParameterList(
-            PartialSpec->getTemplateParameters());
-      if (!ToTPList && PartialSpec->getTemplateParameters())
-        return nullptr;
+      auto ToTPListOrErr = ImportTemplateParameterList(
+      if (!ToTPListOrErr)
+        return ToTPListOrErr.takeError();
 
       if (GetImportedOrCreateDecl<ClassTemplatePartialSpecializationDecl>(
-              D2, D, Importer.getToContext(), D->getTagKind(), DC, StartLoc,
-              IdLoc, ToTPList, ClassTemplate,
+              D2, D, Importer.getToContext(), D->getTagKind(), DC,
+              *BeginLocOrErr, *IdLocOrErr, *ToTPListOrErr, ClassTemplate,
               llvm::makeArrayRef(TemplateArgs.data(), TemplateArgs.size()),
               ToTAInfo, CanonInjType,
               cast_or_null<ClassTemplatePartialSpecializationDecl>(PrevDecl)))
         return D2;
 
       // Update InsertPos, because preceding import calls may have invalidated
       // it by adding new specializations.
       if (!ClassTemplate->findPartialSpecialization(TemplateArgs, InsertPos))
         // Add this partial specialization to the class template.
         ClassTemplate->AddPartialSpecialization(
             cast<ClassTemplatePartialSpecializationDecl>(D2), InsertPos);
 
     } else { // Not a partial specialization.
       if (GetImportedOrCreateDecl(
-              D2, D, Importer.getToContext(), D->getTagKind(), DC, StartLoc,
-              IdLoc, ClassTemplate, TemplateArgs, PrevDecl))
+              D2, D, Importer.getToContext(), D->getTagKind(), DC,
+              *BeginLocOrErr, *IdLocOrErr, ClassTemplate, TemplateArgs,
+              PrevDecl))
         return D2;
 
       // Update InsertPos, because preceding import calls may have invalidated
       // it by adding new specializations.
       if (!ClassTemplate->findSpecialization(TemplateArgs, InsertPos))
         // Add this specialization to the class template.
         ClassTemplate->AddSpecialization(D2, InsertPos);
     }
 
     D2->setSpecializationKind(D->getSpecializationKind());
 
     // Import the qualifier, if any.
-    D2->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
+    if (auto LocOrErr = import(D->getQualifierLoc()))
+      D2->setQualifierInfo(*LocOrErr);
+    else
+      return LocOrErr.takeError();
 
     if (auto *TSI = D->getTypeAsWritten()) {
-      TypeSourceInfo *TInfo = Importer.Import(TSI);
-      if (!TInfo)
-        return nullptr;
-      D2->setTypeAsWritten(TInfo);
-      D2->setTemplateKeywordLoc(Importer.Import(D->getTemplateKeywordLoc()));
-      D2->setExternLoc(Importer.Import(D->getExternLoc()));
+      if (auto TInfoOrErr = import(TSI))
+        D2->setTypeAsWritten(*TInfoOrErr);
+      else
+        return TInfoOrErr.takeError();
+
+      if (auto LocOrErr = import(D->getTemplateKeywordLoc()))
+        D2->setTemplateKeywordLoc(*LocOrErr);
+      else
+        return LocOrErr.takeError();
+
+      if (auto LocOrErr = import(D->getExternLoc()))
+        D2->setExternLoc(*LocOrErr);
+      else
+        return LocOrErr.takeError();
     }
 
-    SourceLocation POI = Importer.Import(D->getPointOfInstantiation());
-    if (POI.isValid())
-      D2->setPointOfInstantiation(POI);
-    else if (D->getPointOfInstantiation().isValid())
-      return nullptr;
+    if (D->getPointOfInstantiation().isValid()) {
+      if (auto POIOrErr = import(D->getPointOfInstantiation()))
+        D2->setPointOfInstantiation(*POIOrErr);
+      else
+        return POIOrErr.takeError();
+    }
 
     D2->setTemplateSpecializationKind(D->getTemplateSpecializationKind());
 
     // Set the context of this specialization/instantiation.
     D2->setLexicalDeclContext(LexicalDC);
 
     // Add to the DC only if it was an explicit specialization/instantiation.
     if (D2->isExplicitInstantiationOrSpecialization()) {
       LexicalDC->addDeclInternal(D2);
     }
   }
-  if (D->isCompleteDefinition() && ImportDefinition(D, D2))
-    return nullptr;
+  if (D->isCompleteDefinition())
+    if (Error Err = ImportDefinition(D, D2))
+      return std::move(Err);
 
   return D2;
 }
 
-Decl *ASTNodeImporter::VisitVarTemplateDecl(VarTemplateDecl *D) {
+ExpectedDecl ASTNodeImporter::VisitVarTemplateDecl(VarTemplateDecl *D) {
   // If this variable has a definition in the translation unit we're coming
   // from,
   // but this particular declaration is not that definition, import the
   // definition and map to that.
   auto *Definition =
       cast_or_null<VarDecl>(D->getTemplatedDecl()->getDefinition());
   if (Definition && Definition != D->getTemplatedDecl()) {
-    Decl *ImportedDef = Importer.Import(Definition->getDescribedVarTemplate());
-    if (!ImportedDef)
-      return nullptr;
-
-    return Importer.MapImported(D, ImportedDef);
+    if (ExpectedDecl ImportedDefOrErr = import(
+        Definition->getDescribedVarTemplate()))
+      return Importer.MapImported(D, *ImportedDefOrErr);
+    else
+      return ImportedDefOrErr.takeError();
   }
 
   // Import the major distinguishing characteristics of this variable template.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
   if (ToD)
     return ToD;
 
   // We may already have a template of the same name; try to find and match it.
   assert(!DC->isFunctionOrMethod() &&
          "Variable templates cannot be declared at function scope");
   SmallVector<NamedDecl *, 4> ConflictingDecls;
   SmallVector<NamedDecl *, 2> FoundDecls;
   DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
   for (auto *FoundDecl : FoundDecls) {
     if (!FoundDecl->isInIdentifierNamespace(Decl::IDNS_Ordinary))
       continue;
 
     Decl *Found = FoundDecl;
-    if (auto *FoundTemplate = dyn_cast<VarTemplateDecl>(Found)) {
+    if (VarTemplateDecl *FoundTemplate = dyn_cast<VarTemplateDecl>(Found)) {
       if (IsStructuralMatch(D, FoundTemplate)) {
         // The variable templates structurally match; call it the same template.
         Importer.MapImported(D->getTemplatedDecl(),
                              FoundTemplate->getTemplatedDecl());
         return Importer.MapImported(D, FoundTemplate);
       }
     }
 
     ConflictingDecls.push_back(FoundDecl);
   }
 
   if (!ConflictingDecls.empty()) {
     Name = Importer.HandleNameConflict(Name, DC, Decl::IDNS_Ordinary,
                                        ConflictingDecls.data(),
                                        ConflictingDecls.size());
   }
 
   if (!Name)
-    return nullptr;
+    // FIXME: Is it possible to get other error than name conflict?
+    // (Put this `if` into the previous `if`?)
+    return make_error<ImportError>(ImportError::NameConflict);
 
   VarDecl *DTemplated = D->getTemplatedDecl();
 
   // Import the type.
-  QualType T = Importer.Import(DTemplated->getType());
-  if (T.isNull())
-    return nullptr;
+  // FIXME: Value not used?
+  ExpectedType TypeOrErr = import(DTemplated->getType());
+  if (!TypeOrErr)
+    return TypeOrErr.takeError();
 
   // Create the declaration that is being templated.
-  auto *ToTemplated = dyn_cast_or_null<VarDecl>(Importer.Import(DTemplated));
-  if (!ToTemplated)
-    return nullptr;
+  VarDecl *ToTemplated;
+  if (Error Err = importInto(ToTemplated, DTemplated))
+    return std::move(Err);
 
   // Create the variable template declaration itself.
-  TemplateParameterList *TemplateParams =
-      ImportTemplateParameterList(D->getTemplateParameters());
-  if (!TemplateParams)
-    return nullptr;
+  auto TemplateParamsOrErr = ImportTemplateParameterList(
+      D->getTemplateParameters());
+  if (!TemplateParamsOrErr)
+    return TemplateParamsOrErr.takeError();
 
   VarTemplateDecl *ToVarTD;
   if (GetImportedOrCreateDecl(ToVarTD, D, Importer.getToContext(), DC, Loc,
-                              Name, TemplateParams, ToTemplated))
+                              Name, *TemplateParamsOrErr, ToTemplated))
     return ToVarTD;
 
   ToTemplated->setDescribedVarTemplate(ToVarTD);
 
   ToVarTD->setAccess(D->getAccess());
   ToVarTD->setLexicalDeclContext(LexicalDC);
   LexicalDC->addDeclInternal(ToVarTD);
 
   if (DTemplated->isThisDeclarationADefinition() &&
       !ToTemplated->isThisDeclarationADefinition()) {
     // FIXME: Import definition!
   }
 
   return ToVarTD;
 }
 
-Decl *ASTNodeImporter::VisitVarTemplateSpecializationDecl(
+ExpectedDecl ASTNodeImporter::VisitVarTemplateSpecializationDecl(
     VarTemplateSpecializationDecl *D) {
   // If this record has a definition in the translation unit we're coming from,
   // but this particular declaration is not that definition, import the
   // definition and map to that.
   VarDecl *Definition = D->getDefinition();
   if (Definition && Definition != D) {
-    Decl *ImportedDef = Importer.Import(Definition);
-    if (!ImportedDef)
-      return nullptr;
-
-    return Importer.MapImported(D, ImportedDef);
+    if (ExpectedDecl ImportedDefOrErr = import(Definition))
+      return Importer.MapImported(D, *ImportedDefOrErr);
+    else
+      return ImportedDefOrErr.takeError();
   }
 
-  auto *VarTemplate = cast_or_null<VarTemplateDecl>(
-      Importer.Import(D->getSpecializedTemplate()));
-  if (!VarTemplate)
-    return nullptr;
+  VarTemplateDecl *VarTemplate;
+  if (Error Err = importInto(VarTemplate, D->getSpecializedTemplate()))
+    return std::move(Err);
 
   // Import the context of this declaration.
-  DeclContext *DC = VarTemplate->getDeclContext();
-  if (!DC)
-    return nullptr;
-
-  DeclContext *LexicalDC = DC;
-  if (D->getDeclContext() != D->getLexicalDeclContext()) {
-    LexicalDC = Importer.ImportContext(D->getLexicalDeclContext());
-    if (!LexicalDC)
-      return nullptr;
-  }
+  DeclContext *DC, *LexicalDC;
+  if (Error Err = ImportDeclContext(D, DC, LexicalDC))
+    return std::move(Err);
 
   // Import the location of this declaration.
-  SourceLocation StartLoc = Importer.Import(D->getBeginLoc());
-  SourceLocation IdLoc = Importer.Import(D->getLocation());
+  ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
+  if (!BeginLocOrErr)
+    return BeginLocOrErr.takeError();
+
+  auto IdLocOrErr = import(D->getLocation());
+  if (!IdLocOrErr)
+    return IdLocOrErr.takeError();
 
   // Import template arguments.
   SmallVector<TemplateArgument, 2> TemplateArgs;
-  if (ImportTemplateArguments(D->getTemplateArgs().data(),
-                              D->getTemplateArgs().size(), TemplateArgs))
-    return nullptr;
+  if (Error Err = ImportTemplateArguments(
+      D->getTemplateArgs().data(), D->getTemplateArgs().size(), TemplateArgs))
+    return std::move(Err);
 
   // Try to find an existing specialization with these template arguments.
   void *InsertPos = nullptr;
   VarTemplateSpecializationDecl *D2 = VarTemplate->findSpecialization(
       TemplateArgs, InsertPos);
   if (D2) {
     // We already have a variable template specialization with these template
     // arguments.
 
     // FIXME: Check for specialization vs. instantiation errors.
 
     if (VarDecl *FoundDef = D2->getDefinition()) {
       if (!D->isThisDeclarationADefinition() ||
           IsStructuralMatch(D, FoundDef)) {
         // The record types structurally match, or the "from" translation
         // unit only had a forward declaration anyway; call it the same
         // variable.
         return Importer.MapImported(D, FoundDef);
       }
     }
   } else {
     // Import the type.
-    QualType T = Importer.Import(D->getType());
-    if (T.isNull())
-      return nullptr;
-
-    TypeSourceInfo *TInfo = Importer.Import(D->getTypeSourceInfo());
-    if (D->getTypeSourceInfo() && !TInfo)
-      return nullptr;
+    QualType T;
+    if (Error Err = importInto(T, D->getType()))
+      return std::move(Err);
+
+    auto TInfoOrErr = import(D->getTypeSourceInfo());
+    if (!TInfoOrErr)
+      return TInfoOrErr.takeError();
 
     TemplateArgumentListInfo ToTAInfo;
-    if (ImportTemplateArgumentListInfo(D->getTemplateArgsInfo(), ToTAInfo))
-      return nullptr;
+    if (Error Err = ImportTemplateArgumentListInfo(
+        D->getTemplateArgsInfo(), ToTAInfo))
+      return std::move(Err);
 
     using PartVarSpecDecl = VarTemplatePartialSpecializationDecl;
     // Create a new specialization.
     if (auto *FromPartial = dyn_cast<PartVarSpecDecl>(D)) {
       // Import TemplateArgumentListInfo
       TemplateArgumentListInfo ArgInfos;
       const auto *FromTAArgsAsWritten = FromPartial->getTemplateArgsAsWritten();
       // NOTE: FromTAArgsAsWritten and template parameter list are non-null.
-      if (ImportTemplateArgumentListInfo(*FromTAArgsAsWritten, ArgInfos))
-        return nullptr;
+      if (Error Err = ImportTemplateArgumentListInfo(
+          *FromTAArgsAsWritten, ArgInfos))
+        return std::move(Err);
 
-      TemplateParameterList *ToTPList = ImportTemplateParameterList(
-            FromPartial->getTemplateParameters());
-      if (!ToTPList)
-        return nullptr;
+      auto ToTPListOrErr = ImportTemplateParameterList(
+      if (!ToTPListOrErr)
+        return ToTPListOrErr.takeError();
 
       PartVarSpecDecl *ToPartial;
       if (GetImportedOrCreateDecl(ToPartial, D, Importer.getToContext(), DC,
-                                  StartLoc, IdLoc, ToTPList, VarTemplate, T,
-                                  TInfo, D->getStorageClass(), TemplateArgs,
-                                  ArgInfos))
+                                  *BeginLocOrErr, *IdLocOrErr, *ToTPListOrErr,
+                                  VarTemplate, T, *TInfoOrErr,
+                                  D->getStorageClass(), TemplateArgs, ArgInfos))
         return ToPartial;
 
-      auto *FromInst = FromPartial->getInstantiatedFromMember();
-      auto *ToInst = cast_or_null<PartVarSpecDecl>(Importer.Import(FromInst));
-      if (FromInst && !ToInst)
-        return nullptr;
+      if (Expected<PartVarSpecDecl *> ToInstOrErr = import(
+          FromPartial->getInstantiatedFromMember()))
+        ToPartial->setInstantiatedFromMember(*ToInstOrErr);
+      else
+        return ToInstOrErr.takeError();
 
-      ToPartial->setInstantiatedFromMember(ToInst);
       if (FromPartial->isMemberSpecialization())
         ToPartial->setMemberSpecialization();
 
       D2 = ToPartial;
+
     } else { // Full specialization
-      if (GetImportedOrCreateDecl(D2, D, Importer.getToContext(), DC, StartLoc,
-                                  IdLoc, VarTemplate, T, TInfo,
+      if (GetImportedOrCreateDecl(D2, D, Importer.getToContext(), DC,
+                                  *BeginLocOrErr, *IdLocOrErr, VarTemplate,
+                                  T, *TInfoOrErr,
                                   D->getStorageClass(), TemplateArgs))
         return D2;
     }
 
-    SourceLocation POI = D->getPointOfInstantiation();
-    if (POI.isValid())
-      D2->setPointOfInstantiation(Importer.Import(POI));
+    if (D->getPointOfInstantiation().isValid()) {
+      if (ExpectedSLoc POIOrErr = import(D->getPointOfInstantiation()))
+        D2->setPointOfInstantiation(*POIOrErr);
+      else
+        return POIOrErr.takeError();
+    }
 
     D2->setSpecializationKind(D->getSpecializationKind());
     D2->setTemplateArgsInfo(ToTAInfo);
 
     // Add this specialization to the class template.
     VarTemplate->AddSpecialization(D2, InsertPos);
 
     // Import the qualifier, if any.
-    D2->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
+    if (auto LocOrErr = import(D->getQualifierLoc()))
+      D2->setQualifierInfo(*LocOrErr);
+    else
+      return LocOrErr.takeError();
 
     if (D->isConstexpr())
       D2->setConstexpr(true);
 
     // Add the specialization to this context.
     D2->setLexicalDeclContext(LexicalDC);
     LexicalDC->addDeclInternal(D2);
 
     D2->setAccess(D->getAccess());
   }
 
-  if (ImportInitializer(D, D2))
-    return nullptr;
+  if (Error Err = ImportInitializer(D, D2))
+    return std::move(Err);
 
   return D2;
 }
 
-Decl *ASTNodeImporter::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
+ExpectedDecl
+ASTNodeImporter::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
   SourceLocation Loc;
   NamedDecl *ToD;
 
-  if (ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
-    return nullptr;
+  if (Error Err = ImportDeclParts(D, DC, LexicalDC, Name, ToD, Loc))
+    return std::move(Err);
 
   if (ToD)
     return ToD;
 
   // Try to find a function in our own ("to") context with the same name, same
   // type, and in the same context as the function we're importing.
   if (!LexicalDC->isFunctionOrMethod()) {
     unsigned IDNS = Decl::IDNS_Ordinary;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
     for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
 
-      if (auto *FoundFunction = dyn_cast<FunctionTemplateDecl>(FoundDecl)) {
+      if (auto *FoundFunction =
+          dyn_cast<FunctionTemplateDecl>(FoundDecl)) {
         if (FoundFunction->hasExternalFormalLinkage() &&
             D->hasExternalFormalLinkage()) {
           if (IsStructuralMatch(D, FoundFunction)) {
             Importer.MapImported(D, FoundFunction);
             // FIXME: Actually try to merge the body and other attributes.
             return FoundFunction;
           }
         }
       }
+      // TODO: handle conflicting names
     }
   }
 
-  TemplateParameterList *Params =
-      ImportTemplateParameterList(D->getTemplateParameters());
-  if (!Params)
-    return nullptr;
+  auto ParamsOrErr = ImportTemplateParameterList(
+      D->getTemplateParameters());
+  if (!ParamsOrErr)
+    return ParamsOrErr.takeError();
 
-  auto *TemplatedFD =
-      cast_or_null<FunctionDecl>(Importer.Import(D->getTemplatedDecl()));
-  if (!TemplatedFD)
-    return nullptr;
+  FunctionDecl *TemplatedFD;
+  if (Error Err = importInto(TemplatedFD, D->getTemplatedDecl()))
+    return std::move(Err);
 
   FunctionTemplateDecl *ToFunc;
   if (GetImportedOrCreateDecl(ToFunc, D, Importer.getToContext(), DC, Loc, Name,
-                              Params, TemplatedFD))
+                              *ParamsOrErr, TemplatedFD))
     return ToFunc;
 
   TemplatedFD->setDescribedFunctionTemplate(ToFunc);
   ToFunc->setAccess(D->getAccess());
   ToFunc->setLexicalDeclContext(LexicalDC);
 
   LexicalDC->addDeclInternal(ToFunc);
   return ToFunc;
 }
 
 //----------------------------------------------------------------------------
 // Import Statements
 //----------------------------------------------------------------------------
 
-DeclGroupRef ASTNodeImporter::ImportDeclGroup(DeclGroupRef DG) {
-  if (DG.isNull())
-    return DeclGroupRef::Create(Importer.getToContext(), nullptr, 0);
-  size_t NumDecls = DG.end() - DG.begin();
-  SmallVector<Decl *, 1> ToDecls(NumDecls);
-  auto &_Importer = this->Importer;
-  std::transform(DG.begin(), DG.end(), ToDecls.begin(),
-    [&_Importer](Decl *D) -> Decl * {
-      return _Importer.Import(D);
-    });
-  return DeclGroupRef::Create(Importer.getToContext(),
-                              ToDecls.begin(),
-                              NumDecls);
-}
-
-Stmt *ASTNodeImporter::VisitStmt(Stmt *S) {
+ExpectedStmt ASTNodeImporter::VisitStmt(Stmt *S) {
   Importer.FromDiag(S->getBeginLoc(), diag::err_unsupported_ast_node)
       << S->getStmtClassName();
-  return nullptr;
+  return make_error<ImportError>(ImportError::UnsupportedConstruct);
 }
 
-Stmt *ASTNodeImporter::VisitGCCAsmStmt(GCCAsmStmt *S) {
+
+ExpectedStmt ASTNodeImporter::VisitGCCAsmStmt(GCCAsmStmt *S) {
   SmallVector<IdentifierInfo *, 4> Names;
   for (unsigned I = 0, E = S->getNumOutputs(); I != E; I++) {
     IdentifierInfo *ToII = Importer.Import(S->getOutputIdentifier(I));
     // ToII is nullptr when no symbolic name is given for output operand
     // see ParseStmtAsm::ParseAsmOperandsOpt
-    if (!ToII && S->getOutputIdentifier(I))
-      return nullptr;
     Names.push_back(ToII);
   }
+
   for (unsigned I = 0, E = S->getNumInputs(); I != E; I++) {
     IdentifierInfo *ToII = Importer.Import(S->getInputIdentifier(I));
     // ToII is nullptr when no symbolic name is given for input operand
     // see ParseStmtAsm::ParseAsmOperandsOpt
-    if (!ToII && S->getInputIdentifier(I))
-      return nullptr;
     Names.push_back(ToII);
   }
 
   SmallVector<StringLiteral *, 4> Clobbers;
   for (unsigned I = 0, E = S->getNumClobbers(); I != E; I++) {
-    auto *Clobber = cast_or_null<StringLiteral>(
-        Importer.Import(S->getClobberStringLiteral(I)));
-    if (!Clobber)
-      return nullptr;
-    Clobbers.push_back(Clobber);
+    if (auto ClobberOrErr = import(S->getClobberStringLiteral(I)))
+      Clobbers.push_back(*ClobberOrErr);
+    else
+      return ClobberOrErr.takeError();
+
   }
 
   SmallVector<StringLiteral *, 4> Constraints;
   for (unsigned I = 0, E = S->getNumOutputs(); I != E; I++) {
-    auto *Output = cast_or_null<StringLiteral>(
-        Importer.Import(S->getOutputConstraintLiteral(I)));
-    if (!Output)
-      return nullptr;
-    Constraints.push_back(Output);
+    if (auto OutputOrErr = import(S->getOutputConstraintLiteral(I)))
+      Constraints.push_back(*OutputOrErr);
+    else
+      return OutputOrErr.takeError();
   }
 
   for (unsigned I = 0, E = S->getNumInputs(); I != E; I++) {
-    auto *Input = cast_or_null<StringLiteral>(
-        Importer.Import(S->getInputConstraintLiteral(I)));
-    if (!Input)
-      return nullptr;
-    Constraints.push_back(Input);
+    if (auto InputOrErr = import(S->getInputConstraintLiteral(I)))
+      Constraints.push_back(*InputOrErr);
+    else
+      return InputOrErr.takeError();
   }
 
   SmallVector<Expr *, 4> Exprs(S->getNumOutputs() + S->getNumInputs());
-  if (ImportContainerChecked(S->outputs(), Exprs))
-    return nullptr;
-
-  if (ImportArrayChecked(S->inputs(), Exprs.begin() + S->getNumOutputs()))
-    return nullptr;
+  if (Error Err = ImportContainerChecked(S->outputs(), Exprs))
+    return std::move(Err);
 
-  auto *AsmStr = cast_or_null<StringLiteral>(
-      Importer.Import(S->getAsmString()));
-  if (!AsmStr)
-    return nullptr;
+  if (Error Err = ImportArrayChecked(
+      S->inputs(), Exprs.begin() + S->getNumOutputs()))
+    return std::move(Err);
+
+  ExpectedSLoc AsmLocOrErr = import(S->getAsmLoc());
+  if (!AsmLocOrErr)
+    return AsmLocOrErr.takeError();
+  auto AsmStrOrErr = import(S->getAsmString());
+  if (!AsmStrOrErr)
+    return AsmStrOrErr.takeError();
+  ExpectedSLoc RParenLocOrErr = import(S->getRParenLoc());
+  if (!RParenLocOrErr)
+    return RParenLocOrErr.takeError();
 
   return new (Importer.getToContext()) GCCAsmStmt(
-        Importer.getToContext(),
-        Importer.Import(S->getAsmLoc()),
-        S->isSimple(),
-        S->isVolatile(),
-        S->getNumOutputs(),
-        S->getNumInputs(),
-        Names.data(),
-        Constraints.data(),
-        Exprs.data(),
-        AsmStr,
-        S->getNumClobbers(),
-        Clobbers.data(),
-        Importer.Import(S->getRParenLoc()));
+      *AsmLocOrErr,
+      *AsmStrOrErr,
+      *RParenLocOrErr);
 }
 
-Stmt *ASTNodeImporter::VisitDeclStmt(DeclStmt *S) {
-  DeclGroupRef ToDG = ImportDeclGroup(S->getDeclGroup());
-  for (auto *ToD : ToDG) {
-    if (!ToD)
-      return nullptr;
-  }
-  SourceLocation ToStartLoc = Importer.Import(S->getBeginLoc());
-  SourceLocation ToEndLoc = Importer.Import(S->getEndLoc());
-  return new (Importer.getToContext()) DeclStmt(ToDG, ToStartLoc, ToEndLoc);
+ExpectedStmt ASTNodeImporter::VisitDeclStmt(DeclStmt *S) {
+  auto Imp = importSeq(S->getDeclGroup(), S->getBeginLoc(), S->getEndLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  DeclGroupRef ToDG;
+  SourceLocation ToBeginLoc, ToEndLoc;
+  std::tie(ToDG, ToBeginLoc, ToEndLoc) = *Imp;
+
+  return new (Importer.getToContext()) DeclStmt(ToDG, ToBeginLoc, ToEndLoc);
 }
 
-Stmt *ASTNodeImporter::VisitNullStmt(NullStmt *S) {
-  SourceLocation ToSemiLoc = Importer.Import(S->getSemiLoc());
-  return new (Importer.getToContext()) NullStmt(ToSemiLoc,
-                                                S->hasLeadingEmptyMacro());
+ExpectedStmt ASTNodeImporter::VisitNullStmt(NullStmt *S) {
+  ExpectedSLoc ToSemiLocOrErr = import(S->getSemiLoc());
+  if (!ToSemiLocOrErr)
+    return ToSemiLocOrErr.takeError();
+  return new (Importer.getToContext()) NullStmt(
+      *ToSemiLocOrErr, S->hasLeadingEmptyMacro());
 }
 
-Stmt *ASTNodeImporter::VisitCompoundStmt(CompoundStmt *S) {
+ExpectedStmt ASTNodeImporter::VisitCompoundStmt(CompoundStmt *S) {
   SmallVector<Stmt *, 8> ToStmts(S->size());
 
-  if (ImportContainerChecked(S->body(), ToStmts))
-    return nullptr;
+  if (Error Err = ImportContainerChecked(S->body(), ToStmts))
+    return std::move(Err);
 
-  SourceLocation ToLBraceLoc = Importer.Import(S->getLBracLoc());
-  SourceLocation ToRBraceLoc = Importer.Import(S->getRBracLoc());
-  return CompoundStmt::Create(Importer.getToContext(), ToStmts, ToLBraceLoc,
-                              ToRBraceLoc);
-}
+  ExpectedSLoc ToLBracLocOrErr = import(S->getLBracLoc());
+  if (!ToLBracLocOrErr)
+    return ToLBracLocOrErr.takeError();
+
+  ExpectedSLoc ToRBracLocOrErr = import(S->getRBracLoc());
+  if (!ToRBracLocOrErr)
+    return ToRBracLocOrErr.takeError();
+
+  return CompoundStmt::Create(
+      Importer.getToContext(), ToStmts,
+      *ToLBracLocOrErr, *ToRBracLocOrErr);
+}
+
+ExpectedStmt ASTNodeImporter::VisitCaseStmt(CaseStmt *S) {
+  auto Imp = importSeq(
+      S->getLHS(), S->getRHS(), S->getSubStmt(), S->getCaseLoc(),
+      S->getEllipsisLoc(), S->getColonLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToLHS, *ToRHS;
+  Stmt *ToSubStmt;
+  SourceLocation ToCaseLoc, ToEllipsisLoc, ToColonLoc;
+  std::tie(ToLHS, ToRHS, ToSubStmt, ToCaseLoc, ToEllipsisLoc, ToColonLoc) =
+      *Imp;
 
-Stmt *ASTNodeImporter::VisitCaseStmt(CaseStmt *S) {
-  Expr *ToLHS = Importer.Import(S->getLHS());
-  if (!ToLHS)
-    return nullptr;
-  Expr *ToRHS = Importer.Import(S->getRHS());
-  if (!ToRHS && S->getRHS())
-    return nullptr;
-  Stmt *ToSubStmt = Importer.Import(S->getSubStmt());
-  if (!ToSubStmt && S->getSubStmt())
-    return nullptr;
-  SourceLocation ToCaseLoc = Importer.Import(S->getCaseLoc());
-  SourceLocation ToEllipsisLoc = Importer.Import(S->getEllipsisLoc());
-  SourceLocation ToColonLoc = Importer.Import(S->getColonLoc());
-  auto *ToStmt = new (Importer.getToContext())
-      CaseStmt(ToLHS, ToRHS, ToCaseLoc, ToEllipsisLoc, ToColonLoc);
+  auto *ToStmt = new (Importer.getToContext()) CaseStmt(
+      ToLHS, ToRHS, ToCaseLoc, ToEllipsisLoc, ToColonLoc);
   ToStmt->setSubStmt(ToSubStmt);
+
   return ToStmt;
 }
 
-Stmt *ASTNodeImporter::VisitDefaultStmt(DefaultStmt *S) {
-  SourceLocation ToDefaultLoc = Importer.Import(S->getDefaultLoc());
-  SourceLocation ToColonLoc = Importer.Import(S->getColonLoc());
-  Stmt *ToSubStmt = Importer.Import(S->getSubStmt());
-  if (!ToSubStmt && S->getSubStmt())
-    return nullptr;
-  return new (Importer.getToContext()) DefaultStmt(ToDefaultLoc, ToColonLoc,
-                                                   ToSubStmt);
+ExpectedStmt ASTNodeImporter::VisitDefaultStmt(DefaultStmt *S) {
+  auto Imp = importSeq(S->getDefaultLoc(), S->getColonLoc(), S->getSubStmt());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToDefaultLoc, ToColonLoc;
+  Stmt *ToSubStmt;
+  std::tie(ToDefaultLoc, ToColonLoc, ToSubStmt) = *Imp;
+
+  return new (Importer.getToContext()) DefaultStmt(
+    ToDefaultLoc, ToColonLoc, ToSubStmt);
 }
 
-Stmt *ASTNodeImporter::VisitLabelStmt(LabelStmt *S) {
-  SourceLocation ToIdentLoc = Importer.Import(S->getIdentLoc());
-  auto *ToLabelDecl = cast_or_null<LabelDecl>(Importer.Import(S->getDecl()));
-  if (!ToLabelDecl && S->getDecl())
-    return nullptr;
-  Stmt *ToSubStmt = Importer.Import(S->getSubStmt());
-  if (!ToSubStmt && S->getSubStmt())
-    return nullptr;
-  return new (Importer.getToContext()) LabelStmt(ToIdentLoc, ToLabelDecl,
-                                                 ToSubStmt);
+ExpectedStmt ASTNodeImporter::VisitLabelStmt(LabelStmt *S) {
+  auto Imp = importSeq(S->getIdentLoc(), S->getDecl(), S->getSubStmt());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToIdentLoc;
+  LabelDecl *ToLabelDecl;
+  Stmt *ToSubStmt;
+  std::tie(ToIdentLoc, ToLabelDecl, ToSubStmt) = *Imp;
+
+  return new (Importer.getToContext()) LabelStmt(
+      ToIdentLoc, ToLabelDecl, ToSubStmt);
 }
 
-Stmt *ASTNodeImporter::VisitAttributedStmt(AttributedStmt *S) {
-  SourceLocation ToAttrLoc = Importer.Import(S->getAttrLoc());
+ExpectedStmt ASTNodeImporter::VisitAttributedStmt(AttributedStmt *S) {
+  ExpectedSLoc ToAttrLocOrErr = import(S->getAttrLoc());
+  if (!ToAttrLocOrErr)
+    return ToAttrLocOrErr.takeError();
   ArrayRef<const Attr*> FromAttrs(S->getAttrs());
   SmallVector<const Attr *, 1> ToAttrs(FromAttrs.size());
-  if (ImportContainerChecked(FromAttrs, ToAttrs))
-    return nullptr;
-  Stmt *ToSubStmt = Importer.Import(S->getSubStmt());
-  if (!ToSubStmt && S->getSubStmt())
-    return nullptr;
-  return AttributedStmt::Create(Importer.getToContext(), ToAttrLoc,
-                                ToAttrs, ToSubStmt);
-}
+  if (Error Err = ImportContainerChecked(FromAttrs, ToAttrs))
+    return std::move(Err);
+  ExpectedStmt ToSubStmtOrErr = import(S->getSubStmt());
+  if (!ToSubStmtOrErr)
+    return ToSubStmtOrErr.takeError();
+
+  return AttributedStmt::Create(
+      Importer.getToContext(), *ToAttrLocOrErr, ToAttrs, *ToSubStmtOrErr);
+}
+
+ExpectedStmt ASTNodeImporter::VisitIfStmt(IfStmt *S) {
+  auto Imp = importSeq(
+      S->getIfLoc(), S->getInit(), S->getConditionVariable(), S->getCond(),
+      S->getThen(), S->getElseLoc(), S->getElse());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToIfLoc, ToElseLoc;
+  Stmt *ToInit, *ToThen, *ToElse;
+  VarDecl *ToConditionVariable;
+  Expr *ToCond;
+  std::tie(
+      ToIfLoc, ToInit, ToConditionVariable, ToCond, ToThen, ToElseLoc, ToElse) =
+          *Imp;
 
-Stmt *ASTNodeImporter::VisitIfStmt(IfStmt *S) {
-  SourceLocation ToIfLoc = Importer.Import(S->getIfLoc());
-  Stmt *ToInit = Importer.Import(S->getInit());
-  if (!ToInit && S->getInit())
-    return nullptr;
-  VarDecl *ToConditionVariable = nullptr;
-  if (VarDecl *FromConditionVariable = S->getConditionVariable()) {
-    ToConditionVariable =
-      dyn_cast_or_null<VarDecl>(Importer.Import(FromConditionVariable));
-    if (!ToConditionVariable)
-      return nullptr;
-  }
-  Expr *ToCondition = Importer.Import(S->getCond());
-  if (!ToCondition && S->getCond())
-    return nullptr;
-  Stmt *ToThenStmt = Importer.Import(S->getThen());
-  if (!ToThenStmt && S->getThen())
-    return nullptr;
-  SourceLocation ToElseLoc = Importer.Import(S->getElseLoc());
-  Stmt *ToElseStmt = Importer.Import(S->getElse());
-  if (!ToElseStmt && S->getElse())
-    return nullptr;
-  return new (Importer.getToContext()) IfStmt(Importer.getToContext(),
-                                              ToIfLoc, S->isConstexpr(),
-                                              ToInit,
-                                              ToConditionVariable,
-                                              ToCondition, ToThenStmt,
-                                              ToElseLoc, ToElseStmt);
+  return new (Importer.getToContext()) IfStmt(
+      Importer.getToContext(),
+      ToIfLoc, S->isConstexpr(), ToInit, ToConditionVariable, ToCond,
+      ToThen, ToElseLoc, ToElse);
 }
 
-Stmt *ASTNodeImporter::VisitSwitchStmt(SwitchStmt *S) {
-  Stmt *ToInit = Importer.Import(S->getInit());
-  if (!ToInit && S->getInit())
-    return nullptr;
-  VarDecl *ToConditionVariable = nullptr;
-  if (VarDecl *FromConditionVariable = S->getConditionVariable()) {
-    ToConditionVariable =
-      dyn_cast_or_null<VarDecl>(Importer.Import(FromConditionVariable));
-    if (!ToConditionVariable)
-      return nullptr;
-  }
-  Expr *ToCondition = Importer.Import(S->getCond());
-  if (!ToCondition && S->getCond())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitSwitchStmt(SwitchStmt *S) {
+  auto Imp = importSeq(
+      S->getInit(), S->getConditionVariable(), S->getCond(),
+      S->getBody(), S->getSwitchLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Stmt *ToInit, *ToBody;
+  VarDecl *ToConditionVariable;
+  Expr *ToCond;
+  SourceLocation ToSwitchLoc;
+  std::tie(ToInit, ToConditionVariable, ToCond, ToBody, ToSwitchLoc) = *Imp;
+
   auto *ToStmt = new (Importer.getToContext()) SwitchStmt(
-                         Importer.getToContext(), ToInit,
-                         ToConditionVariable, ToCondition);
-  Stmt *ToBody = Importer.Import(S->getBody());
-  if (!ToBody && S->getBody())
-    return nullptr;
+      Importer.getToContext(), ToInit, ToConditionVariable, ToCond);
   ToStmt->setBody(ToBody);
-  ToStmt->setSwitchLoc(Importer.Import(S->getSwitchLoc()));
+  ToStmt->setSwitchLoc(ToSwitchLoc);
+
   // Now we have to re-chain the cases.
   SwitchCase *LastChainedSwitchCase = nullptr;
   for (SwitchCase *SC = S->getSwitchCaseList(); SC != nullptr;
        SC = SC->getNextSwitchCase()) {
-    auto *ToSC = dyn_cast_or_null<SwitchCase>(Importer.Import(SC));
-    if (!ToSC)
-      return nullptr;
+    Expected<SwitchCase *> ToSCOrErr = import(SC);
+    if (!ToSCOrErr)
+      return ToSCOrErr.takeError();
     if (LastChainedSwitchCase)
-      LastChainedSwitchCase->setNextSwitchCase(ToSC);
+      LastChainedSwitchCase->setNextSwitchCase(*ToSCOrErr);
     else
-      ToStmt->setSwitchCaseList(ToSC);
-    LastChainedSwitchCase = ToSC;
+      ToStmt->setSwitchCaseList(*ToSCOrErr);
+    LastChainedSwitchCase = *ToSCOrErr;
   }
+
   return ToStmt;
 }
 
-Stmt *ASTNodeImporter::VisitWhileStmt(WhileStmt *S) {
-  VarDecl *ToConditionVariable = nullptr;
-  if (VarDecl *FromConditionVariable = S->getConditionVariable()) {
-    ToConditionVariable =
-      dyn_cast_or_null<VarDecl>(Importer.Import(FromConditionVariable));
-    if (!ToConditionVariable)
-      return nullptr;
-  }
-  Expr *ToCondition = Importer.Import(S->getCond());
-  if (!ToCondition && S->getCond())
-    return nullptr;
-  Stmt *ToBody = Importer.Import(S->getBody());
-  if (!ToBody && S->getBody())
-    return nullptr;
-  SourceLocation ToWhileLoc = Importer.Import(S->getWhileLoc());
-  return new (Importer.getToContext()) WhileStmt(Importer.getToContext(),
-                                                 ToConditionVariable,
-                                                 ToCondition, ToBody,
-                                                 ToWhileLoc);
-}
+ExpectedStmt ASTNodeImporter::VisitWhileStmt(WhileStmt *S) {
+  auto Imp = importSeq(
+      S->getConditionVariable(), S->getCond(), S->getBody(), S->getWhileLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  VarDecl *ToConditionVariable;
+  Expr *ToCond;
+  Stmt *ToBody;
+  SourceLocation ToWhileLoc;
+  std::tie(ToConditionVariable, ToCond, ToBody, ToWhileLoc) = *Imp;
 
-Stmt *ASTNodeImporter::VisitDoStmt(DoStmt *S) {
-  Stmt *ToBody = Importer.Import(S->getBody());
-  if (!ToBody && S->getBody())
-    return nullptr;
-  Expr *ToCondition = Importer.Import(S->getCond());
-  if (!ToCondition && S->getCond())
-    return nullptr;
-  SourceLocation ToDoLoc = Importer.Import(S->getDoLoc());
-  SourceLocation ToWhileLoc = Importer.Import(S->getWhileLoc());
-  SourceLocation ToRParenLoc = Importer.Import(S->getRParenLoc());
-  return new (Importer.getToContext()) DoStmt(ToBody, ToCondition,
-                                              ToDoLoc, ToWhileLoc,
-                                              ToRParenLoc);
+  return new (Importer.getToContext()) WhileStmt(
+      Importer.getToContext(),
+      ToConditionVariable, ToCond, ToBody, ToWhileLoc);
 }
 
-Stmt *ASTNodeImporter::VisitForStmt(ForStmt *S) {
-  Stmt *ToInit = Importer.Import(S->getInit());
-  if (!ToInit && S->getInit())
-    return nullptr;
-  Expr *ToCondition = Importer.Import(S->getCond());
-  if (!ToCondition && S->getCond())
-    return nullptr;
-  VarDecl *ToConditionVariable = nullptr;
-  if (VarDecl *FromConditionVariable = S->getConditionVariable()) {
-    ToConditionVariable =
-      dyn_cast_or_null<VarDecl>(Importer.Import(FromConditionVariable));
-    if (!ToConditionVariable)
-      return nullptr;
-  }
-  Expr *ToInc = Importer.Import(S->getInc());
-  if (!ToInc && S->getInc())
-    return nullptr;
-  Stmt *ToBody = Importer.Import(S->getBody());
-  if (!ToBody && S->getBody())
-    return nullptr;
-  SourceLocation ToForLoc = Importer.Import(S->getForLoc());
-  SourceLocation ToLParenLoc = Importer.Import(S->getLParenLoc());
-  SourceLocation ToRParenLoc = Importer.Import(S->getRParenLoc());
-  return new (Importer.getToContext()) ForStmt(Importer.getToContext(),
-                                               ToInit, ToCondition,
-                                               ToConditionVariable,
-                                               ToInc, ToBody,
-                                               ToForLoc, ToLParenLoc,
-                                               ToRParenLoc);
+ExpectedStmt ASTNodeImporter::VisitDoStmt(DoStmt *S) {
+  auto Imp = importSeq(
+      S->getBody(), S->getCond(), S->getDoLoc(), S->getWhileLoc(),
+      S->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Stmt *ToBody;
+  Expr *ToCond;
+  SourceLocation ToDoLoc, ToWhileLoc, ToRParenLoc;
+  std::tie(ToBody, ToCond, ToDoLoc, ToWhileLoc, ToRParenLoc) = *Imp;
+
+  return new (Importer.getToContext()) DoStmt(
+      ToBody, ToCond, ToDoLoc, ToWhileLoc, ToRParenLoc);
+}
+
+ExpectedStmt ASTNodeImporter::VisitForStmt(ForStmt *S) {
+  auto Imp = importSeq(
+      S->getInit(), S->getCond(), S->getConditionVariable(), S->getInc(),
+      S->getBody(), S->getForLoc(), S->getLParenLoc(), S->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Stmt *ToInit;
+  Expr *ToCond, *ToInc;
+  VarDecl *ToConditionVariable;
+  Stmt *ToBody;
+  SourceLocation ToForLoc, ToLParenLoc, ToRParenLoc;
+  std::tie(
+      ToInit, ToCond, ToConditionVariable,  ToInc, ToBody, ToForLoc,
+      ToLParenLoc, ToRParenLoc) = *Imp;
+
+  return new (Importer.getToContext()) ForStmt(
+      Importer.getToContext(),
+      ToInit, ToCond, ToConditionVariable, ToInc, ToBody, ToForLoc, ToLParenLoc,
 }
 
-Stmt *ASTNodeImporter::VisitGotoStmt(GotoStmt *S) {
-  LabelDecl *ToLabel = nullptr;
-  if (LabelDecl *FromLabel = S->getLabel()) {
-    ToLabel = dyn_cast_or_null<LabelDecl>(Importer.Import(FromLabel));
-    if (!ToLabel)
-      return nullptr;
-  }
-  SourceLocation ToGotoLoc = Importer.Import(S->getGotoLoc());
-  SourceLocation ToLabelLoc = Importer.Import(S->getLabelLoc());
-  return new (Importer.getToContext()) GotoStmt(ToLabel,
-                                                ToGotoLoc, ToLabelLoc);
-}
+ExpectedStmt ASTNodeImporter::VisitGotoStmt(GotoStmt *S) {
+  auto Imp = importSeq(S->getLabel(), S->getGotoLoc(), S->getLabelLoc());
+  if (!Imp)
+    return Imp.takeError();
 
-Stmt *ASTNodeImporter::VisitIndirectGotoStmt(IndirectGotoStmt *S) {
-  SourceLocation ToGotoLoc = Importer.Import(S->getGotoLoc());
-  SourceLocation ToStarLoc = Importer.Import(S->getStarLoc());
-  Expr *ToTarget = Importer.Import(S->getTarget());
-  if (!ToTarget && S->getTarget())
-    return nullptr;
-  return new (Importer.getToContext()) IndirectGotoStmt(ToGotoLoc, ToStarLoc,
-                                                        ToTarget);
+  LabelDecl *ToLabel;
+  SourceLocation ToGotoLoc, ToLabelLoc;
+  std::tie(ToLabel, ToGotoLoc, ToLabelLoc) = *Imp;
+
+  return new (Importer.getToContext()) GotoStmt(
+      ToLabel, ToGotoLoc, ToLabelLoc);
 }
 
-Stmt *ASTNodeImporter::VisitContinueStmt(ContinueStmt *S) {
-  SourceLocation ToContinueLoc = Importer.Import(S->getContinueLoc());
-  return new (Importer.getToContext()) ContinueStmt(ToContinueLoc);
+ExpectedStmt ASTNodeImporter::VisitIndirectGotoStmt(IndirectGotoStmt *S) {
+  auto Imp = importSeq(S->getGotoLoc(), S->getStarLoc(), S->getTarget());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToGotoLoc, ToStarLoc;
+  Expr *ToTarget;
+  std::tie(ToGotoLoc, ToStarLoc, ToTarget) = *Imp;
+
+  return new (Importer.getToContext()) IndirectGotoStmt(
+      ToGotoLoc, ToStarLoc, ToTarget);
 }
 
-Stmt *ASTNodeImporter::VisitBreakStmt(BreakStmt *S) {
-  SourceLocation ToBreakLoc = Importer.Import(S->getBreakLoc());
-  return new (Importer.getToContext()) BreakStmt(ToBreakLoc);
+ExpectedStmt ASTNodeImporter::VisitContinueStmt(ContinueStmt *S) {
+  ExpectedSLoc ToContinueLocOrErr = import(S->getContinueLoc());
+  if (!ToContinueLocOrErr)
+    return ToContinueLocOrErr.takeError();
+  return new (Importer.getToContext()) ContinueStmt(*ToContinueLocOrErr);
 }
 
-Stmt *ASTNodeImporter::VisitReturnStmt(ReturnStmt *S) {
-  SourceLocation ToRetLoc = Importer.Import(S->getReturnLoc());
-  Expr *ToRetExpr = Importer.Import(S->getRetValue());
-  if (!ToRetExpr && S->getRetValue())
-    return nullptr;
-  auto *NRVOCandidate = const_cast<VarDecl *>(S->getNRVOCandidate());
-  auto *ToNRVOCandidate = cast_or_null<VarDecl>(Importer.Import(NRVOCandidate));
-  if (!ToNRVOCandidate && NRVOCandidate)
-    return nullptr;
-  return new (Importer.getToContext()) ReturnStmt(ToRetLoc, ToRetExpr,
-                                                  ToNRVOCandidate);
+ExpectedStmt ASTNodeImporter::VisitBreakStmt(BreakStmt *S) {
+  auto ToBreakLocOrErr = import(S->getBreakLoc());
+  if (!ToBreakLocOrErr)
+    return ToBreakLocOrErr.takeError();
+  return new (Importer.getToContext()) BreakStmt(*ToBreakLocOrErr);
 }
 
-Stmt *ASTNodeImporter::VisitCXXCatchStmt(CXXCatchStmt *S) {
-  SourceLocation ToCatchLoc = Importer.Import(S->getCatchLoc());
-  VarDecl *ToExceptionDecl = nullptr;
-  if (VarDecl *FromExceptionDecl = S->getExceptionDecl()) {
-    ToExceptionDecl =
-      dyn_cast_or_null<VarDecl>(Importer.Import(FromExceptionDecl));
-    if (!ToExceptionDecl)
-      return nullptr;
-  }
-  Stmt *ToHandlerBlock = Importer.Import(S->getHandlerBlock());
-  if (!ToHandlerBlock && S->getHandlerBlock())
-    return nullptr;
-  return new (Importer.getToContext()) CXXCatchStmt(ToCatchLoc,
-                                                    ToExceptionDecl,
-                                                    ToHandlerBlock);
+ExpectedStmt ASTNodeImporter::VisitReturnStmt(ReturnStmt *S) {
+  auto Imp = importSeq(
+      S->getReturnLoc(), S->getRetValue(), S->getNRVOCandidate());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToReturnLoc;
+  Expr *ToRetValue;
+  const VarDecl *ToNRVOCandidate;
+  std::tie(ToReturnLoc, ToRetValue, ToNRVOCandidate) = *Imp;
+
+  return new (Importer.getToContext()) ReturnStmt(
+      ToReturnLoc, ToRetValue, ToNRVOCandidate);
+
+ExpectedStmt ASTNodeImporter::VisitCXXCatchStmt(CXXCatchStmt *S) {
+  auto Imp = importSeq(
+      S->getCatchLoc(), S->getExceptionDecl(), S->getHandlerBlock());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToCatchLoc;
+  VarDecl *ToExceptionDecl;
+  Stmt *ToHandlerBlock;
+  std::tie(ToCatchLoc, ToExceptionDecl, ToHandlerBlock) = *Imp;
+
+  return new (Importer.getToContext()) CXXCatchStmt (
+      ToCatchLoc, ToExceptionDecl, ToHandlerBlock);
 }
 
-Stmt *ASTNodeImporter::VisitCXXTryStmt(CXXTryStmt *S) {
-  SourceLocation ToTryLoc = Importer.Import(S->getTryLoc());
-  Stmt *ToTryBlock = Importer.Import(S->getTryBlock());
-  if (!ToTryBlock && S->getTryBlock())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXTryStmt(CXXTryStmt *S) {
+  ExpectedSLoc ToTryLocOrErr = import(S->getTryLoc());
+  if (!ToTryLocOrErr)
+    return ToTryLocOrErr.takeError();
+
+  ExpectedStmt ToTryBlockOrErr = import(S->getTryBlock());
+  if (!ToTryBlockOrErr)
+    return ToTryBlockOrErr.takeError();
+
   SmallVector<Stmt *, 1> ToHandlers(S->getNumHandlers());
   for (unsigned HI = 0, HE = S->getNumHandlers(); HI != HE; ++HI) {
     CXXCatchStmt *FromHandler = S->getHandler(HI);
-    if (Stmt *ToHandler = Importer.Import(FromHandler))
-      ToHandlers[HI] = ToHandler;
+    if (auto ToHandlerOrErr = import(FromHandler))
+      ToHandlers[HI] = *ToHandlerOrErr;
     else
-      return nullptr;
+      return ToHandlerOrErr.takeError();
   }
-  return CXXTryStmt::Create(Importer.getToContext(), ToTryLoc, ToTryBlock,
-                            ToHandlers);
+
+  return CXXTryStmt::Create(
+      Importer.getToContext(), *ToTryLocOrErr,*ToTryBlockOrErr, ToHandlers);
 }
 
-Stmt *ASTNodeImporter::VisitCXXForRangeStmt(CXXForRangeStmt *S) {
-  auto *ToInit = dyn_cast_or_null<Stmt>(Importer.Import(S->getInit()));
-  if (!ToInit && S->getInit())
-    return nullptr;
-  auto *ToRange =
-    dyn_cast_or_null<DeclStmt>(Importer.Import(S->getRangeStmt()));
-  if (!ToRange && S->getRangeStmt())
-    return nullptr;
-  auto *ToBegin =
-    dyn_cast_or_null<DeclStmt>(Importer.Import(S->getBeginStmt()));
-  if (!ToBegin && S->getBeginStmt())
-    return nullptr;
-  auto *ToEnd =
-    dyn_cast_or_null<DeclStmt>(Importer.Import(S->getEndStmt()));
-  if (!ToEnd && S->getEndStmt())
-    return nullptr;
-  Expr *ToCond = Importer.Import(S->getCond());
-  if (!ToCond && S->getCond())
-    return nullptr;
-  Expr *ToInc = Importer.Import(S->getInc());
-  if (!ToInc && S->getInc())
-    return nullptr;
-  auto *ToLoopVar =
-    dyn_cast_or_null<DeclStmt>(Importer.Import(S->getLoopVarStmt()));
-  if (!ToLoopVar && S->getLoopVarStmt())
-    return nullptr;
-  Stmt *ToBody = Importer.Import(S->getBody());
-  if (!ToBody && S->getBody())
-    return nullptr;
-  SourceLocation ToForLoc = Importer.Import(S->getForLoc());
-  SourceLocation ToCoawaitLoc = Importer.Import(S->getCoawaitLoc());
-  SourceLocation ToColonLoc = Importer.Import(S->getColonLoc());
-  SourceLocation ToRParenLoc = Importer.Import(S->getRParenLoc());
-  return new (Importer.getToContext())
-      CXXForRangeStmt(ToInit, ToRange, ToBegin, ToEnd, ToCond, ToInc, ToLoopVar,
-                      ToBody, ToForLoc, ToCoawaitLoc, ToColonLoc, ToRParenLoc);
+ExpectedStmt ASTNodeImporter::VisitCXXForRangeStmt(CXXForRangeStmt *S) {
+  auto Imp1 = importSeq(
+      S->getInit(), S->getRangeStmt(), S->getBeginStmt(), S->getEndStmt(),
+      S->getCond(), S->getInc(), S->getLoopVarStmt(), S->getBody());
+  if (!Imp1)
+    return Imp1.takeError();
+  auto Imp2 = importSeq(
+      S->getForLoc(), S->getCoawaitLoc(), S->getColonLoc(), S->getRParenLoc());
+  if (!Imp2)
+    return Imp2.takeError();
+
+  DeclStmt *ToRangeStmt, *ToBeginStmt, *ToEndStmt, *ToLoopVarStmt;
+  Expr *ToCond, *ToInc;
+  Stmt *ToInit, *ToBody;
+  std::tie(
+      ToInit, ToRangeStmt, ToBeginStmt, ToEndStmt, ToCond, ToInc, ToLoopVarStmt,
+      ToBody) = *Imp1;
+  SourceLocation ToForLoc, ToCoawaitLoc, ToColonLoc, ToRParenLoc;
+  std::tie(ToForLoc, ToCoawaitLoc, ToColonLoc, ToRParenLoc) = *Imp2;
+
+  return new (Importer.getToContext()) CXXForRangeStmt(
+      ToInit, ToRangeStmt, ToBeginStmt, ToEndStmt, ToCond, ToInc, ToLoopVarStmt,
 }
 
-Stmt *ASTNodeImporter::VisitObjCForCollectionStmt(ObjCForCollectionStmt *S) {
-  Stmt *ToElem = Importer.Import(S->getElement());
-  if (!ToElem && S->getElement())
-    return nullptr;
-  Expr *ToCollect = Importer.Import(S->getCollection());
-  if (!ToCollect && S->getCollection())
-    return nullptr;
-  Stmt *ToBody = Importer.Import(S->getBody());
-  if (!ToBody && S->getBody())
-    return nullptr;
-  SourceLocation ToForLoc = Importer.Import(S->getForLoc());
-  SourceLocation ToRParenLoc = Importer.Import(S->getRParenLoc());
-  return new (Importer.getToContext()) ObjCForCollectionStmt(ToElem,
-                                                             ToCollect,
-                                                             ToBody, ToForLoc,
+ExpectedStmt
+ASTNodeImporter::VisitObjCForCollectionStmt(ObjCForCollectionStmt *S) {
+  auto Imp = importSeq(
+      S->getElement(), S->getCollection(), S->getBody(),
+      S->getForLoc(), S->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Stmt *ToElement, *ToBody;
+  Expr *ToCollection;
+  SourceLocation ToForLoc, ToRParenLoc;
+  std::tie(ToElement, ToCollection, ToBody, ToForLoc, ToRParenLoc) = *Imp;
+
+  return new (Importer.getToContext()) ObjCForCollectionStmt(ToElement,
+                                                             ToCollection,
+                                                             ToBody,
+                                                             ToForLoc,
                                                              ToRParenLoc);
 }
 
-Stmt *ASTNodeImporter::VisitObjCAtCatchStmt(ObjCAtCatchStmt *S) {
-  SourceLocation ToAtCatchLoc = Importer.Import(S->getAtCatchLoc());
-  SourceLocation ToRParenLoc = Importer.Import(S->getRParenLoc());
-  VarDecl *ToExceptionDecl = nullptr;
-  if (VarDecl *FromExceptionDecl = S->getCatchParamDecl()) {
-    ToExceptionDecl =
-      dyn_cast_or_null<VarDecl>(Importer.Import(FromExceptionDecl));
-    if (!ToExceptionDecl)
-      return nullptr;
-  }
-  Stmt *ToBody = Importer.Import(S->getCatchBody());
-  if (!ToBody && S->getCatchBody())
-    return nullptr;
-  return new (Importer.getToContext()) ObjCAtCatchStmt(ToAtCatchLoc,
-                                                       ToRParenLoc,
-                                                       ToExceptionDecl,
-                                                       ToBody);
-}
+ExpectedStmt ASTNodeImporter::VisitObjCAtCatchStmt(ObjCAtCatchStmt *S) {
+  auto Imp = importSeq(
+      S->getAtCatchLoc(), S->getRParenLoc(), S->getCatchParamDecl(),
+      S->getCatchBody());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToAtCatchLoc, ToRParenLoc;
+  VarDecl *ToCatchParamDecl;
+  Stmt *ToCatchBody;
+  std::tie(ToAtCatchLoc, ToRParenLoc, ToCatchParamDecl, ToCatchBody) = *Imp;
+
+  return new (Importer.getToContext()) ObjCAtCatchStmt (
+      ToAtCatchLoc, ToRParenLoc, ToCatchParamDecl, ToCatchBody);
+}
+
+ExpectedStmt ASTNodeImporter::VisitObjCAtFinallyStmt(ObjCAtFinallyStmt *S) {
+  ExpectedSLoc ToAtFinallyLocOrErr = import(S->getAtFinallyLoc());
+  if (!ToAtFinallyLocOrErr)
+    return ToAtFinallyLocOrErr.takeError();
+  ExpectedStmt ToAtFinallyStmtOrErr = import(S->getFinallyBody());
+  if (!ToAtFinallyStmtOrErr)
+    return ToAtFinallyStmtOrErr.takeError();
+  return new (Importer.getToContext()) ObjCAtFinallyStmt(*ToAtFinallyLocOrErr,
+                                                         *ToAtFinallyStmtOrErr);
+}
+
+ExpectedStmt ASTNodeImporter::VisitObjCAtTryStmt(ObjCAtTryStmt *S) {
+  auto Imp = importSeq(
+      S->getAtTryLoc(), S->getTryBody(), S->getFinallyStmt());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToAtTryLoc;
+  Stmt *ToTryBody, *ToFinallyStmt;
+  std::tie(ToAtTryLoc, ToTryBody, ToFinallyStmt) = *Imp;
 
-Stmt *ASTNodeImporter::VisitObjCAtFinallyStmt(ObjCAtFinallyStmt *S) {
-  SourceLocation ToAtFinallyLoc = Importer.Import(S->getAtFinallyLoc());
-  Stmt *ToAtFinallyStmt = Importer.Import(S->getFinallyBody());
-  if (!ToAtFinallyStmt && S->getFinallyBody())
-    return nullptr;
-  return new (Importer.getToContext()) ObjCAtFinallyStmt(ToAtFinallyLoc,
-                                                         ToAtFinallyStmt);
-}
-
-Stmt *ASTNodeImporter::VisitObjCAtTryStmt(ObjCAtTryStmt *S) {
-  SourceLocation ToAtTryLoc = Importer.Import(S->getAtTryLoc());
-  Stmt *ToAtTryStmt = Importer.Import(S->getTryBody());
-  if (!ToAtTryStmt && S->getTryBody())
-    return nullptr;
   SmallVector<Stmt *, 1> ToCatchStmts(S->getNumCatchStmts());
   for (unsigned CI = 0, CE = S->getNumCatchStmts(); CI != CE; ++CI) {
     ObjCAtCatchStmt *FromCatchStmt = S->getCatchStmt(CI);
-    if (Stmt *ToCatchStmt = Importer.Import(FromCatchStmt))
-      ToCatchStmts[CI] = ToCatchStmt;
+    if (ExpectedStmt ToCatchStmtOrErr = import(FromCatchStmt))
+      ToCatchStmts[CI] = *ToCatchStmtOrErr;
     else
-      return nullptr;
+      return ToCatchStmtOrErr.takeError();
   }
-  Stmt *ToAtFinallyStmt = Importer.Import(S->getFinallyStmt());
-  if (!ToAtFinallyStmt && S->getFinallyStmt())
-    return nullptr;
+
   return ObjCAtTryStmt::Create(Importer.getToContext(),
-                               ToAtTryLoc, ToAtTryStmt,
+                               ToAtTryLoc, ToTryBody,
                                ToCatchStmts.begin(), ToCatchStmts.size(),
-                               ToAtFinallyStmt);
+                               ToFinallyStmt);
 }
 
-Stmt *ASTNodeImporter::VisitObjCAtSynchronizedStmt
+ExpectedStmt ASTNodeImporter::VisitObjCAtSynchronizedStmt
   (ObjCAtSynchronizedStmt *S) {
-  SourceLocation ToAtSynchronizedLoc =
-    Importer.Import(S->getAtSynchronizedLoc());
-  Expr *ToSynchExpr = Importer.Import(S->getSynchExpr());
-  if (!ToSynchExpr && S->getSynchExpr())
-    return nullptr;
-  Stmt *ToSynchBody = Importer.Import(S->getSynchBody());
-  if (!ToSynchBody && S->getSynchBody())
-    return nullptr;
+  auto Imp = importSeq(
+      S->getAtSynchronizedLoc(), S->getSynchExpr(), S->getSynchBody());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToAtSynchronizedLoc;
+  Expr *ToSynchExpr;
+  Stmt *ToSynchBody;
+  std::tie(ToAtSynchronizedLoc, ToSynchExpr, ToSynchBody) = *Imp;
+
   return new (Importer.getToContext()) ObjCAtSynchronizedStmt(
     ToAtSynchronizedLoc, ToSynchExpr, ToSynchBody);
 }
 
-Stmt *ASTNodeImporter::VisitObjCAtThrowStmt(ObjCAtThrowStmt *S) {
-  SourceLocation ToAtThrowLoc = Importer.Import(S->getThrowLoc());
-  Expr *ToThrow = Importer.Import(S->getThrowExpr());
-  if (!ToThrow && S->getThrowExpr())
-    return nullptr;
-  return new (Importer.getToContext()) ObjCAtThrowStmt(ToAtThrowLoc, ToThrow);
-}
-
-Stmt *ASTNodeImporter::VisitObjCAutoreleasePoolStmt
-  (ObjCAutoreleasePoolStmt *S) {
-  SourceLocation ToAtLoc = Importer.Import(S->getAtLoc());
-  Stmt *ToSubStmt = Importer.Import(S->getSubStmt());
-  if (!ToSubStmt && S->getSubStmt())
-    return nullptr;
-  return new (Importer.getToContext()) ObjCAutoreleasePoolStmt(ToAtLoc,
-                                                               ToSubStmt);
+ExpectedStmt ASTNodeImporter::VisitObjCAtThrowStmt(ObjCAtThrowStmt *S) {
+  ExpectedSLoc ToThrowLocOrErr = import(S->getThrowLoc());
+  if (!ToThrowLocOrErr)
+    return ToThrowLocOrErr.takeError();
+  ExpectedExpr ToThrowExprOrErr = import(S->getThrowExpr());
+  if (!ToThrowExprOrErr)
+    return ToThrowExprOrErr.takeError();
+  return new (Importer.getToContext()) ObjCAtThrowStmt(
+      *ToThrowLocOrErr, *ToThrowExprOrErr);
+}
+
+ExpectedStmt ASTNodeImporter::VisitObjCAutoreleasePoolStmt(
+    ObjCAutoreleasePoolStmt *S) {
+  ExpectedSLoc ToAtLocOrErr = import(S->getAtLoc());
+  if (!ToAtLocOrErr)
+    return ToAtLocOrErr.takeError();
+  ExpectedStmt ToSubStmtOrErr = import(S->getSubStmt());
+  if (!ToSubStmtOrErr)
+    return ToSubStmtOrErr.takeError();
+  return new (Importer.getToContext()) ObjCAutoreleasePoolStmt(*ToAtLocOrErr,
+                                                               *ToSubStmtOrErr);
 }
 
 //----------------------------------------------------------------------------
 // Import Expressions
 //----------------------------------------------------------------------------
-Expr *ASTNodeImporter::VisitExpr(Expr *E) {
+ExpectedStmt ASTNodeImporter::VisitExpr(Expr *E) {
   Importer.FromDiag(E->getBeginLoc(), diag::err_unsupported_ast_node)
       << E->getStmtClassName();
-  return nullptr;
+  return make_error<ImportError>(ImportError::UnsupportedConstruct);
 }
 
-Expr *ASTNodeImporter::VisitVAArgExpr(VAArgExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr && E->getSubExpr())
-    return nullptr;
-
-  TypeSourceInfo *TInfo = Importer.Import(E->getWrittenTypeInfo());
-  if (!TInfo)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitVAArgExpr(VAArgExpr *E) {
+  auto Imp = importSeq(
+      E->getBuiltinLoc(), E->getSubExpr(), E->getWrittenTypeInfo(),
+      E->getRParenLoc(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToBuiltinLoc, ToRParenLoc;
+  Expr *ToSubExpr;
+  TypeSourceInfo *ToWrittenTypeInfo;
+  QualType ToType;
+  std::tie(ToBuiltinLoc, ToSubExpr, ToWrittenTypeInfo, ToRParenLoc, ToType) =
+      *Imp;
 
   return new (Importer.getToContext()) VAArgExpr(
-        Importer.Import(E->getBuiltinLoc()), SubExpr, TInfo,
-        Importer.Import(E->getRParenLoc()), T, E->isMicrosoftABI());
+      ToBuiltinLoc, ToSubExpr, ToWrittenTypeInfo, ToRParenLoc, ToType,
+      E->isMicrosoftABI());
 }
 
-Expr *ASTNodeImporter::VisitGNUNullExpr(GNUNullExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
 
-  return new (Importer.getToContext())
-      GNUNullExpr(T, Importer.Import(E->getBeginLoc()));
+ExpectedStmt ASTNodeImporter::VisitGNUNullExpr(GNUNullExpr *E) {
+  ExpectedType TypeOrErr = import(E->getType());
+  if (!TypeOrErr)
+    return TypeOrErr.takeError();
+
+  ExpectedSLoc BeginLocOrErr = import(E->getBeginLoc());
+  if (!BeginLocOrErr)
+    return BeginLocOrErr.takeError();
+
+  return new (Importer.getToContext()) GNUNullExpr(*TypeOrErr, *BeginLocOrErr);
 }
 
-Expr *ASTNodeImporter::VisitPredefinedExpr(PredefinedExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitPredefinedExpr(PredefinedExpr *E) {
+  auto Imp = importSeq(
+      E->getBeginLoc(), E->getType(), E->getFunctionName());
+  if (!Imp)
+    return Imp.takeError();
 
-  auto *SL = cast_or_null<StringLiteral>(Importer.Import(E->getFunctionName()));
-  if (!SL && E->getFunctionName())
-    return nullptr;
+  SourceLocation ToBeginLoc;
+  QualType ToType;
+  StringLiteral *ToFunctionName;
+  std::tie(ToBeginLoc, ToType, ToFunctionName) = *Imp;
 
   return new (Importer.getToContext()) PredefinedExpr(
-      Importer.Import(E->getBeginLoc()), T, E->getIdentType(), SL);
+      ToBeginLoc, ToType, E->getIdentType(), ToFunctionName);
 }
 
-Expr *ASTNodeImporter::VisitDeclRefExpr(DeclRefExpr *E) {
-  auto *ToD = cast_or_null<ValueDecl>(Importer.Import(E->getDecl()));
-  if (!ToD)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitDeclRefExpr(DeclRefExpr *E) {
+  auto Imp = importSeq(
+      E->getQualifierLoc(), E->getTemplateKeywordLoc(), E->getDecl(),
+      E->getLocation(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  NestedNameSpecifierLoc ToQualifierLoc;
+  SourceLocation ToTemplateKeywordLoc, ToLocation;
+  ValueDecl *ToDecl;
+  QualType ToType;
+  std::tie(ToQualifierLoc, ToTemplateKeywordLoc, ToDecl, ToLocation, ToType) =
+      *Imp;
 
-  NamedDecl *FoundD = nullptr;
+  NamedDecl *ToFoundD = nullptr;
   if (E->getDecl() != E->getFoundDecl()) {
-    FoundD = cast_or_null<NamedDecl>(Importer.Import(E->getFoundDecl()));
-    if (!FoundD)
-      return nullptr;
+    auto FoundDOrErr = import(E->getFoundDecl());
+    if (!FoundDOrErr)
+      return FoundDOrErr.takeError();
+    ToFoundD = *FoundDOrErr;
   }
 
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
   TemplateArgumentListInfo ToTAInfo;
-  TemplateArgumentListInfo *ResInfo = nullptr;
+  TemplateArgumentListInfo *ToResInfo = nullptr;
   if (E->hasExplicitTemplateArgs()) {
-    if (ImportTemplateArgumentListInfo(E->template_arguments(), ToTAInfo))
-      return nullptr;
-    ResInfo = &ToTAInfo;
+    if (Error Err =
+        ImportTemplateArgumentListInfo(E->template_arguments(), ToTAInfo))
+      return std::move(Err);
+    ToResInfo = &ToTAInfo;
   }
 
-  DeclRefExpr *DRE = DeclRefExpr::Create(Importer.getToContext(),
-                                         Importer.Import(E->getQualifierLoc()),
-                                   Importer.Import(E->getTemplateKeywordLoc()),
-                                         ToD,
-                                        E->refersToEnclosingVariableOrCapture(),
-                                         Importer.Import(E->getLocation()),
-                                         T, E->getValueKind(),
-                                         FoundD, ResInfo);
+  auto *ToE = DeclRefExpr::Create(
+      Importer.getToContext(), ToQualifierLoc, ToTemplateKeywordLoc, ToDecl,
+      E->refersToEnclosingVariableOrCapture(), ToLocation, ToType,
+      E->getValueKind(), ToFoundD, ToResInfo);
   if (E->hadMultipleCandidates())
-    DRE->setHadMultipleCandidates(true);
-  return DRE;
+    ToE->setHadMultipleCandidates(true);
+  return ToE;
 }
 
-Expr *ASTNodeImporter::VisitImplicitValueInitExpr(ImplicitValueInitExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitImplicitValueInitExpr(ImplicitValueInitExpr *E) {
+  ExpectedType TypeOrErr = import(E->getType());
+  if (!TypeOrErr)
+    return TypeOrErr.takeError();
 
-  return new (Importer.getToContext()) ImplicitValueInitExpr(T);
-}
-
-ASTNodeImporter::Designator
-ASTNodeImporter::ImportDesignator(const Designator &D) {
-  if (D.isFieldDesignator()) {
-    IdentifierInfo *ToFieldName = Importer.Import(D.getFieldName());
-    // Caller checks for import error
-    return Designator(ToFieldName, Importer.Import(D.getDotLoc()),
-                      Importer.Import(D.getFieldLoc()));
-  }
-  if (D.isArrayDesignator())
-    return Designator(D.getFirstExprIndex(),
-                      Importer.Import(D.getLBracketLoc()),
-                      Importer.Import(D.getRBracketLoc()));
-
-  assert(D.isArrayRangeDesignator());
-  return Designator(D.getFirstExprIndex(),
-                    Importer.Import(D.getLBracketLoc()),
-                    Importer.Import(D.getEllipsisLoc()),
-                    Importer.Import(D.getRBracketLoc()));
+  return new (Importer.getToContext()) ImplicitValueInitExpr(*TypeOrErr);
 }
 
+ExpectedStmt ASTNodeImporter::VisitDesignatedInitExpr(DesignatedInitExpr *E) {
+  ExpectedExpr ToInitOrErr = import(E->getInit());
+  if (!ToInitOrErr)
+    return ToInitOrErr.takeError();
 
-Expr *ASTNodeImporter::VisitDesignatedInitExpr(DesignatedInitExpr *DIE) {
-  auto *Init = cast_or_null<Expr>(Importer.Import(DIE->getInit()));
-  if (!Init)
-    return nullptr;
+  ExpectedSLoc ToEqualOrColonLocOrErr = import(E->getEqualOrColonLoc());
+  if (!ToEqualOrColonLocOrErr)
+    return ToEqualOrColonLocOrErr.takeError();
 
-  SmallVector<Expr *, 4> IndexExprs(DIE->getNumSubExprs() - 1);
+  SmallVector<Expr *, 4> ToIndexExprs(E->getNumSubExprs() - 1);
   // List elements from the second, the first is Init itself
-  for (unsigned I = 1, E = DIE->getNumSubExprs(); I < E; I++) {
-    if (auto *Arg = cast_or_null<Expr>(Importer.Import(DIE->getSubExpr(I))))
-      IndexExprs[I - 1] = Arg;
+  for (unsigned I = 1, N = E->getNumSubExprs(); I < N; I++) {
+    if (ExpectedExpr ToArgOrErr = import(E->getSubExpr(I)))
+      ToIndexExprs[I - 1] = *ToArgOrErr;
     else
-      return nullptr;
+      return ToArgOrErr.takeError();
   }
 
-  SmallVector<Designator, 4> Designators(DIE->size());
-  llvm::transform(DIE->designators(), Designators.begin(),
-                  [this](const Designator &D) -> Designator {
-                    return ImportDesignator(D);
-                  });
-
-  for (const auto &D : DIE->designators())
-    if (D.isFieldDesignator() && !D.getFieldName())
-      return nullptr;
+  SmallVector<Designator, 4> ToDesignators(E->size());
+  if (Error Err = ImportContainerChecked(E->designators(), ToDesignators))
+    return std::move(Err);
 
   return DesignatedInitExpr::Create(
-        Importer.getToContext(), Designators,
-        IndexExprs, Importer.Import(DIE->getEqualOrColonLoc()),
-        DIE->usesGNUSyntax(), Init);
+        Importer.getToContext(), ToDesignators,
+        ToIndexExprs, *ToEqualOrColonLocOrErr,
+        E->usesGNUSyntax(), *ToInitOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCXXNullPtrLiteralExpr(CXXNullPtrLiteralExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt
+ASTNodeImporter::VisitCXXNullPtrLiteralExpr(CXXNullPtrLiteralExpr *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
 
-  return new (Importer.getToContext())
-      CXXNullPtrLiteralExpr(T, Importer.Import(E->getLocation()));
+  ExpectedSLoc ToLocationOrErr = import(E->getLocation());
+  if (!ToLocationOrErr)
+    return ToLocationOrErr.takeError();
+
+  return new (Importer.getToContext()) CXXNullPtrLiteralExpr(
+      *ToTypeOrErr, *ToLocationOrErr);
 }
 
-Expr *ASTNodeImporter::VisitIntegerLiteral(IntegerLiteral *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitIntegerLiteral(IntegerLiteral *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+
+  ExpectedSLoc ToLocationOrErr = import(E->getLocation());
+  if (!ToLocationOrErr)
+    return ToLocationOrErr.takeError();
 
-  return IntegerLiteral::Create(Importer.getToContext(),
-                                E->getValue(), T,
-                                Importer.Import(E->getLocation()));
+  return IntegerLiteral::Create(
+      Importer.getToContext(), E->getValue(), *ToTypeOrErr, *ToLocationOrErr);
 }
 
-Expr *ASTNodeImporter::VisitFloatingLiteral(FloatingLiteral *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
 
-  return FloatingLiteral::Create(Importer.getToContext(),
-                                E->getValue(), E->isExact(), T,
-                                Importer.Import(E->getLocation()));
+ExpectedStmt ASTNodeImporter::VisitFloatingLiteral(FloatingLiteral *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+
+  ExpectedSLoc ToLocationOrErr = import(E->getLocation());
+  if (!ToLocationOrErr)
+    return ToLocationOrErr.takeError();
+
+  return FloatingLiteral::Create(
+      Importer.getToContext(), E->getValue(), E->isExact(),
+      *ToTypeOrErr, *ToLocationOrErr);
 }
 
-Expr *ASTNodeImporter::VisitImaginaryLiteral(ImaginaryLiteral *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitImaginaryLiteral(ImaginaryLiteral *E) {
+  auto ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
 
-  Expr *SubE = Importer.Import(E->getSubExpr());
-  if (!SubE)
-    return nullptr;
+  ExpectedExpr ToSubExprOrErr = import(E->getSubExpr());
+  if (!ToSubExprOrErr)
+    return ToSubExprOrErr.takeError();
 
-  return new (Importer.getToContext()) ImaginaryLiteral(SubE, T);
+  return new (Importer.getToContext()) ImaginaryLiteral(
+      *ToSubExprOrErr, *ToTypeOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCharacterLiteral(CharacterLiteral *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCharacterLiteral(CharacterLiteral *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
 
-  return new (Importer.getToContext()) CharacterLiteral(E->getValue(),
-                                                        E->getKind(), T,
-                                          Importer.Import(E->getLocation()));
+  ExpectedSLoc ToLocationOrErr = import(E->getLocation());
+  if (!ToLocationOrErr)
+    return ToLocationOrErr.takeError();
+
+  return new (Importer.getToContext()) CharacterLiteral(
+      E->getValue(), E->getKind(), *ToTypeOrErr, *ToLocationOrErr);
 }
 
-Expr *ASTNodeImporter::VisitStringLiteral(StringLiteral *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitStringLiteral(StringLiteral *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
 
-  SmallVector<SourceLocation, 4> Locations(E->getNumConcatenated());
-  ImportArray(E->tokloc_begin(), E->tokloc_end(), Locations.begin());
+  SmallVector<SourceLocation, 4> ToLocations(E->getNumConcatenated());
+  if (Error Err = ImportArrayChecked(
+      E->tokloc_begin(), E->tokloc_end(), ToLocations.begin()))
+    return std::move(Err);
 
-  return StringLiteral::Create(Importer.getToContext(), E->getBytes(),
-                               E->getKind(), E->isPascal(), T,
-                               Locations.data(), Locations.size());
+  return StringLiteral::Create(
+      Importer.getToContext(), E->getBytes(), E->getKind(), E->isPascal(),
+      *ToTypeOrErr, ToLocations.data(), ToLocations.size());
 }
 
-Expr *ASTNodeImporter::VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
+  auto Imp = importSeq(
+      E->getLParenLoc(), E->getTypeSourceInfo(), E->getType(),
+      E->getInitializer());
+  if (!Imp)
+    return Imp.takeError();
 
-  TypeSourceInfo *TInfo = Importer.Import(E->getTypeSourceInfo());
-  if (!TInfo)
-    return nullptr;
-
-  Expr *Init = Importer.Import(E->getInitializer());
-  if (!Init)
-    return nullptr;
+  SourceLocation ToLParenLoc;
+  TypeSourceInfo *ToTypeSourceInfo;
+  QualType ToType;
+  Expr *ToInitializer;
+  std::tie(ToLParenLoc, ToTypeSourceInfo, ToType, ToInitializer) = *Imp;
 
   return new (Importer.getToContext()) CompoundLiteralExpr(
-        Importer.Import(E->getLParenLoc()), TInfo, T, E->getValueKind(),
-        Init, E->isFileScope());
+        ToLParenLoc, ToTypeSourceInfo, ToType, E->getValueKind(),
+        ToInitializer, E->isFileScope());
 }
 
-Expr *ASTNodeImporter::VisitAtomicExpr(AtomicExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitAtomicExpr(AtomicExpr *E) {
+  auto Imp = importSeq(
+      E->getBuiltinLoc(), E->getType(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToBuiltinLoc, ToRParenLoc;
+  QualType ToType;
+  std::tie(ToBuiltinLoc, ToType, ToRParenLoc) = *Imp;
 
-  SmallVector<Expr *, 6> Exprs(E->getNumSubExprs());
-  if (ImportArrayChecked(
-        E->getSubExprs(), E->getSubExprs() + E->getNumSubExprs(),
-        Exprs.begin()))
-    return nullptr;
+  SmallVector<Expr *, 6> ToExprs(E->getNumSubExprs());
+  if (Error Err = ImportArrayChecked(
+      ToExprs.begin()))
+    return std::move(Err);
 
   return new (Importer.getToContext()) AtomicExpr(
-        Importer.Import(E->getBuiltinLoc()), Exprs, T, E->getOp(),
-        Importer.Import(E->getRParenLoc()));
+      ToBuiltinLoc, ToExprs, ToType, E->getOp(), ToRParenLoc);
 }
 
-Expr *ASTNodeImporter::VisitAddrLabelExpr(AddrLabelExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitAddrLabelExpr(AddrLabelExpr *E) {
+  auto Imp = importSeq(
+      E->getAmpAmpLoc(), E->getLabelLoc(), E->getLabel(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
 
-  auto *ToLabel = cast_or_null<LabelDecl>(Importer.Import(E->getLabel()));
-  if (!ToLabel)
-    return nullptr;
+  SourceLocation ToAmpAmpLoc, ToLabelLoc;
+  LabelDecl *ToLabel;
+  QualType ToType;
+  std::tie(ToAmpAmpLoc, ToLabelLoc, ToLabel, ToType) = *Imp;
 
   return new (Importer.getToContext()) AddrLabelExpr(
-        Importer.Import(E->getAmpAmpLoc()), Importer.Import(E->getLabelLoc()),
-        ToLabel, T);
+      ToAmpAmpLoc, ToLabelLoc, ToLabel, ToType);
 }
 
-Expr *ASTNodeImporter::VisitParenExpr(ParenExpr *E) {
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitParenExpr(ParenExpr *E) {
+  auto Imp = importSeq(E->getLParen(), E->getRParen(), E->getSubExpr());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToLParen, ToRParen;
+  Expr *ToSubExpr;
+  std::tie(ToLParen, ToRParen, ToSubExpr) = *Imp;
 
   return new (Importer.getToContext())
-                                  ParenExpr(Importer.Import(E->getLParen()),
-                                            Importer.Import(E->getRParen()),
-                                            SubExpr);
+      ParenExpr(ToLParen, ToRParen, ToSubExpr);
 }
 
-Expr *ASTNodeImporter::VisitParenListExpr(ParenListExpr *E) {
-  SmallVector<Expr *, 4> Exprs(E->getNumExprs());
-  if (ImportContainerChecked(E->exprs(), Exprs))
-    return nullptr;
-
-  return new (Importer.getToContext()) ParenListExpr(
-        Importer.getToContext(), Importer.Import(E->getLParenLoc()),
-        Exprs, Importer.Import(E->getLParenLoc()));
-}
+ExpectedStmt ASTNodeImporter::VisitParenListExpr(ParenListExpr *E) {
+  SmallVector<Expr *, 4> ToExprs(E->getNumExprs());
+  if (Error Err = ImportContainerChecked(E->exprs(), ToExprs))
+    return std::move(Err);
 
-Expr *ASTNodeImporter::VisitStmtExpr(StmtExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+  ExpectedSLoc ToLParenLocOrErr = import(E->getLParenLoc());
+  if (!ToLParenLocOrErr)
+    return ToLParenLocOrErr.takeError();
 
-  auto *ToSubStmt = cast_or_null<CompoundStmt>(
-      Importer.Import(E->getSubStmt()));
-  if (!ToSubStmt && E->getSubStmt())
-    return nullptr;
+  ExpectedSLoc ToRParenLocOrErr = import(E->getRParenLoc());
+  if (!ToRParenLocOrErr)
+    return ToRParenLocOrErr.takeError();
 
-  return new (Importer.getToContext()) StmtExpr(ToSubStmt, T,
-        Importer.Import(E->getLParenLoc()), Importer.Import(E->getRParenLoc()));
+  return new (Importer.getToContext()) ParenListExpr(
+      Importer.getToContext(), *ToLParenLocOrErr, ToExprs, *ToRParenLocOrErr);
 }
 
-Expr *ASTNodeImporter::VisitUnaryOperator(UnaryOperator *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitStmtExpr(StmtExpr *E) {
+  auto Imp = importSeq(
+      E->getSubStmt(), E->getType(), E->getLParenLoc(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  CompoundStmt *ToSubStmt;
+  QualType ToType;
+  SourceLocation ToLParenLoc, ToRParenLoc;
+  std::tie(ToSubStmt, ToType, ToLParenLoc, ToRParenLoc) = *Imp;
+
+  return new (Importer.getToContext()) StmtExpr(
+      ToSubStmt, ToType, ToLParenLoc, ToRParenLoc);
+}
+
+ExpectedStmt ASTNodeImporter::VisitUnaryOperator(UnaryOperator *E) {
+  auto Imp = importSeq(
+      E->getSubExpr(), E->getType(), E->getOperatorLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToSubExpr;
+  QualType ToType;
+  SourceLocation ToOperatorLoc;
+  std::tie(ToSubExpr, ToType, ToOperatorLoc) = *Imp;
 
   return new (Importer.getToContext()) UnaryOperator(
-      SubExpr, E->getOpcode(), T, E->getValueKind(), E->getObjectKind(),
-      Importer.Import(E->getOperatorLoc()), E->canOverflow());
+      ToSubExpr, E->getOpcode(), ToType, E->getValueKind(), E->getObjectKind(),
+      ToOperatorLoc, E->canOverflow());
 }
 
-Expr *
+ExpectedStmt
 ASTNodeImporter::VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *E) {
-  QualType ResultType = Importer.Import(E->getType());
+  auto Imp = importSeq(E->getType(), E->getOperatorLoc(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  SourceLocation ToOperatorLoc, ToRParenLoc;
+  std::tie(ToType, ToOperatorLoc, ToRParenLoc) = *Imp;
 
   if (E->isArgumentType()) {
-    TypeSourceInfo *TInfo = Importer.Import(E->getArgumentTypeInfo());
-    if (!TInfo)
-      return nullptr;
-
-    return new (Importer.getToContext()) UnaryExprOrTypeTraitExpr(E->getKind(),
-                                           TInfo, ResultType,
-                                           Importer.Import(E->getOperatorLoc()),
-                                           Importer.Import(E->getRParenLoc()));
-  }
-
-  Expr *SubExpr = Importer.Import(E->getArgumentExpr());
-  if (!SubExpr)
-    return nullptr;
-
-  return new (Importer.getToContext()) UnaryExprOrTypeTraitExpr(E->getKind(),
-                                          SubExpr, ResultType,
-                                          Importer.Import(E->getOperatorLoc()),
-                                          Importer.Import(E->getRParenLoc()));
-}
-
-Expr *ASTNodeImporter::VisitBinaryOperator(BinaryOperator *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *LHS = Importer.Import(E->getLHS());
-  if (!LHS)
-    return nullptr;
-
-  Expr *RHS = Importer.Import(E->getRHS());
-  if (!RHS)
-    return nullptr;
+    Expected<TypeSourceInfo *> ToArgumentTypeInfoOrErr =
+        import(E->getArgumentTypeInfo());
+    if (!ToArgumentTypeInfoOrErr)
+      return ToArgumentTypeInfoOrErr.takeError();
 
-  return new (Importer.getToContext()) BinaryOperator(LHS, RHS, E->getOpcode(),
-                                                      T, E->getValueKind(),
-                                                      E->getObjectKind(),
-                                           Importer.Import(E->getOperatorLoc()),
-                                                      E->getFPFeatures());
+    return new (Importer.getToContext()) UnaryExprOrTypeTraitExpr(
+        E->getKind(), *ToArgumentTypeInfoOrErr, ToType, ToOperatorLoc,
+        ToRParenLoc);
   }
 
-Expr *ASTNodeImporter::VisitConditionalOperator(ConditionalOperator *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *ToLHS = Importer.Import(E->getLHS());
-  if (!ToLHS)
-    return nullptr;
-
-  Expr *ToRHS = Importer.Import(E->getRHS());
-  if (!ToRHS)
-    return nullptr;
-
-  Expr *ToCond = Importer.Import(E->getCond());
-  if (!ToCond)
-    return nullptr;
+  ExpectedExpr ToArgumentExprOrErr = import(E->getArgumentExpr());
+  if (!ToArgumentExprOrErr)
+    return ToArgumentExprOrErr.takeError();
+
+  return new (Importer.getToContext()) UnaryExprOrTypeTraitExpr(
+      E->getKind(), *ToArgumentExprOrErr, ToType, ToOperatorLoc, ToRParenLoc);
+}
+
+ExpectedStmt ASTNodeImporter::VisitBinaryOperator(BinaryOperator *E) {
+  auto Imp = importSeq(
+      E->getLHS(), E->getRHS(), E->getType(), E->getOperatorLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToLHS, *ToRHS;
+  QualType ToType;
+  SourceLocation  ToOperatorLoc;
+  std::tie(ToLHS, ToRHS, ToType, ToOperatorLoc) = *Imp;
+
+  return new (Importer.getToContext()) BinaryOperator(
+      ToLHS, ToRHS, E->getOpcode(), ToType, E->getValueKind(),
+      E->getObjectKind(), ToOperatorLoc, E->getFPFeatures());
+}
+
+ExpectedStmt ASTNodeImporter::VisitConditionalOperator(ConditionalOperator *E) {
+  auto Imp = importSeq(
+      E->getCond(), E->getQuestionLoc(), E->getLHS(), E->getColonLoc(),
+      E->getRHS(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToCond, *ToLHS, *ToRHS;
+  SourceLocation ToQuestionLoc, ToColonLoc;
+  QualType ToType;
+  std::tie(ToCond, ToQuestionLoc, ToLHS, ToColonLoc, ToRHS, ToType) = *Imp;
 
   return new (Importer.getToContext()) ConditionalOperator(
-        ToCond, Importer.Import(E->getQuestionLoc()),
-        ToLHS, Importer.Import(E->getColonLoc()),
-        ToRHS, T, E->getValueKind(), E->getObjectKind());
+      ToCond, ToQuestionLoc, ToLHS, ToColonLoc, ToRHS, ToType,
+      E->getValueKind(), E->getObjectKind());
 }
 
-Expr *ASTNodeImporter::VisitBinaryConditionalOperator(
+ExpectedStmt ASTNodeImporter::VisitBinaryConditionalOperator(
     BinaryConditionalOperator *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *Common = Importer.Import(E->getCommon());
-  if (!Common)
-    return nullptr;
-
-  Expr *Cond = Importer.Import(E->getCond());
-  if (!Cond)
-    return nullptr;
-
-  auto *OpaqueValue = cast_or_null<OpaqueValueExpr>(
-      Importer.Import(E->getOpaqueValue()));
-  if (!OpaqueValue)
-    return nullptr;
-
-  Expr *TrueExpr = Importer.Import(E->getTrueExpr());
-  if (!TrueExpr)
-    return nullptr;
-
-  Expr *FalseExpr = Importer.Import(E->getFalseExpr());
-  if (!FalseExpr)
-    return nullptr;
+  auto Imp = importSeq(
+      E->getCommon(), E->getOpaqueValue(), E->getCond(), E->getTrueExpr(),
+      E->getFalseExpr(), E->getQuestionLoc(), E->getColonLoc(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToCommon, *ToCond, *ToTrueExpr, *ToFalseExpr;
+  OpaqueValueExpr *ToOpaqueValue;
+  SourceLocation ToQuestionLoc, ToColonLoc;
+  QualType ToType;
+  std::tie(
+      ToCommon, ToOpaqueValue, ToCond, ToTrueExpr, ToFalseExpr, ToQuestionLoc,
+      ToColonLoc, ToType) = *Imp;
 
   return new (Importer.getToContext()) BinaryConditionalOperator(
-        Common, OpaqueValue, Cond, TrueExpr, FalseExpr,
-        Importer.Import(E->getQuestionLoc()), Importer.Import(E->getColonLoc()),
-        T, E->getValueKind(), E->getObjectKind());
+      ToCommon, ToOpaqueValue, ToCond, ToTrueExpr, ToFalseExpr,
+      ToQuestionLoc, ToColonLoc, ToType, E->getValueKind(),
+      E->getObjectKind());
 }
 
-Expr *ASTNodeImporter::VisitArrayTypeTraitExpr(ArrayTypeTraitExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  TypeSourceInfo *ToQueried = Importer.Import(E->getQueriedTypeSourceInfo());
-  if (!ToQueried)
-    return nullptr;
-
-  Expr *Dim = Importer.Import(E->getDimensionExpression());
-  if (!Dim && E->getDimensionExpression())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitArrayTypeTraitExpr(ArrayTypeTraitExpr *E) {
+  auto Imp = importSeq(
+      E->getBeginLoc(), E->getQueriedTypeSourceInfo(),
+      E->getDimensionExpression(), E->getEndLoc(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToBeginLoc, ToEndLoc;
+  TypeSourceInfo *ToQueriedTypeSourceInfo;
+  Expr *ToDimensionExpression;
+  QualType ToType;
+  std::tie(
+      ToBeginLoc, ToQueriedTypeSourceInfo, ToDimensionExpression, ToEndLoc,
+      ToType) = *Imp;
 
   return new (Importer.getToContext()) ArrayTypeTraitExpr(
-      Importer.Import(E->getBeginLoc()), E->getTrait(), ToQueried,
-      E->getValue(), Dim, Importer.Import(E->getEndLoc()), T);
+      ToBeginLoc, E->getTrait(), ToQueriedTypeSourceInfo, E->getValue(),
+      ToDimensionExpression, ToEndLoc, ToType);
 }
 
-Expr *ASTNodeImporter::VisitExpressionTraitExpr(ExpressionTraitExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *ToQueried = Importer.Import(E->getQueriedExpression());
-  if (!ToQueried)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitExpressionTraitExpr(ExpressionTraitExpr *E) {
+  auto Imp = importSeq(
+      E->getBeginLoc(), E->getQueriedExpression(), E->getEndLoc(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToBeginLoc, ToEndLoc;
+  Expr *ToQueriedExpression;
+  QualType ToType;
+  std::tie(ToBeginLoc, ToQueriedExpression, ToEndLoc, ToType) = *Imp;
 
   return new (Importer.getToContext()) ExpressionTraitExpr(
-      Importer.Import(E->getBeginLoc()), E->getTrait(), ToQueried,
-      E->getValue(), Importer.Import(E->getEndLoc()), T);
+      ToBeginLoc, E->getTrait(), ToQueriedExpression, E->getValue(),
+      ToEndLoc, ToType);
 }
 
-Expr *ASTNodeImporter::VisitOpaqueValueExpr(OpaqueValueExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *SourceExpr = Importer.Import(E->getSourceExpr());
-  if (!SourceExpr && E->getSourceExpr())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitOpaqueValueExpr(OpaqueValueExpr *E) {
+  auto Imp = importSeq(
+      E->getLocation(), E->getType(), E->getSourceExpr());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToLocation;
+  QualType ToType;
+  Expr *ToSourceExpr;
+  std::tie(ToLocation, ToType, ToSourceExpr) = *Imp;
 
   return new (Importer.getToContext()) OpaqueValueExpr(
-        Importer.Import(E->getLocation()), T, E->getValueKind(),
-        E->getObjectKind(), SourceExpr);
+      ToLocation, ToType, E->getValueKind(), E->getObjectKind(), ToSourceExpr);
 }
 
-Expr *ASTNodeImporter::VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *ToLHS = Importer.Import(E->getLHS());
-  if (!ToLHS)
-    return nullptr;
-
-  Expr *ToRHS = Importer.Import(E->getRHS());
-  if (!ToRHS)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
+  auto Imp = importSeq(
+      E->getLHS(), E->getRHS(), E->getType(), E->getRBracketLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToLHS, *ToRHS;
+  SourceLocation ToRBracketLoc;
+  QualType ToType;
+  std::tie(ToLHS, ToRHS, ToType, ToRBracketLoc) = *Imp;
 
   return new (Importer.getToContext()) ArraySubscriptExpr(
-        ToLHS, ToRHS, T, E->getValueKind(), E->getObjectKind(),
-        Importer.Import(E->getRBracketLoc()));
+      ToLHS, ToRHS, ToType, E->getValueKind(), E->getObjectKind(),
+      ToRBracketLoc);
 }
 
-Expr *ASTNodeImporter::VisitCompoundAssignOperator(CompoundAssignOperator *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  QualType CompLHSType = Importer.Import(E->getComputationLHSType());
-  if (CompLHSType.isNull())
-    return nullptr;
-
-  QualType CompResultType = Importer.Import(E->getComputationResultType());
-  if (CompResultType.isNull())
-    return nullptr;
-
-  Expr *LHS = Importer.Import(E->getLHS());
-  if (!LHS)
-    return nullptr;
-
-  Expr *RHS = Importer.Import(E->getRHS());
-  if (!RHS)
-    return nullptr;
-
-  return new (Importer.getToContext())
-                        CompoundAssignOperator(LHS, RHS, E->getOpcode(),
-                                               T, E->getValueKind(),
-                                               E->getObjectKind(),
-                                               CompLHSType, CompResultType,
-                                           Importer.Import(E->getOperatorLoc()),
-                                               E->getFPFeatures());
-}
-
-bool ASTNodeImporter::ImportCastPath(CastExpr *CE, CXXCastPath &Path) {
+ExpectedStmt
+ASTNodeImporter::VisitCompoundAssignOperator(CompoundAssignOperator *E) {
+  auto Imp = importSeq(
+      E->getLHS(), E->getRHS(), E->getType(), E->getComputationLHSType(),
+      E->getComputationResultType(), E->getOperatorLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToLHS, *ToRHS;
+  QualType ToType, ToComputationLHSType, ToComputationResultType;
+  SourceLocation ToOperatorLoc;
+  std::tie(ToLHS, ToRHS, ToType, ToComputationLHSType, ToComputationResultType,
+      ToOperatorLoc) = *Imp;
+
+  return new (Importer.getToContext()) CompoundAssignOperator(
+      ToLHS, ToRHS, E->getOpcode(), ToType, E->getValueKind(),
+      E->getObjectKind(), ToComputationLHSType, ToComputationResultType,
+      ToOperatorLoc, E->getFPFeatures());
+}
+
+Expected<CXXCastPath>
+ASTNodeImporter::ImportCastPath(CastExpr *CE) {
+  CXXCastPath Path;
   for (auto I = CE->path_begin(), E = CE->path_end(); I != E; ++I) {
-    if (CXXBaseSpecifier *Spec = Importer.Import(*I))
-      Path.push_back(Spec);
+    if (auto SpecOrErr = import(*I))
+      Path.push_back(*SpecOrErr);
     else
-      return true;
+      return SpecOrErr.takeError();
   }
-  return false;
+  return Path;
 }
 
-Expr *ASTNodeImporter::VisitImplicitCastExpr(ImplicitCastExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitImplicitCastExpr(ImplicitCastExpr *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
 
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr)
-    return nullptr;
+  ExpectedExpr ToSubExprOrErr = import(E->getSubExpr());
+  if (!ToSubExprOrErr)
+    return ToSubExprOrErr.takeError();
 
-  CXXCastPath BasePath;
-  if (ImportCastPath(E, BasePath))
-    return nullptr;
+  Expected<CXXCastPath> ToBasePathOrErr = ImportCastPath(E);
+  if (!ToBasePathOrErr)
+    return ToBasePathOrErr.takeError();
 
-  return ImplicitCastExpr::Create(Importer.getToContext(), T, E->getCastKind(),
-                                  SubExpr, &BasePath, E->getValueKind());
+  return ImplicitCastExpr::Create(
+      Importer.getToContext(), *ToTypeOrErr, E->getCastKind(), *ToSubExprOrErr,
+      &(*ToBasePathOrErr), E->getValueKind());
 }
 
-Expr *ASTNodeImporter::VisitExplicitCastExpr(ExplicitCastExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitExplicitCastExpr(ExplicitCastExpr *E) {
+  auto Imp1 = importSeq(
+      E->getType(), E->getSubExpr(), E->getTypeInfoAsWritten());
+  if (!Imp1)
+    return Imp1.takeError();
 
-  TypeSourceInfo *TInfo = Importer.Import(E->getTypeInfoAsWritten());
-  if (!TInfo && E->getTypeInfoAsWritten())
-    return nullptr;
+  QualType ToType;
+  Expr *ToSubExpr;
+  TypeSourceInfo *ToTypeInfoAsWritten;
+  std::tie(ToType, ToSubExpr, ToTypeInfoAsWritten) = *Imp1;
 
-  CXXCastPath BasePath;
-  if (ImportCastPath(E, BasePath))
-    return nullptr;
+  Expected<CXXCastPath> ToBasePathOrErr = ImportCastPath(E);
+  if (!ToBasePathOrErr)
+    return ToBasePathOrErr.takeError();
+  CXXCastPath *ToBasePath = &(*ToBasePathOrErr);
 
   switch (E->getStmtClass()) {
   case Stmt::CStyleCastExprClass: {
     auto *CCE = cast<CStyleCastExpr>(E);
-    return CStyleCastExpr::Create(Importer.getToContext(), T,
-                                  E->getValueKind(), E->getCastKind(),
-                                  SubExpr, &BasePath, TInfo,
-                                  Importer.Import(CCE->getLParenLoc()),
-                                  Importer.Import(CCE->getRParenLoc()));
+    ExpectedSLoc ToLParenLocOrErr = import(CCE->getLParenLoc());
+    if (!ToLParenLocOrErr)
+      return ToLParenLocOrErr.takeError();
+    ExpectedSLoc ToRParenLocOrErr = import(CCE->getRParenLoc());
+    if (!ToRParenLocOrErr)
+      return ToRParenLocOrErr.takeError();
+    return CStyleCastExpr::Create(
+        Importer.getToContext(), ToType, E->getValueKind(), E->getCastKind(),
+        ToSubExpr, ToBasePath, ToTypeInfoAsWritten, *ToLParenLocOrErr,
+        *ToRParenLocOrErr);
   }
 
   case Stmt::CXXFunctionalCastExprClass: {
     auto *FCE = cast<CXXFunctionalCastExpr>(E);
-    return CXXFunctionalCastExpr::Create(Importer.getToContext(), T,
-                                         E->getValueKind(), TInfo,
-                                         E->getCastKind(), SubExpr, &BasePath,
-                                         Importer.Import(FCE->getLParenLoc()),
-                                         Importer.Import(FCE->getRParenLoc()));
+    ExpectedSLoc ToLParenLocOrErr = import(FCE->getLParenLoc());
+    if (!ToLParenLocOrErr)
+      return ToLParenLocOrErr.takeError();
+    ExpectedSLoc ToRParenLocOrErr = import(FCE->getRParenLoc());
+    if (!ToRParenLocOrErr)
+      return ToRParenLocOrErr.takeError();
+    return CXXFunctionalCastExpr::Create(
+        Importer.getToContext(), ToType, E->getValueKind(), ToTypeInfoAsWritten,
+        E->getCastKind(), ToSubExpr, ToBasePath, *ToLParenLocOrErr,
+        *ToRParenLocOrErr);
   }
 
   case Stmt::ObjCBridgedCastExprClass: {
-      auto *OCE = cast<ObjCBridgedCastExpr>(E);
-      return new (Importer.getToContext()) ObjCBridgedCastExpr(
-            Importer.Import(OCE->getLParenLoc()), OCE->getBridgeKind(),
-            E->getCastKind(), Importer.Import(OCE->getBridgeKeywordLoc()),
-            TInfo, SubExpr);
+    ExpectedSLoc ToLParenLocOrErr = import(OCE->getLParenLoc());
+    if (!ToLParenLocOrErr)
+      return ToLParenLocOrErr.takeError();
+    ExpectedSLoc ToBridgeKeywordLocOrErr = import(OCE->getBridgeKeywordLoc());
+    if (!ToBridgeKeywordLocOrErr)
+      return ToBridgeKeywordLocOrErr.takeError();
+        *ToLParenLocOrErr, OCE->getBridgeKind(), E->getCastKind(),
+        *ToBridgeKeywordLocOrErr, ToTypeInfoAsWritten, ToSubExpr);
   }
   default:
     llvm_unreachable("Cast expression of unsupported type!");
-    return nullptr;
+    return make_error<ImportError>(ImportError::UnsupportedConstruct);
   }
 }
 
-Expr *ASTNodeImporter::VisitOffsetOfExpr(OffsetOfExpr *OE) {
-  QualType T = Importer.Import(OE->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitOffsetOfExpr(OffsetOfExpr *E) {
+  SmallVector<OffsetOfNode, 4> ToNodes;
+  for (int I = 0, N = E->getNumComponents(); I < N; ++I) {
+    const OffsetOfNode &FromNode = E->getComponent(I);
 
-  SmallVector<OffsetOfNode, 4> Nodes;
-  for (int I = 0, E = OE->getNumComponents(); I < E; ++I) {
-    const OffsetOfNode &Node = OE->getComponent(I);
+    SourceLocation ToBeginLoc, ToEndLoc;
+    if (FromNode.getKind() != OffsetOfNode::Base) {
+      auto Imp = importSeq(FromNode.getBeginLoc(), FromNode.getEndLoc());
+      if (!Imp)
+        return Imp.takeError();
+      std::tie(ToBeginLoc, ToEndLoc) = *Imp;
+    }
 
-    switch (Node.getKind()) {
+    switch (FromNode.getKind()) {
     case OffsetOfNode::Array:
-      Nodes.push_back(OffsetOfNode(Importer.Import(Node.getBeginLoc()),
-                                   Node.getArrayExprIndex(),
-                                   Importer.Import(Node.getEndLoc())));
+      ToNodes.push_back(
+          OffsetOfNode(ToBeginLoc, FromNode.getArrayExprIndex(), ToEndLoc));
       break;
-
     case OffsetOfNode::Base: {
-      CXXBaseSpecifier *BS = Importer.Import(Node.getBase());
-      if (!BS && Node.getBase())
-        return nullptr;
-      Nodes.push_back(OffsetOfNode(BS));
+      auto ToBSOrErr = import(FromNode.getBase());
+      if (!ToBSOrErr)
+        return ToBSOrErr.takeError();
+      ToNodes.push_back(OffsetOfNode(*ToBSOrErr));
       break;
     }
     case OffsetOfNode::Field: {
-      auto *FD = cast_or_null<FieldDecl>(Importer.Import(Node.getField()));
-      if (!FD)
-        return nullptr;
-      Nodes.push_back(OffsetOfNode(Importer.Import(Node.getBeginLoc()), FD,
-                                   Importer.Import(Node.getEndLoc())));
+      auto ToFieldOrErr = import(FromNode.getField());
+      if (!ToFieldOrErr)
+        return ToFieldOrErr.takeError();
+      ToNodes.push_back(OffsetOfNode(ToBeginLoc, *ToFieldOrErr, ToEndLoc));
       break;
     }
     case OffsetOfNode::Identifier: {
-      IdentifierInfo *ToII = Importer.Import(Node.getFieldName());
-      if (!ToII)
-        return nullptr;
-      Nodes.push_back(OffsetOfNode(Importer.Import(Node.getBeginLoc()), ToII,
-                                   Importer.Import(Node.getEndLoc())));
+      IdentifierInfo *ToII = Importer.Import(FromNode.getFieldName());
+      ToNodes.push_back(OffsetOfNode(ToBeginLoc, ToII, ToEndLoc));
       break;
     }
     }
   }
 
-  SmallVector<Expr *, 4> Exprs(OE->getNumExpressions());
-  for (int I = 0, E = OE->getNumExpressions(); I < E; ++I) {
-    Expr *ToIndexExpr = Importer.Import(OE->getIndexExpr(I));
-    if (!ToIndexExpr)
-      return nullptr;
-    Exprs[I] = ToIndexExpr;
-  }
-
-  TypeSourceInfo *TInfo = Importer.Import(OE->getTypeSourceInfo());
-  if (!TInfo && OE->getTypeSourceInfo())
-    return nullptr;
-
-  return OffsetOfExpr::Create(Importer.getToContext(), T,
-                              Importer.Import(OE->getOperatorLoc()),
-                              TInfo, Nodes, Exprs,
-                              Importer.Import(OE->getRParenLoc()));
-}
-
-Expr *ASTNodeImporter::VisitCXXNoexceptExpr(CXXNoexceptExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *Operand = Importer.Import(E->getOperand());
-  if (!Operand)
-    return nullptr;
+  SmallVector<Expr *, 4> ToExprs(E->getNumExpressions());
+  for (int I = 0, N = E->getNumExpressions(); I < N; ++I) {
+    ExpectedExpr ToIndexExprOrErr = import(E->getIndexExpr(I));
+    if (!ToIndexExprOrErr)
+      return ToIndexExprOrErr.takeError();
+    ToExprs[I] = *ToIndexExprOrErr;
+  }
+
+  auto Imp = importSeq(
+      E->getType(), E->getTypeSourceInfo(), E->getOperatorLoc(),
+      E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  SourceLocation ToOperatorLoc, ToRParenLoc;
+  std::tie(ToType, ToTypeSourceInfo, ToOperatorLoc, ToRParenLoc) = *Imp;
+
+  return OffsetOfExpr::Create(
+      Importer.getToContext(), ToType, ToOperatorLoc, ToTypeSourceInfo, ToNodes,
+      ToExprs, ToRParenLoc);
+}
+
+ExpectedStmt ASTNodeImporter::VisitCXXNoexceptExpr(CXXNoexceptExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getOperand(), E->getBeginLoc(), E->getEndLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  Expr *ToOperand;
+  SourceLocation ToBeginLoc, ToEndLoc;
+  std::tie(ToType, ToOperand, ToBeginLoc, ToEndLoc) = *Imp;
 
-  CanThrowResult CanThrow;
+  CanThrowResult ToCanThrow;
   if (E->isValueDependent())
-    CanThrow = CT_Dependent;
+    ToCanThrow = CT_Dependent;
   else
-    CanThrow = E->getValue() ? CT_Can : CT_Cannot;
+    ToCanThrow = E->getValue() ? CT_Can : CT_Cannot;
 
-  return new (Importer.getToContext())
-      CXXNoexceptExpr(T, Operand, CanThrow, Importer.Import(E->getBeginLoc()),
-                      Importer.Import(E->getEndLoc()));
+  return new (Importer.getToContext()) CXXNoexceptExpr(
+      ToType, ToOperand, ToCanThrow, ToBeginLoc, ToEndLoc);
 }
 
-Expr *ASTNodeImporter::VisitCXXThrowExpr(CXXThrowExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXThrowExpr(CXXThrowExpr *E) {
+  auto Imp = importSeq(E->getSubExpr(), E->getType(), E->getThrowLoc());
+  if (!Imp)
+    return Imp.takeError();
 
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr && E->getSubExpr())
-    return nullptr;
+  Expr *ToSubExpr;
+  QualType ToType;
+  SourceLocation ToThrowLoc;
+  std::tie(ToSubExpr, ToType, ToThrowLoc) = *Imp;
 
   return new (Importer.getToContext()) CXXThrowExpr(
-        SubExpr, T, Importer.Import(E->getThrowLoc()),
-        E->isThrownVariableInScope());
+      ToSubExpr, ToType, ToThrowLoc, E->isThrownVariableInScope());
 }
 
-Expr *ASTNodeImporter::VisitCXXDefaultArgExpr(CXXDefaultArgExpr *E) {
-  auto *Param = cast_or_null<ParmVarDecl>(Importer.Import(E->getParam()));
-  if (!Param)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXDefaultArgExpr(CXXDefaultArgExpr *E) {
+  ExpectedSLoc ToUsedLocOrErr = import(E->getUsedLocation());
+  if (!ToUsedLocOrErr)
+    return ToUsedLocOrErr.takeError();
+
+  auto ToParamOrErr = import(E->getParam());
+  if (!ToParamOrErr)
+    return ToParamOrErr.takeError();
 
   return CXXDefaultArgExpr::Create(
-        Importer.getToContext(), Importer.Import(E->getUsedLocation()), Param);
+      Importer.getToContext(), *ToUsedLocOrErr, *ToParamOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  TypeSourceInfo *TypeInfo = Importer.Import(E->getTypeSourceInfo());
-  if (!TypeInfo)
-    return nullptr;
+ExpectedStmt
+ASTNodeImporter::VisitCXXScalarValueInitExpr(CXXScalarValueInitExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getTypeSourceInfo(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  SourceLocation ToRParenLoc;
+  std::tie(ToType, ToTypeSourceInfo, ToRParenLoc) = *Imp;
 
   return new (Importer.getToContext()) CXXScalarValueInitExpr(
-        T, TypeInfo, Importer.Import(E->getRParenLoc()));
+      ToType, ToTypeSourceInfo, ToRParenLoc);
 }
 
-Expr *ASTNodeImporter::VisitCXXBindTemporaryExpr(CXXBindTemporaryExpr *E) {
-  Expr *SubExpr = Importer.Import(E->getSubExpr());
-  if (!SubExpr)
-    return nullptr;
-
-  auto *Dtor = cast_or_null<CXXDestructorDecl>(
-        Importer.Import(const_cast<CXXDestructorDecl *>(
-                          E->getTemporary()->getDestructor())));
-  if (!Dtor)
-    return nullptr;
+ExpectedStmt
+ASTNodeImporter::VisitCXXBindTemporaryExpr(CXXBindTemporaryExpr *E) {
+  ExpectedExpr ToSubExprOrErr = import(E->getSubExpr());
+  if (!ToSubExprOrErr)
+    return ToSubExprOrErr.takeError();
+
+  auto ToDtorOrErr = import(E->getTemporary()->getDestructor());
+  if (!ToDtorOrErr)
+    return ToDtorOrErr.takeError();
 
   ASTContext &ToCtx = Importer.getToContext();
-  CXXTemporary *Temp = CXXTemporary::Create(ToCtx, Dtor);
-  return CXXBindTemporaryExpr::Create(ToCtx, Temp, SubExpr);
+  CXXTemporary *Temp = CXXTemporary::Create(ToCtx, *ToDtorOrErr);
+  return CXXBindTemporaryExpr::Create(ToCtx, Temp, *ToSubExprOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCXXTemporaryObjectExpr(CXXTemporaryObjectExpr *CE) {
-  QualType T = Importer.Import(CE->getType());
-  if (T.isNull())
-    return nullptr;
-
-  TypeSourceInfo *TInfo = Importer.Import(CE->getTypeSourceInfo());
-  if (!TInfo)
-    return nullptr;
-
-  SmallVector<Expr *, 8> Args(CE->getNumArgs());
-  if (ImportContainerChecked(CE->arguments(), Args))
-    return nullptr;
-
-  auto *Ctor = cast_or_null<CXXConstructorDecl>(
-        Importer.Import(CE->getConstructor()));
-  if (!Ctor)
-    return nullptr;
+ExpectedStmt
+ASTNodeImporter::VisitCXXTemporaryObjectExpr(CXXTemporaryObjectExpr *E) {
+  auto Imp = importSeq(
+      E->getConstructor(), E->getType(), E->getTypeSourceInfo(),
+      E->getParenOrBraceRange());
+  if (!Imp)
+    return Imp.takeError();
+
+  CXXConstructorDecl *ToConstructor;
+  QualType ToType;
+  TypeSourceInfo *ToTypeSourceInfo;
+  SourceRange ToParenOrBraceRange;
+  std::tie(ToConstructor, ToType, ToTypeSourceInfo, ToParenOrBraceRange) = *Imp;
+
+  SmallVector<Expr *, 8> ToArgs(E->getNumArgs());
+  if (Error Err = ImportContainerChecked(E->arguments(), ToArgs))
+    return std::move(Err);
 
   return new (Importer.getToContext()) CXXTemporaryObjectExpr(
-      Importer.getToContext(), Ctor, T, TInfo, Args,
-      Importer.Import(CE->getParenOrBraceRange()), CE->hadMultipleCandidates(),
-      CE->isListInitialization(), CE->isStdInitListInitialization(),
-      CE->requiresZeroInitialization());
+      Importer.getToContext(), ToConstructor, ToType, ToTypeSourceInfo, ToArgs,
+      ToParenOrBraceRange, E->hadMultipleCandidates(),
+      E->isListInitialization(), E->isStdInitListInitialization(),
+      E->requiresZeroInitialization());
 }
 
-Expr *
+ExpectedStmt
 ASTNodeImporter::VisitMaterializeTemporaryExpr(MaterializeTemporaryExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *TempE = Importer.Import(E->GetTemporaryExpr());
-  if (!TempE)
-    return nullptr;
-
-  auto *ExtendedBy = cast_or_null<ValueDecl>(
-        Importer.Import(const_cast<ValueDecl *>(E->getExtendingDecl())));
-  if (!ExtendedBy && E->getExtendingDecl())
-    return nullptr;
+  auto Imp = importSeq(
+      E->getType(), E->GetTemporaryExpr(), E->getExtendingDecl());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  Expr *ToTemporaryExpr;
+  const ValueDecl *ToExtendingDecl;
+  std::tie(ToType, ToTemporaryExpr, ToExtendingDecl) = *Imp;
 
   auto *ToMTE =  new (Importer.getToContext()) MaterializeTemporaryExpr(
-        T, TempE, E->isBoundToLvalueReference());
+      ToType, ToTemporaryExpr, E->isBoundToLvalueReference());
 
   // FIXME: Should ManglingNumber get numbers associated with 'to' context?
-  ToMTE->setExtendingDecl(ExtendedBy, E->getManglingNumber());
+  ToMTE->setExtendingDecl(ToExtendingDecl, E->getManglingNumber());
   return ToMTE;
 }
 
-Expr *ASTNodeImporter::VisitPackExpansionExpr(PackExpansionExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *Pattern = Importer.Import(E->getPattern());
-  if (!Pattern)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitPackExpansionExpr(PackExpansionExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getPattern(), E->getEllipsisLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  Expr *ToPattern;
+  SourceLocation ToEllipsisLoc;
+  std::tie(ToType, ToPattern, ToEllipsisLoc) = *Imp;
 
   return new (Importer.getToContext()) PackExpansionExpr(
-        T, Pattern, Importer.Import(E->getEllipsisLoc()),
-        E->getNumExpansions());
+      ToType, ToPattern, ToEllipsisLoc, E->getNumExpansions());
 }
 
-Expr *ASTNodeImporter::VisitSizeOfPackExpr(SizeOfPackExpr *E) {
-  auto *Pack = cast_or_null<NamedDecl>(Importer.Import(E->getPack()));
-  if (!Pack)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitSizeOfPackExpr(SizeOfPackExpr *E) {
+  auto Imp = importSeq(
+      E->getOperatorLoc(), E->getPack(), E->getPackLoc(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToOperatorLoc, ToPackLoc, ToRParenLoc;
+  NamedDecl *ToPack;
+  std::tie(ToOperatorLoc, ToPack, ToPackLoc, ToRParenLoc) = *Imp;
 
   Optional<unsigned> Length;
-
   if (!E->isValueDependent())
     Length = E->getPackLength();
 
-  SmallVector<TemplateArgument, 8> PartialArguments;
+  SmallVector<TemplateArgument, 8> ToPartialArguments;
   if (E->isPartiallySubstituted()) {
-    if (ImportTemplateArguments(E->getPartialArguments().data(),
-                                E->getPartialArguments().size(),
-                                PartialArguments))
-      return nullptr;
+    if (Error Err = ImportTemplateArguments(
+        E->getPartialArguments().data(),
+        ToPartialArguments))
+      return std::move(Err);
   }
 
   return SizeOfPackExpr::Create(
-      Importer.getToContext(), Importer.Import(E->getOperatorLoc()), Pack,
-      Importer.Import(E->getPackLoc()), Importer.Import(E->getRParenLoc()),
-      Length, PartialArguments);
+      Importer.getToContext(), ToOperatorLoc, ToPack, ToPackLoc, ToRParenLoc,
+      Length, ToPartialArguments);
 }
 
-Expr *ASTNodeImporter::VisitCXXNewExpr(CXXNewExpr *CE) {
-  QualType T = Importer.Import(CE->getType());
-  if (T.isNull())
-    return nullptr;
-
-  SmallVector<Expr *, 4> PlacementArgs(CE->getNumPlacementArgs());
-  if (ImportContainerChecked(CE->placement_arguments(), PlacementArgs))
-    return nullptr;
-
-  auto *OperatorNewDecl = cast_or_null<FunctionDecl>(
-        Importer.Import(CE->getOperatorNew()));
-  if (!OperatorNewDecl && CE->getOperatorNew())
-    return nullptr;
-
-  auto *OperatorDeleteDecl = cast_or_null<FunctionDecl>(
-        Importer.Import(CE->getOperatorDelete()));
-  if (!OperatorDeleteDecl && CE->getOperatorDelete())
-    return nullptr;
 
-  Expr *ToInit = Importer.Import(CE->getInitializer());
-  if (!ToInit && CE->getInitializer())
-    return nullptr;
-
-  TypeSourceInfo *TInfo = Importer.Import(CE->getAllocatedTypeSourceInfo());
-  if (!TInfo)
-    return nullptr;
-
-  Expr *ToArrSize = Importer.Import(CE->getArraySize());
-  if (!ToArrSize && CE->getArraySize())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXNewExpr(CXXNewExpr *E) {
+  auto Imp = importSeq(
+      E->getOperatorNew(), E->getOperatorDelete(), E->getTypeIdParens(),
+      E->getArraySize(), E->getInitializer(), E->getType(),
+      E->getAllocatedTypeSourceInfo(), E->getSourceRange(),
+      E->getDirectInitRange());
+  if (!Imp)
+    return Imp.takeError();
+
+  FunctionDecl *ToOperatorNew, *ToOperatorDelete;
+  SourceRange ToTypeIdParens, ToSourceRange, ToDirectInitRange;
+  Expr *ToArraySize, *ToInitializer;
+  QualType ToType;
+  TypeSourceInfo *ToAllocatedTypeSourceInfo;
+  std::tie(
+    ToOperatorNew, ToOperatorDelete, ToTypeIdParens, ToArraySize, ToInitializer,
+    ToType, ToAllocatedTypeSourceInfo, ToSourceRange, ToDirectInitRange) = *Imp;
+
+  SmallVector<Expr *, 4> ToPlacementArgs(E->getNumPlacementArgs());
+  if (Error Err =
+      ImportContainerChecked(E->placement_arguments(), ToPlacementArgs))
+    return std::move(Err);
 
   return new (Importer.getToContext()) CXXNewExpr(
-        Importer.getToContext(),
-        CE->isGlobalNew(),
-        OperatorNewDecl, OperatorDeleteDecl,
-        CE->passAlignment(),
-        CE->doesUsualArrayDeleteWantSize(),
-        PlacementArgs,
-        Importer.Import(CE->getTypeIdParens()),
-        ToArrSize, CE->getInitializationStyle(), ToInit, T, TInfo,
-        Importer.Import(CE->getSourceRange()),
-        Importer.Import(CE->getDirectInitRange()));
-}
-
-Expr *ASTNodeImporter::VisitCXXDeleteExpr(CXXDeleteExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  auto *OperatorDeleteDecl = cast_or_null<FunctionDecl>(
-        Importer.Import(E->getOperatorDelete()));
-  if (!OperatorDeleteDecl && E->getOperatorDelete())
-    return nullptr;
-
-  Expr *ToArg = Importer.Import(E->getArgument());
-  if (!ToArg && E->getArgument())
-    return nullptr;
+      Importer.getToContext(), E->isGlobalNew(), ToOperatorNew,
+      ToOperatorDelete, E->passAlignment(), E->doesUsualArrayDeleteWantSize(),
+      ToPlacementArgs, ToTypeIdParens, ToArraySize, E->getInitializationStyle(),
+      ToInitializer, ToType, ToAllocatedTypeSourceInfo, ToSourceRange,
+      ToDirectInitRange);
+}
+
+ExpectedStmt ASTNodeImporter::VisitCXXDeleteExpr(CXXDeleteExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getOperatorDelete(), E->getArgument(), E->getBeginLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  FunctionDecl *ToOperatorDelete;
+  Expr *ToArgument;
+  SourceLocation ToBeginLoc;
+  std::tie(ToType, ToOperatorDelete, ToArgument, ToBeginLoc) = *Imp;
 
   return new (Importer.getToContext()) CXXDeleteExpr(
-      T, E->isGlobalDelete(), E->isArrayForm(), E->isArrayFormAsWritten(),
-      E->doesUsualArrayDeleteWantSize(), OperatorDeleteDecl, ToArg,
-      Importer.Import(E->getBeginLoc()));
-}
-
-Expr *ASTNodeImporter::VisitCXXConstructExpr(CXXConstructExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  auto *ToCCD =
-    dyn_cast_or_null<CXXConstructorDecl>(Importer.Import(E->getConstructor()));
-  if (!ToCCD)
-    return nullptr;
+      ToType, E->isGlobalDelete(), E->isArrayForm(), E->isArrayFormAsWritten(),
+      E->doesUsualArrayDeleteWantSize(), ToOperatorDelete, ToArgument,
+      ToBeginLoc);
+}
+
+ExpectedStmt ASTNodeImporter::VisitCXXConstructExpr(CXXConstructExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getLocation(), E->getConstructor(),
+      E->getParenOrBraceRange());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  SourceLocation ToLocation;
+  CXXConstructorDecl *ToConstructor;
+  SourceRange ToParenOrBraceRange;
+  std::tie(ToType, ToLocation, ToConstructor, ToParenOrBraceRange) = *Imp;
 
   SmallVector<Expr *, 6> ToArgs(E->getNumArgs());
-  if (ImportContainerChecked(E->arguments(), ToArgs))
-    return nullptr;
+  if (Error Err = ImportContainerChecked(E->arguments(), ToArgs))
+    return std::move(Err);
 
-  return CXXConstructExpr::Create(Importer.getToContext(), T,
-                                  Importer.Import(E->getLocation()),
-                                  ToCCD, E->isElidable(),
-                                  ToArgs, E->hadMultipleCandidates(),
-                                  E->isListInitialization(),
-                                  E->isStdInitListInitialization(),
-                                  E->requiresZeroInitialization(),
-                                  E->getConstructionKind(),
-                                  Importer.Import(E->getParenOrBraceRange()));
-}
-
-Expr *ASTNodeImporter::VisitExprWithCleanups(ExprWithCleanups *EWC) {
-  Expr *SubExpr = Importer.Import(EWC->getSubExpr());
-  if (!SubExpr && EWC->getSubExpr())
-    return nullptr;
-
-  SmallVector<ExprWithCleanups::CleanupObject, 8> Objs(EWC->getNumObjects());
-  for (unsigned I = 0, E = EWC->getNumObjects(); I < E; I++)
-    if (ExprWithCleanups::CleanupObject Obj =
-        cast_or_null<BlockDecl>(Importer.Import(EWC->getObject(I))))
-      Objs[I] = Obj;
-    else
-      return nullptr;
-
-  return ExprWithCleanups::Create(Importer.getToContext(),
-                                  SubExpr, EWC->cleanupsHaveSideEffects(),
-                                  Objs);
-}
-
-Expr *ASTNodeImporter::VisitCXXMemberCallExpr(CXXMemberCallExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *ToFn = Importer.Import(E->getCallee());
-  if (!ToFn)
-    return nullptr;
+  return CXXConstructExpr::Create(
+      Importer.getToContext(), ToType, ToLocation, ToConstructor,
+      E->isElidable(), ToArgs, E->hadMultipleCandidates(),
+      E->isListInitialization(), E->isStdInitListInitialization(),
+      E->requiresZeroInitialization(), E->getConstructionKind(),
+      ToParenOrBraceRange);
+}
+
+ExpectedStmt ASTNodeImporter::VisitExprWithCleanups(ExprWithCleanups *E) {
+  ExpectedExpr ToSubExprOrErr = import(E->getSubExpr());
+  if (!ToSubExprOrErr)
+    return ToSubExprOrErr.takeError();
+
+  SmallVector<ExprWithCleanups::CleanupObject, 8> ToObjects(E->getNumObjects());
+  if (Error Err = ImportContainerChecked(E->getObjects(), ToObjects))
+    return std::move(Err);
+
+  return ExprWithCleanups::Create(
+      Importer.getToContext(), *ToSubExprOrErr, E->cleanupsHaveSideEffects(),
+      ToObjects);
+}
+
+ExpectedStmt ASTNodeImporter::VisitCXXMemberCallExpr(CXXMemberCallExpr *E) {
+  auto Imp = importSeq(
+      E->getCallee(), E->getType(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToCallee;
+  QualType ToType;
+  SourceLocation ToRParenLoc;
+  std::tie(ToCallee, ToType, ToRParenLoc) = *Imp;
 
   SmallVector<Expr *, 4> ToArgs(E->getNumArgs());
-  if (ImportContainerChecked(E->arguments(), ToArgs))
-    return nullptr;
+  if (Error Err = ImportContainerChecked(E->arguments(), ToArgs))
+    return std::move(Err);
 
   return new (Importer.getToContext()) CXXMemberCallExpr(
-        Importer.getToContext(), ToFn, ToArgs, T, E->getValueKind(),
-        Importer.Import(E->getRParenLoc()));
-}
-
-Expr *ASTNodeImporter::VisitCXXThisExpr(CXXThisExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  return new (Importer.getToContext())
-  CXXThisExpr(Importer.Import(E->getLocation()), T, E->isImplicit());
-}
-
-Expr *ASTNodeImporter::VisitCXXBoolLiteralExpr(CXXBoolLiteralExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  return new (Importer.getToContext())
-  CXXBoolLiteralExpr(E->getValue(), T, Importer.Import(E->getLocation()));
+      Importer.getToContext(), ToCallee, ToArgs, ToType, E->getValueKind(),
+      ToRParenLoc);
 }
 
-
-Expr *ASTNodeImporter::VisitMemberExpr(MemberExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *ToBase = Importer.Import(E->getBase());
-  if (!ToBase && E->getBase())
-    return nullptr;
-
-  auto *ToMember = dyn_cast<ValueDecl>(Importer.Import(E->getMemberDecl()));
-  if (!ToMember && E->getMemberDecl())
-    return nullptr;
-
-  auto *ToDecl =
-      dyn_cast_or_null<NamedDecl>(Importer.Import(E->getFoundDecl().getDecl()));
-  if (!ToDecl && E->getFoundDecl().getDecl())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXThisExpr(CXXThisExpr *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+
+  ExpectedSLoc ToLocationOrErr = import(E->getLocation());
+  if (!ToLocationOrErr)
+    return ToLocationOrErr.takeError();
+
+  return new (Importer.getToContext()) CXXThisExpr(
+      *ToLocationOrErr, *ToTypeOrErr, E->isImplicit());
+}
+
+ExpectedStmt ASTNodeImporter::VisitCXXBoolLiteralExpr(CXXBoolLiteralExpr *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+
+  ExpectedSLoc ToLocationOrErr = import(E->getLocation());
+  if (!ToLocationOrErr)
+    return ToLocationOrErr.takeError();
+
+  return new (Importer.getToContext()) CXXBoolLiteralExpr(
+      E->getValue(), *ToTypeOrErr, *ToLocationOrErr);
+}
+
+ExpectedStmt ASTNodeImporter::VisitMemberExpr(MemberExpr *E) {
+  auto Imp1 = importSeq(
+      E->getBase(), E->getOperatorLoc(), E->getQualifierLoc(),
+      E->getTemplateKeywordLoc(), E->getMemberDecl(), E->getType());
+  if (!Imp1)
+    return Imp1.takeError();
+
+  Expr *ToBase;
+  SourceLocation ToOperatorLoc, ToTemplateKeywordLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  ValueDecl *ToMemberDecl;
+  QualType ToType;
+  std::tie(
+      ToBase, ToOperatorLoc, ToQualifierLoc, ToTemplateKeywordLoc, ToMemberDecl,
+      ToType) = *Imp1;
+
+  auto Imp2 = importSeq(
+      E->getFoundDecl().getDecl(), E->getMemberNameInfo().getName(),
+      E->getMemberNameInfo().getLoc(), E->getLAngleLoc(), E->getRAngleLoc());
+  if (!Imp2)
+    return Imp2.takeError();
+  NamedDecl *ToDecl;
+  DeclarationName ToName;
+  SourceLocation ToLoc, ToLAngleLoc, ToRAngleLoc;
+  std::tie(ToDecl, ToName, ToLoc, ToLAngleLoc, ToRAngleLoc) = *Imp2;
 
   DeclAccessPair ToFoundDecl =
       DeclAccessPair::make(ToDecl, E->getFoundDecl().getAccess());
 
-  DeclarationNameInfo ToMemberNameInfo(
-    Importer.Import(E->getMemberNameInfo().getName()),
-    Importer.Import(E->getMemberNameInfo().getLoc()));
+  DeclarationNameInfo ToMemberNameInfo(ToName, ToLoc);
 
   if (E->hasExplicitTemplateArgs()) {
-    return nullptr; // FIXME: handle template arguments
+    // FIXME: handle template arguments
+    return make_error<ImportError>(ImportError::UnsupportedConstruct);
   }
 
-  return MemberExpr::Create(Importer.getToContext(), ToBase,
-                            E->isArrow(),
-                            Importer.Import(E->getOperatorLoc()),
-                            Importer.Import(E->getQualifierLoc()),
-                            Importer.Import(E->getTemplateKeywordLoc()),
-                            ToMember, ToFoundDecl, ToMemberNameInfo,
-                            nullptr, T, E->getValueKind(),
-                            E->getObjectKind());
-}
-
-Expr *ASTNodeImporter::VisitCXXPseudoDestructorExpr(
-    CXXPseudoDestructorExpr *E) {
-  Expr *BaseE = Importer.Import(E->getBase());
-  if (!BaseE)
-    return nullptr;
-
-  TypeSourceInfo *ScopeInfo = Importer.Import(E->getScopeTypeInfo());
-  if (!ScopeInfo && E->getScopeTypeInfo())
-    return nullptr;
+  return MemberExpr::Create(
+      Importer.getToContext(), ToBase, E->isArrow(), ToOperatorLoc,
+      ToQualifierLoc, ToTemplateKeywordLoc, ToMemberDecl, ToFoundDecl,
+      ToMemberNameInfo, nullptr, ToType, E->getValueKind(), E->getObjectKind());
+}
+
+ExpectedStmt
+ASTNodeImporter::VisitCXXPseudoDestructorExpr(CXXPseudoDestructorExpr *E) {
+  auto Imp = importSeq(
+      E->getBase(), E->getOperatorLoc(), E->getQualifierLoc(),
+      E->getScopeTypeInfo(), E->getColonColonLoc(), E->getTildeLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToBase;
+  SourceLocation ToOperatorLoc, ToColonColonLoc, ToTildeLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  TypeSourceInfo *ToScopeTypeInfo;
+  std::tie(
+      ToBase, ToOperatorLoc, ToQualifierLoc, ToScopeTypeInfo, ToColonColonLoc,
+      ToTildeLoc) = *Imp;
 
   PseudoDestructorTypeStorage Storage;
   if (IdentifierInfo *FromII = E->getDestroyedTypeIdentifier()) {
     IdentifierInfo *ToII = Importer.Import(FromII);
-    if (!ToII)
-      return nullptr;
-    Storage = PseudoDestructorTypeStorage(
-          ToII, Importer.Import(E->getDestroyedTypeLoc()));
+    ExpectedSLoc ToDestroyedTypeLocOrErr = import(E->getDestroyedTypeLoc());
+    if (!ToDestroyedTypeLocOrErr)
+      return ToDestroyedTypeLocOrErr.takeError();
+    Storage = PseudoDestructorTypeStorage(ToII, *ToDestroyedTypeLocOrErr);
   } else {
-    TypeSourceInfo *TI = Importer.Import(E->getDestroyedTypeInfo());
-    if (!TI)
-      return nullptr;
-    Storage = PseudoDestructorTypeStorage(TI);
+    if (auto ToTIOrErr = import(E->getDestroyedTypeInfo()))
+      Storage = PseudoDestructorTypeStorage(*ToTIOrErr);
+    else
+      return ToTIOrErr.takeError();
   }
 
   return new (Importer.getToContext()) CXXPseudoDestructorExpr(
-        Importer.getToContext(), BaseE, E->isArrow(),
-        Importer.Import(E->getOperatorLoc()),
-        Importer.Import(E->getQualifierLoc()),
-        ScopeInfo, Importer.Import(E->getColonColonLoc()),
-        Importer.Import(E->getTildeLoc()), Storage);
+      Importer.getToContext(), ToBase, E->isArrow(), ToOperatorLoc,
+      ToQualifierLoc, ToScopeTypeInfo, ToColonColonLoc, ToTildeLoc, Storage);
 }
 
-Expr *ASTNodeImporter::VisitCXXDependentScopeMemberExpr(
+ExpectedStmt ASTNodeImporter::VisitCXXDependentScopeMemberExpr(
     CXXDependentScopeMemberExpr *E) {
-  Expr *Base = nullptr;
+  auto Imp = importSeq(
+      E->getType(), E->getOperatorLoc(), E->getQualifierLoc(),
+      E->getTemplateKeywordLoc(), E->getFirstQualifierFoundInScope());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  SourceLocation ToOperatorLoc, ToTemplateKeywordLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  NamedDecl *ToFirstQualifierFoundInScope;
+  std::tie(
+      ToType, ToOperatorLoc, ToQualifierLoc, ToTemplateKeywordLoc,
+      ToFirstQualifierFoundInScope) = *Imp;
+
+  Expr *ToBase = nullptr;
   if (!E->isImplicitAccess()) {
-    Base = Importer.Import(E->getBase());
-    if (!Base)
-      return nullptr;
+    if (ExpectedExpr ToBaseOrErr = import(E->getBase()))
+      ToBase = *ToBaseOrErr;
+    else
+      return ToBaseOrErr.takeError();
   }
 
-  QualType BaseType = Importer.Import(E->getBaseType());
-  if (BaseType.isNull())
-    return nullptr;
-
   TemplateArgumentListInfo ToTAInfo, *ResInfo = nullptr;
   if (E->hasExplicitTemplateArgs()) {
-    if (ImportTemplateArgumentListInfo(E->getLAngleLoc(), E->getRAngleLoc(),
-                                       E->template_arguments(), ToTAInfo))
-      return nullptr;
+    if (Error Err = ImportTemplateArgumentListInfo(
+        E->getLAngleLoc(), E->getRAngleLoc(), E->template_arguments(),
+        ToTAInfo))
+      return std::move(Err);
     ResInfo = &ToTAInfo;
   }
 
-  DeclarationName Name = Importer.Import(E->getMember());
-  if (!E->getMember().isEmpty() && Name.isEmpty())
-    return nullptr;
-
-  DeclarationNameInfo MemberNameInfo(Name, Importer.Import(E->getMemberLoc()));
+  auto ToMemberNameInfoOrErr = importSeq(E->getMember(), E->getMemberLoc());
+  if (!ToMemberNameInfoOrErr)
+    return ToMemberNameInfoOrErr.takeError();
+  DeclarationNameInfo ToMemberNameInfo(
+      std::get<0>(*ToMemberNameInfoOrErr), std::get<1>(*ToMemberNameInfoOrErr));
   // Import additional name location/type info.
-  ImportDeclarationNameLoc(E->getMemberNameInfo(), MemberNameInfo);
-  auto ToFQ = Importer.Import(E->getFirstQualifierFoundInScope());
-  if (!ToFQ && E->getFirstQualifierFoundInScope())
-    return nullptr;
+  if (Error Err = ImportDeclarationNameLoc(
+      E->getMemberNameInfo(), ToMemberNameInfo))
+    return std::move(Err);
 
   return CXXDependentScopeMemberExpr::Create(
-      Importer.getToContext(), Base, BaseType, E->isArrow(),
-      Importer.Import(E->getOperatorLoc()),
-      Importer.Import(E->getQualifierLoc()),
-      Importer.Import(E->getTemplateKeywordLoc()),
-      cast_or_null<NamedDecl>(ToFQ), MemberNameInfo, ResInfo);
+      Importer.getToContext(), ToBase, ToType, E->isArrow(), ToOperatorLoc,
+      ToQualifierLoc, ToTemplateKeywordLoc, ToFirstQualifierFoundInScope,
+      ToMemberNameInfo, ResInfo);
 }
 
-Expr *
+ExpectedStmt
 ASTNodeImporter::VisitDependentScopeDeclRefExpr(DependentScopeDeclRefExpr *E) {
-  DeclarationName Name = Importer.Import(E->getDeclName());
-  if (!E->getDeclName().isEmpty() && Name.isEmpty())
-    return nullptr;
-
-  DeclarationNameInfo NameInfo(Name, Importer.Import(E->getExprLoc()));
-  ImportDeclarationNameLoc(E->getNameInfo(), NameInfo);
+  auto Imp = importSeq(
+      E->getQualifierLoc(), E->getTemplateKeywordLoc(), E->getDeclName(),
+      E->getExprLoc(), E->getLAngleLoc(), E->getRAngleLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  NestedNameSpecifierLoc ToQualifierLoc;
+  SourceLocation ToTemplateKeywordLoc, ToExprLoc, ToLAngleLoc, ToRAngleLoc;
+  DeclarationName ToDeclName;
+  std::tie(
+      ToQualifierLoc, ToTemplateKeywordLoc, ToDeclName, ToExprLoc,
+      ToLAngleLoc, ToRAngleLoc) = *Imp;
+
+  DeclarationNameInfo ToNameInfo(ToDeclName, ToExprLoc);
+  if (Error Err = ImportDeclarationNameLoc(E->getNameInfo(), ToNameInfo))
+    return std::move(Err);
 
-  TemplateArgumentListInfo ToTAInfo(Importer.Import(E->getLAngleLoc()),
-                                    Importer.Import(E->getRAngleLoc()));
+  TemplateArgumentListInfo ToTAInfo(ToLAngleLoc, ToRAngleLoc);
   TemplateArgumentListInfo *ResInfo = nullptr;
   if (E->hasExplicitTemplateArgs()) {
-    if (ImportTemplateArgumentListInfo(E->template_arguments(), ToTAInfo))
-      return nullptr;
+    if (Error Err =
+        ImportTemplateArgumentListInfo(E->template_arguments(), ToTAInfo))
+      return std::move(Err);
     ResInfo = &ToTAInfo;
   }
 
   return DependentScopeDeclRefExpr::Create(
-      Importer.getToContext(), Importer.Import(E->getQualifierLoc()),
-      Importer.Import(E->getTemplateKeywordLoc()), NameInfo, ResInfo);
+      Importer.getToContext(), ToQualifierLoc, ToTemplateKeywordLoc,
+      ToNameInfo, ResInfo);
 }
 
-Expr *ASTNodeImporter::VisitCXXUnresolvedConstructExpr(
-    CXXUnresolvedConstructExpr *CE) {
-  unsigned NumArgs = CE->arg_size();
-
-  SmallVector<Expr *, 8> ToArgs(NumArgs);
-  if (ImportArrayChecked(CE->arg_begin(), CE->arg_end(), ToArgs.begin()))
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXUnresolvedConstructExpr(
+    CXXUnresolvedConstructExpr *E) {
+  auto Imp = importSeq(
+      E->getLParenLoc(), E->getRParenLoc(), E->getTypeSourceInfo());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToLParenLoc, ToRParenLoc;
+  TypeSourceInfo *ToTypeSourceInfo;
+  std::tie(ToLParenLoc, ToRParenLoc, ToTypeSourceInfo) = *Imp;
+
+  SmallVector<Expr *, 8> ToArgs(E->arg_size());
+  if (Error Err =
+      ImportArrayChecked(E->arg_begin(), E->arg_end(), ToArgs.begin()))
+    return std::move(Err);
 
   return CXXUnresolvedConstructExpr::Create(
-      Importer.getToContext(), Importer.Import(CE->getTypeSourceInfo()),
-      Importer.Import(CE->getLParenLoc()), llvm::makeArrayRef(ToArgs),
-      Importer.Import(CE->getRParenLoc()));
+      Importer.getToContext(), ToTypeSourceInfo, ToLParenLoc,
+      llvm::makeArrayRef(ToArgs), ToRParenLoc);
 }
 
-Expr *ASTNodeImporter::VisitUnresolvedLookupExpr(UnresolvedLookupExpr *E) {
-  auto *NamingClass =
-      cast_or_null<CXXRecordDecl>(Importer.Import(E->getNamingClass()));
-  if (E->getNamingClass() && !NamingClass)
-    return nullptr;
-
-  DeclarationName Name = Importer.Import(E->getName());
-  if (E->getName() && !Name)
-    return nullptr;
-
-  DeclarationNameInfo NameInfo(Name, Importer.Import(E->getNameLoc()));
+ExpectedStmt
+ASTNodeImporter::VisitUnresolvedLookupExpr(UnresolvedLookupExpr *E) {
+  Expected<CXXRecordDecl *> ToNamingClassOrErr = import(E->getNamingClass());
+  if (!ToNamingClassOrErr)
+    return ToNamingClassOrErr.takeError();
+
+  auto ToQualifierLocOrErr = import(E->getQualifierLoc());
+  if (!ToQualifierLocOrErr)
+    return ToQualifierLocOrErr.takeError();
+
+  auto ToNameInfoOrErr = importSeq(E->getName(), E->getNameLoc());
+  if (!ToNameInfoOrErr)
+    return ToNameInfoOrErr.takeError();
+  DeclarationNameInfo ToNameInfo(
+      std::get<0>(*ToNameInfoOrErr), std::get<1>(*ToNameInfoOrErr));
   // Import additional name location/type info.
-  ImportDeclarationNameLoc(E->getNameInfo(), NameInfo);
+  if (Error Err = ImportDeclarationNameLoc(E->getNameInfo(), ToNameInfo))
+    return std::move(Err);
 
   UnresolvedSet<8> ToDecls;
-  for (auto *D : E->decls()) {
-    if (auto *To = cast_or_null<NamedDecl>(Importer.Import(D)))
-      ToDecls.addDecl(To);
+  for (auto *D : E->decls())
+    if (auto ToDOrErr = import(D))
+      ToDecls.addDecl(cast<NamedDecl>(*ToDOrErr));
     else
-      return nullptr;
-  }
+      return ToDOrErr.takeError();
 
-  TemplateArgumentListInfo ToTAInfo, *ResInfo = nullptr;
-  if (E->hasExplicitTemplateArgs()) {
-    if (ImportTemplateArgumentListInfo(E->getLAngleLoc(), E->getRAngleLoc(),
-                                       E->template_arguments(), ToTAInfo))
-      return nullptr;
-    ResInfo = &ToTAInfo;
-  }
+  if (E->hasExplicitTemplateArgs() && E->getTemplateKeywordLoc().isValid()) {
+    TemplateArgumentListInfo ToTAInfo;
+    if (Error Err = ImportTemplateArgumentListInfo(
+        E->getLAngleLoc(), E->getRAngleLoc(), E->template_arguments(),
+        ToTAInfo))
+      return std::move(Err);
 
-  if (ResInfo || E->getTemplateKeywordLoc().isValid())
-    return UnresolvedLookupExpr::Create(
-        Importer.getToContext(), NamingClass,
-        Importer.Import(E->getQualifierLoc()),
-        Importer.Import(E->getTemplateKeywordLoc()), NameInfo, E->requiresADL(),
-        ResInfo, ToDecls.begin(), ToDecls.end());
+    ExpectedSLoc ToTemplateKeywordLocOrErr = import(E->getTemplateKeywordLoc());
+    if (!ToTemplateKeywordLocOrErr)
+      return ToTemplateKeywordLocOrErr.takeError();
 
     return UnresolvedLookupExpr::Create(
-      Importer.getToContext(), NamingClass,
-      Importer.Import(E->getQualifierLoc()), NameInfo, E->requiresADL(),
-      E->isOverloaded(), ToDecls.begin(), ToDecls.end());
+        Importer.getToContext(), *ToNamingClassOrErr, *ToQualifierLocOrErr,
+        *ToTemplateKeywordLocOrErr, ToNameInfo, E->requiresADL(), &ToTAInfo,
+        ToDecls.begin(), ToDecls.end());
   }
 
-Expr *ASTNodeImporter::VisitUnresolvedMemberExpr(UnresolvedMemberExpr *E) {
-  DeclarationName Name = Importer.Import(E->getName());
-  if (!E->getName().isEmpty() && Name.isEmpty())
-    return nullptr;
-  DeclarationNameInfo NameInfo(Name, Importer.Import(E->getNameLoc()));
+  return UnresolvedLookupExpr::Create(
+      Importer.getToContext(), *ToNamingClassOrErr, *ToQualifierLocOrErr,
+      ToNameInfo, E->requiresADL(), E->isOverloaded(), ToDecls.begin(),
+      ToDecls.end());
+}
+
+ExpectedStmt
+ASTNodeImporter::VisitUnresolvedMemberExpr(UnresolvedMemberExpr *E) {
+  auto Imp1 = importSeq(
+      E->getType(), E->getOperatorLoc(), E->getQualifierLoc(),
+      E->getTemplateKeywordLoc());
+  if (!Imp1)
+    return Imp1.takeError();
+
+  QualType ToType;
+  SourceLocation ToOperatorLoc, ToTemplateKeywordLoc;
+  NestedNameSpecifierLoc ToQualifierLoc;
+  std::tie(ToType, ToOperatorLoc, ToQualifierLoc, ToTemplateKeywordLoc) = *Imp1;
+
+  auto Imp2 = importSeq(E->getName(), E->getNameLoc());
+  if (!Imp2)
+    return Imp2.takeError();
+  DeclarationNameInfo ToNameInfo(std::get<0>(*Imp2), std::get<1>(*Imp2));
   // Import additional name location/type info.
-  ImportDeclarationNameLoc(E->getNameInfo(), NameInfo);
-
-  QualType BaseType = Importer.Import(E->getType());
-  if (!E->getType().isNull() && BaseType.isNull())
-    return nullptr;
+  if (Error Err = ImportDeclarationNameLoc(E->getNameInfo(), ToNameInfo))
+    return std::move(Err);
 
   UnresolvedSet<8> ToDecls;
-  for (Decl *D : E->decls()) {
-    if (NamedDecl *To = cast_or_null<NamedDecl>(Importer.Import(D)))
-      ToDecls.addDecl(To);
+  for (Decl *D : E->decls())
+    if (auto ToDOrErr = import(D))
+      ToDecls.addDecl(cast<NamedDecl>(*ToDOrErr));
     else
-      return nullptr;
-  }
+      return ToDOrErr.takeError();
 
   TemplateArgumentListInfo ToTAInfo;
   TemplateArgumentListInfo *ResInfo = nullptr;
   if (E->hasExplicitTemplateArgs()) {
-    if (ImportTemplateArgumentListInfo(E->template_arguments(), ToTAInfo))
-      return nullptr;
+    if (Error Err =
+        ImportTemplateArgumentListInfo(E->template_arguments(), ToTAInfo))
+      return std::move(Err);
     ResInfo = &ToTAInfo;
   }
 
-  Expr *BaseE = E->isImplicitAccess() ? nullptr : Importer.Import(E->getBase());
-  if (!BaseE && !E->isImplicitAccess() && E->getBase()) {
-    return nullptr;
+  Expr *ToBase = nullptr;
+  if (!E->isImplicitAccess()) {
+    if (ExpectedExpr ToBaseOrErr = import(E->getBase()))
+      ToBase = *ToBaseOrErr;
+    else
+      return ToBaseOrErr.takeError();
   }
 
   return UnresolvedMemberExpr::Create(
-      Importer.getToContext(), E->hasUnresolvedUsing(), BaseE, BaseType,
-      E->isArrow(), Importer.Import(E->getOperatorLoc()),
-      Importer.Import(E->getQualifierLoc()),
-      Importer.Import(E->getTemplateKeywordLoc()), NameInfo, ResInfo,
-      ToDecls.begin(), ToDecls.end());
+      Importer.getToContext(), E->hasUnresolvedUsing(), ToBase, ToType,
+      E->isArrow(), ToOperatorLoc, ToQualifierLoc, ToTemplateKeywordLoc,
+      ToNameInfo, ResInfo, ToDecls.begin(), ToDecls.end());
 }
 
-Expr *ASTNodeImporter::VisitCallExpr(CallExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  Expr *ToCallee = Importer.Import(E->getCallee());
-  if (!ToCallee && E->getCallee())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCallExpr(CallExpr *E) {
+  auto Imp = importSeq(E->getCallee(), E->getType(), E->getRParenLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  Expr *ToCallee;
+  QualType ToType;
+  SourceLocation ToRParenLoc;
+  std::tie(ToCallee, ToType, ToRParenLoc) = *Imp;
 
   unsigned NumArgs = E->getNumArgs();
-  SmallVector<Expr *, 2> ToArgs(NumArgs);
-  if (ImportContainerChecked(E->arguments(), ToArgs))
-     return nullptr;
-
-  auto **ToArgs_Copied = new (Importer.getToContext()) Expr*[NumArgs];
-
-  for (unsigned ai = 0, ae = NumArgs; ai != ae; ++ai)
-    ToArgs_Copied[ai] = ToArgs[ai];
+  llvm::SmallVector<Expr *, 2> ToArgs(NumArgs);
+  if (Error Err = ImportContainerChecked(E->arguments(), ToArgs))
+     return std::move(Err);
 
   if (const auto *OCE = dyn_cast<CXXOperatorCallExpr>(E)) {
     return new (Importer.getToContext()) CXXOperatorCallExpr(
-          Importer.getToContext(), OCE->getOperator(), ToCallee, ToArgs, T,
-          OCE->getValueKind(), Importer.Import(OCE->getRParenLoc()),
-          OCE->getFPFeatures());
-  }
-
-  return new (Importer.getToContext())
-    CallExpr(Importer.getToContext(), ToCallee,
-             llvm::makeArrayRef(ToArgs_Copied, NumArgs), T, E->getValueKind(),
-             Importer.Import(E->getRParenLoc()));
-}
-
-Optional<LambdaCapture>
-ASTNodeImporter::ImportLambdaCapture(const LambdaCapture &From) {
-  VarDecl *Var = nullptr;
-  if (From.capturesVariable()) {
-    Var = cast_or_null<VarDecl>(Importer.Import(From.getCapturedVar()));
-    if (!Var)
-      return None;
+        Importer.getToContext(), OCE->getOperator(), ToCallee, ToArgs, ToType,
+        OCE->getValueKind(), ToRParenLoc, OCE->getFPFeatures());
   }
 
-  return LambdaCapture(Importer.Import(From.getLocation()), From.isImplicit(),
-                       From.getCaptureKind(), Var,
-                       From.isPackExpansion()
-                         ? Importer.Import(From.getEllipsisLoc())
-                         : SourceLocation());
+  return new (Importer.getToContext()) CallExpr(
+      Importer.getToContext(), ToCallee, ToArgs, ToType, E->getValueKind(),
+      ToRParenLoc);
 }
 
-Expr *ASTNodeImporter::VisitLambdaExpr(LambdaExpr *LE) {
-  CXXRecordDecl *FromClass = LE->getLambdaClass();
-  auto *ToClass = dyn_cast_or_null<CXXRecordDecl>(Importer.Import(FromClass));
-  if (!ToClass)
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitLambdaExpr(LambdaExpr *E) {
+  CXXRecordDecl *FromClass = E->getLambdaClass();
+  auto ToClassOrErr = import(FromClass);
+  if (!ToClassOrErr)
+    return ToClassOrErr.takeError();
+  CXXRecordDecl *ToClass = *ToClassOrErr;
 
   // NOTE: lambda classes are created with BeingDefined flag set up.
   // It means that ImportDefinition doesn't work for them and we should fill it
   // manually.
   if (ToClass->isBeingDefined()) {
     for (auto FromField : FromClass->fields()) {
-      auto *ToField = cast_or_null<FieldDecl>(Importer.Import(FromField));
-      if (!ToField)
-        return nullptr;
+      auto ToFieldOrErr = import(FromField);
+      if (!ToFieldOrErr)
+        return ToFieldOrErr.takeError();
     }
   }
 
-  auto *ToCallOp = dyn_cast_or_null<CXXMethodDecl>(
-        Importer.Import(LE->getCallOperator()));
-  if (!ToCallOp)
-    return nullptr;
+  auto ToCallOpOrErr = import(E->getCallOperator());
+  if (!ToCallOpOrErr)
+    return ToCallOpOrErr.takeError();
 
   ToClass->completeDefinition();
 
-  unsigned NumCaptures = LE->capture_size();
-  SmallVector<LambdaCapture, 8> Captures;
-  Captures.reserve(NumCaptures);
-  for (const auto &FromCapture : LE->captures()) {
-    if (auto ToCapture = ImportLambdaCapture(FromCapture))
-      Captures.push_back(*ToCapture);
+  SmallVector<LambdaCapture, 8> ToCaptures;
+  ToCaptures.reserve(E->capture_size());
+  for (const auto &FromCapture : E->captures()) {
+    if (auto ToCaptureOrErr = import(FromCapture))
+      ToCaptures.push_back(*ToCaptureOrErr);
     else
-      return nullptr;
+      return ToCaptureOrErr.takeError();
   }
 
-  SmallVector<Expr *, 8> InitCaptures(NumCaptures);
-  if (ImportContainerChecked(LE->capture_inits(), InitCaptures))
-    return nullptr;
+  SmallVector<Expr *, 8> ToCaptureInits(E->capture_size());
+  if (Error Err = ImportContainerChecked(E->capture_inits(), ToCaptureInits))
+    return std::move(Err);
+
+  auto Imp = importSeq(
+      E->getIntroducerRange(), E->getCaptureDefaultLoc(), E->getEndLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceRange ToIntroducerRange;
+  SourceLocation ToCaptureDefaultLoc, ToEndLoc;
+  std::tie(ToIntroducerRange, ToCaptureDefaultLoc, ToEndLoc) = *Imp;
 
   return LambdaExpr::Create(
-      Importer.getToContext(), ToClass,
-      Importer.Import(LE->getIntroducerRange()), LE->getCaptureDefault(),
-      Importer.Import(LE->getCaptureDefaultLoc()), Captures,
-      LE->hasExplicitParameters(), LE->hasExplicitResultType(), InitCaptures,
-      Importer.Import(LE->getEndLoc()), LE->containsUnexpandedParameterPack());
+      Importer.getToContext(), ToClass, ToIntroducerRange,
+      E->getCaptureDefault(), ToCaptureDefaultLoc, ToCaptures,
+      E->hasExplicitParameters(), E->hasExplicitResultType(), ToCaptureInits,
+      ToEndLoc, E->containsUnexpandedParameterPack());
 }
 
-Expr *ASTNodeImporter::VisitInitListExpr(InitListExpr *ILE) {
-  QualType T = Importer.Import(ILE->getType());
-  if (T.isNull())
-    return nullptr;
 
-  SmallVector<Expr *, 4> Exprs(ILE->getNumInits());
-  if (ImportContainerChecked(ILE->inits(), Exprs))
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitInitListExpr(InitListExpr *E) {
+  auto Imp = importSeq(E->getLBraceLoc(), E->getRBraceLoc(), E->getType());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToLBraceLoc, ToRBraceLoc;
+  QualType ToType;
+  std::tie(ToLBraceLoc, ToRBraceLoc, ToType) = *Imp;
+
+  SmallVector<Expr *, 4> ToExprs(E->getNumInits());
+  if (Error Err = ImportContainerChecked(E->inits(), ToExprs))
+    return std::move(Err);
 
   ASTContext &ToCtx = Importer.getToContext();
   InitListExpr *To = new (ToCtx) InitListExpr(
-        ToCtx, Importer.Import(ILE->getLBraceLoc()),
-        Exprs, Importer.Import(ILE->getLBraceLoc()));
-  To->setType(T);
-
-  if (ILE->hasArrayFiller()) {
-    Expr *Filler = Importer.Import(ILE->getArrayFiller());
-    if (!Filler)
-      return nullptr;
-    To->setArrayFiller(Filler);
+      ToCtx, ToLBraceLoc, ToExprs, ToRBraceLoc);
+  To->setType(ToType);
+
+  if (E->hasArrayFiller()) {
+    if (ExpectedExpr ToFillerOrErr = import(E->getArrayFiller()))
+      To->setArrayFiller(*ToFillerOrErr);
+    else
+      return ToFillerOrErr.takeError();
   }
 
-  if (FieldDecl *FromFD = ILE->getInitializedFieldInUnion()) {
-    auto *ToFD = cast_or_null<FieldDecl>(Importer.Import(FromFD));
-    if (!ToFD)
-      return nullptr;
-    To->setInitializedFieldInUnion(ToFD);
+  if (FieldDecl *FromFD = E->getInitializedFieldInUnion()) {
+    if (auto ToFDOrErr = import(FromFD))
+      To->setInitializedFieldInUnion(*ToFDOrErr);
+    else
+      return ToFDOrErr.takeError();
   }
 
-  if (InitListExpr *SyntForm = ILE->getSyntacticForm()) {
-    auto *ToSyntForm = cast_or_null<InitListExpr>(Importer.Import(SyntForm));
-    if (!ToSyntForm)
-      return nullptr;
-    To->setSyntacticForm(ToSyntForm);
+  if (InitListExpr *SyntForm = E->getSyntacticForm()) {
+    if (auto ToSyntFormOrErr = import(SyntForm))
+      To->setSyntacticForm(*ToSyntFormOrErr);
+    else
+      return ToSyntFormOrErr.takeError();
   }
 
   // Copy InitListExprBitfields, which are not handled in the ctor of
   // InitListExpr.
-  To->sawArrayRangeDesignator(ILE->hadArrayRangeDesignator());
+  To->sawArrayRangeDesignator(E->hadArrayRangeDesignator());
 
   return To;
 }
 
-Expr *ASTNodeImporter::VisitCXXStdInitializerListExpr(
+ExpectedStmt ASTNodeImporter::VisitCXXStdInitializerListExpr(
     CXXStdInitializerListExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
 
-  Expr *SE = Importer.Import(E->getSubExpr());
-  if (!SE)
-    return nullptr;
+  ExpectedExpr ToSubExprOrErr = import(E->getSubExpr());
+  if (!ToSubExprOrErr)
+    return ToSubExprOrErr.takeError();
 
-  return new (Importer.getToContext()) CXXStdInitializerListExpr(T, SE);
+  return new (Importer.getToContext()) CXXStdInitializerListExpr(
+      *ToTypeOrErr, *ToSubExprOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCXXInheritedCtorInitExpr(
+ExpectedStmt ASTNodeImporter::VisitCXXInheritedCtorInitExpr(
     CXXInheritedCtorInitExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  auto *Ctor = cast_or_null<CXXConstructorDecl>(Importer.Import(
-      E->getConstructor()));
-  if (!Ctor)
-    return nullptr;
+  auto Imp = importSeq(E->getLocation(), E->getType(), E->getConstructor());
+  if (!Imp)
+    return Imp.takeError();
+
+  SourceLocation ToLocation;
+  QualType ToType;
+  CXXConstructorDecl *ToConstructor;
+  std::tie(ToLocation, ToType, ToConstructor) = *Imp;
 
   return new (Importer.getToContext()) CXXInheritedCtorInitExpr(
-      Importer.Import(E->getLocation()), T, Ctor,
-      E->constructsVBase(), E->inheritedFromVBase());
+      ToLocation, ToType, ToConstructor, E->constructsVBase(),
+      E->inheritedFromVBase());
 }
 
-Expr *ASTNodeImporter::VisitArrayInitLoopExpr(ArrayInitLoopExpr *E) {
-  QualType ToType = Importer.Import(E->getType());
-  if (ToType.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitArrayInitLoopExpr(ArrayInitLoopExpr *E) {
+  auto Imp = importSeq(E->getType(), E->getCommonExpr(), E->getSubExpr());
+  if (!Imp)
+    return Imp.takeError();
 
-  Expr *ToCommon = Importer.Import(E->getCommonExpr());
-  if (!ToCommon && E->getCommonExpr())
-    return nullptr;
+  QualType ToType;
+  Expr *ToCommonExpr, *ToSubExpr;
+  std::tie(ToType, ToCommonExpr, ToSubExpr) = *Imp;
 
-  Expr *ToSubExpr = Importer.Import(E->getSubExpr());
-  if (!ToSubExpr && E->getSubExpr())
-    return nullptr;
-
-  return new (Importer.getToContext())
-      ArrayInitLoopExpr(ToType, ToCommon, ToSubExpr);
+  return new (Importer.getToContext()) ArrayInitLoopExpr(
+      ToType, ToCommonExpr, ToSubExpr);
 }
 
-Expr *ASTNodeImporter::VisitArrayInitIndexExpr(ArrayInitIndexExpr *E) {
-  QualType ToType = Importer.Import(E->getType());
-  if (ToType.isNull())
-    return nullptr;
-  return new (Importer.getToContext()) ArrayInitIndexExpr(ToType);
+ExpectedStmt ASTNodeImporter::VisitArrayInitIndexExpr(ArrayInitIndexExpr *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+  return new (Importer.getToContext()) ArrayInitIndexExpr(*ToTypeOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCXXDefaultInitExpr(CXXDefaultInitExpr *DIE) {
-  auto *ToField = dyn_cast_or_null<FieldDecl>(Importer.Import(DIE->getField()));
-  if (!ToField && DIE->getField())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXDefaultInitExpr(CXXDefaultInitExpr *E) {
+  ExpectedSLoc ToBeginLocOrErr = import(E->getBeginLoc());
+  if (!ToBeginLocOrErr)
+    return ToBeginLocOrErr.takeError();
+
+  auto ToFieldOrErr = import(E->getField());
+  if (!ToFieldOrErr)
+    return ToFieldOrErr.takeError();
 
   return CXXDefaultInitExpr::Create(
-      Importer.getToContext(), Importer.Import(DIE->getBeginLoc()), ToField);
+      Importer.getToContext(), *ToBeginLocOrErr, *ToFieldOrErr);
 }
 
-Expr *ASTNodeImporter::VisitCXXNamedCastExpr(CXXNamedCastExpr *E) {
-  QualType ToType = Importer.Import(E->getType());
-  if (ToType.isNull() && !E->getType().isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXNamedCastExpr(CXXNamedCastExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getSubExpr(), E->getTypeInfoAsWritten(),
+      E->getOperatorLoc(), E->getRParenLoc(), E->getAngleBrackets());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  Expr *ToSubExpr;
+  TypeSourceInfo *ToTypeInfoAsWritten;
+  SourceLocation ToOperatorLoc, ToRParenLoc;
+  SourceRange ToAngleBrackets;
+  std::tie(
+      ToType, ToSubExpr, ToTypeInfoAsWritten, ToOperatorLoc, ToRParenLoc,
+      ToAngleBrackets) = *Imp;
+
   ExprValueKind VK = E->getValueKind();
   CastKind CK = E->getCastKind();
-  Expr *ToOp = Importer.Import(E->getSubExpr());
-  if (!ToOp && E->getSubExpr())
-    return nullptr;
-  CXXCastPath BasePath;
-  if (ImportCastPath(E, BasePath))
-    return nullptr;
-  TypeSourceInfo *ToWritten = Importer.Import(E->getTypeInfoAsWritten());
-  SourceLocation ToOperatorLoc = Importer.Import(E->getOperatorLoc());
-  SourceLocation ToRParenLoc = Importer.Import(E->getRParenLoc());
-  SourceRange ToAngleBrackets = Importer.Import(E->getAngleBrackets());
+  auto ToBasePathOrErr = ImportCastPath(E);
+  if (!ToBasePathOrErr)
+    return ToBasePathOrErr.takeError();
 
   if (isa<CXXStaticCastExpr>(E)) {
     return CXXStaticCastExpr::Create(
-        Importer.getToContext(), ToType, VK, CK, ToOp, &BasePath,
-        ToWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
+        Importer.getToContext(), ToType, VK, CK, ToSubExpr, &(*ToBasePathOrErr),
+        ToTypeInfoAsWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
   } else if (isa<CXXDynamicCastExpr>(E)) {
     return CXXDynamicCastExpr::Create(
-        Importer.getToContext(), ToType, VK, CK, ToOp, &BasePath,
-        ToWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
+        Importer.getToContext(), ToType, VK, CK, ToSubExpr, &(*ToBasePathOrErr),
+        ToTypeInfoAsWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
   } else if (isa<CXXReinterpretCastExpr>(E)) {
     return CXXReinterpretCastExpr::Create(
-        Importer.getToContext(), ToType, VK, CK, ToOp, &BasePath,
-        ToWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
+        Importer.getToContext(), ToType, VK, CK, ToSubExpr, &(*ToBasePathOrErr),
+        ToTypeInfoAsWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
   } else if (isa<CXXConstCastExpr>(E)) {
-    return CXXConstCastExpr::Create(Importer.getToContext(), ToType, VK, ToOp,
-                                    ToWritten, ToOperatorLoc, ToRParenLoc,
-                                    ToAngleBrackets);
+    return CXXConstCastExpr::Create(
+        Importer.getToContext(), ToType, VK, ToSubExpr, ToTypeInfoAsWritten,
+        ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
   } else {
-    return nullptr;
+    llvm_unreachable("Unknown cast type");
+    return make_error<ImportError>();
   }
 }
 
-Expr *ASTNodeImporter::VisitSubstNonTypeTemplateParmExpr(
+ExpectedStmt ASTNodeImporter::VisitSubstNonTypeTemplateParmExpr(
     SubstNonTypeTemplateParmExpr *E) {
-  QualType T = Importer.Import(E->getType());
-  if (T.isNull())
-    return nullptr;
-
-  auto *Param = cast_or_null<NonTypeTemplateParmDecl>(
-        Importer.Import(E->getParameter()));
-  if (!Param)
-    return nullptr;
-
-  Expr *Replacement = Importer.Import(E->getReplacement());
-  if (!Replacement)
-    return nullptr;
+  auto Imp = importSeq(
+      E->getType(), E->getExprLoc(), E->getParameter(), E->getReplacement());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  SourceLocation ToExprLoc;
+  NonTypeTemplateParmDecl *ToParameter;
+  Expr *ToReplacement;
+  std::tie(ToType, ToExprLoc, ToParameter, ToReplacement) = *Imp;
 
   return new (Importer.getToContext()) SubstNonTypeTemplateParmExpr(
-        T, E->getValueKind(), Importer.Import(E->getExprLoc()), Param,
-        Replacement);
+      ToType, E->getValueKind(), ToExprLoc, ToParameter, ToReplacement);
 }
 
-Expr *ASTNodeImporter::VisitTypeTraitExpr(TypeTraitExpr *E) {
-  QualType ToType = Importer.Import(E->getType());
-  if (ToType.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitTypeTraitExpr(TypeTraitExpr *E) {
+  auto Imp = importSeq(
+      E->getType(), E->getBeginLoc(), E->getEndLoc());
+  if (!Imp)
+    return Imp.takeError();
+
+  QualType ToType;
+  SourceLocation ToBeginLoc, ToEndLoc;
+  std::tie(ToType, ToBeginLoc, ToEndLoc) = *Imp;
 
   SmallVector<TypeSourceInfo *, 4> ToArgs(E->getNumArgs());
-  if (ImportContainerChecked(E->getArgs(), ToArgs))
-    return nullptr;
+  if (Error Err = ImportContainerChecked(E->getArgs(), ToArgs))
+    return std::move(Err);
 
   // According to Sema::BuildTypeTrait(), if E is value-dependent,
   // Value is always false.
-  bool ToValue = false;
-  if (!E->isValueDependent())
-    ToValue = E->getValue();
+  bool ToValue = (E->isValueDependent() ? false : E->getValue());
 
   return TypeTraitExpr::Create(
-      Importer.getToContext(), ToType, Importer.Import(E->getBeginLoc()),
-      E->getTrait(), ToArgs, Importer.Import(E->getEndLoc()), ToValue);
+      Importer.getToContext(), ToType, ToBeginLoc, E->getTrait(), ToArgs,
+      ToEndLoc, ToValue);
 }
 
-Expr *ASTNodeImporter::VisitCXXTypeidExpr(CXXTypeidExpr *E) {
-  QualType ToType = Importer.Import(E->getType());
-  if (ToType.isNull())
-    return nullptr;
+ExpectedStmt ASTNodeImporter::VisitCXXTypeidExpr(CXXTypeidExpr *E) {
+  ExpectedType ToTypeOrErr = import(E->getType());
+  if (!ToTypeOrErr)
+    return ToTypeOrErr.takeError();
+
+  auto ToSourceRangeOrErr = import(E->getSourceRange());
+  if (!ToSourceRangeOrErr)
+    return ToSourceRangeOrErr.takeError();
 
   if (E->isTypeOperand()) {
-    TypeSourceInfo *TSI = Importer.Import(E->getTypeOperandSourceInfo());
-    if (!TSI)
-      return nullptr;
-
-    return new (Importer.getToContext())
-        CXXTypeidExpr(ToType, TSI, Importer.Import(E->getSourceRange()));
+    if (auto ToTSIOrErr = import(E->getTypeOperandSourceInfo()))
+      return new (Importer.getToContext()) CXXTypeidExpr(
+          *ToTypeOrErr, *ToTSIOrErr, *ToSourceRangeOrErr);
+    else
+      return ToTSIOrErr.takeError();
   }
 
-  Expr *Op = Importer.Import(E->getExprOperand());
-  if (!Op)
-    return nullptr;
+  ExpectedExpr ToExprOperandOrErr = import(E->getExprOperand());
+  if (!ToExprOperandOrErr)
+    return ToExprOperandOrErr.takeError();
 
-  return new (Importer.getToContext())
-      CXXTypeidExpr(ToType, Op, Importer.Import(E->getSourceRange()));
+  return new (Importer.getToContext()) CXXTypeidExpr(
+      *ToTypeOrErr, *ToExprOperandOrErr, *ToSourceRangeOrErr);
 }
 
 void ASTNodeImporter::ImportOverrides(CXXMethodDecl *ToMethod,
                                       CXXMethodDecl *FromMethod) {
-  for (auto *FromOverriddenMethod : FromMethod->overridden_methods())
-    ToMethod->addOverriddenMethod(
-      cast<CXXMethodDecl>(Importer.Import(const_cast<CXXMethodDecl*>(
-                                            FromOverriddenMethod))));
+  for (auto *FromOverriddenMethod : FromMethod->overridden_methods()) {
+    if (auto ImportedOrErr = import(FromOverriddenMethod))
+      ToMethod->getCanonicalDecl()->addOverriddenMethod(cast<CXXMethodDecl>(
+          (*ImportedOrErr)->getCanonicalDecl()));
+    else
+      consumeError(ImportedOrErr.takeError());
+  }
 }
 
 ASTImporter::ASTImporter(ASTContext &ToContext, FileManager &ToFileManager,
                          ASTContext &FromContext, FileManager &FromFileManager,
                          bool MinimalImport)
     : ToContext(ToContext), FromContext(FromContext),
       ToFileManager(ToFileManager), FromFileManager(FromFileManager),
       Minimal(MinimalImport) {
   ImportedDecls[FromContext.getTranslationUnitDecl()]
     = ToContext.getTranslationUnitDecl();
 }
 
 ASTImporter::~ASTImporter() = default;
 
 QualType ASTImporter::Import(QualType FromT) {
   if (FromT.isNull())
     return {};
 
-  const Type *fromTy = FromT.getTypePtr();
+  const Type *FromTy = FromT.getTypePtr();
 
   // Check whether we've already imported this type.
   llvm::DenseMap<const Type *, const Type *>::iterator Pos
-    = ImportedTypes.find(fromTy);
+    = ImportedTypes.find(FromTy);
   if (Pos != ImportedTypes.end())
     return ToContext.getQualifiedType(Pos->second, FromT.getLocalQualifiers());
 
   // Import the type
   ASTNodeImporter Importer(*this);
-  QualType ToT = Importer.Visit(fromTy);
-  if (ToT.isNull())
-    return ToT;
+  ExpectedType ToTOrErr = Importer.Visit(FromTy);
+  if (!ToTOrErr) {
+    llvm::consumeError(ToTOrErr.takeError());
+    return {};
+  }
 
   // Record the imported type.
-  ImportedTypes[fromTy] = ToT.getTypePtr();
+  ImportedTypes[FromTy] = (*ToTOrErr).getTypePtr();
 
-  return ToContext.getQualifiedType(ToT, FromT.getLocalQualifiers());
+  return ToContext.getQualifiedType(*ToTOrErr, FromT.getLocalQualifiers());
 }
 
 TypeSourceInfo *ASTImporter::Import(TypeSourceInfo *FromTSI) {
   if (!FromTSI)
     return FromTSI;
 
   // FIXME: For now we just create a "trivial" type source info based
   // on the type and a single location. Implement a real version of this.
Attr *ASTImporter::Import(const Attr *FromAttr) {
   return ToAttr;
 }
 
 Decl *ASTImporter::GetAlreadyImportedOrNull(Decl *FromD) {
   llvm::DenseMap<Decl *, Decl *>::iterator Pos = ImportedDecls.find(FromD);
   if (Pos != ImportedDecls.end()) {
     Decl *ToD = Pos->second;
     // FIXME: move this call to ImportDeclParts().
-    ASTNodeImporter(*this).ImportDefinitionIfNeeded(FromD, ToD);
+    if (Error Err = ASTNodeImporter(*this).ImportDefinitionIfNeeded(FromD, ToD))
+      llvm::consumeError(std::move(Err));
     return ToD;
   } else {
     return nullptr;
   }
 }
 
 Decl *ASTImporter::Import(Decl *FromD) {
   if (!FromD)
     return nullptr;
 
   ASTNodeImporter Importer(*this);
 
   // Check whether we've already imported this declaration.
   Decl *ToD = GetAlreadyImportedOrNull(FromD);
   if (ToD) {
     // If FromD has some updated flags after last import, apply it
     updateFlags(FromD, ToD);
     return ToD;
   }
 
   // Import the type.
-  ToD = Importer.Visit(FromD);
-  if (!ToD)
+  ExpectedDecl ToDOrErr = Importer.Visit(FromD);
+  if (!ToDOrErr) {
+    llvm::consumeError(ToDOrErr.takeError());
     return nullptr;
+  }
+  ToD = *ToDOrErr;
 
   // Notify subclasses.
   Imported(FromD, ToD);
 
   updateFlags(FromD, ToD);
   return ToD;
 }
 
-DeclContext *ASTImporter::ImportContext(DeclContext *FromDC) {
+Expected<DeclContext *> ASTImporter::ImportContext(DeclContext *FromDC) {
   if (!FromDC)
     return FromDC;
 
   auto *ToDC = cast_or_null<DeclContext>(Import(cast<Decl>(FromDC)));
   if (!ToDC)
     return nullptr;
 
   // When we're using a record/enum/Objective-C class/protocol as a context, we
   // need it to have a definition.
   if (auto *ToRecord = dyn_cast<RecordDecl>(ToDC)) {
     auto *FromRecord = cast<RecordDecl>(FromDC);
     if (ToRecord->isCompleteDefinition()) {
       // Do nothing.
     } else if (FromRecord->isCompleteDefinition()) {
-      ASTNodeImporter(*this).ImportDefinition(FromRecord, ToRecord,
-                                              ASTNodeImporter::IDK_Basic);
+      if (Error Err = ASTNodeImporter(*this).ImportDefinition(
+          FromRecord, ToRecord, ASTNodeImporter::IDK_Basic))
+        return std::move(Err);
     } else {
       CompleteDecl(ToRecord);
     }
   } else if (auto *ToEnum = dyn_cast<EnumDecl>(ToDC)) {
     auto *FromEnum = cast<EnumDecl>(FromDC);
     if (ToEnum->isCompleteDefinition()) {
       // Do nothing.
     } else if (FromEnum->isCompleteDefinition()) {
-      ASTNodeImporter(*this).ImportDefinition(FromEnum, ToEnum,
-                                              ASTNodeImporter::IDK_Basic);
+      if (Error Err = ASTNodeImporter(*this).ImportDefinition(
+          FromEnum, ToEnum, ASTNodeImporter::IDK_Basic))
+        return std::move(Err);
     } else {
       CompleteDecl(ToEnum);
     }
   } else if (auto *ToClass = dyn_cast<ObjCInterfaceDecl>(ToDC)) {
     auto *FromClass = cast<ObjCInterfaceDecl>(FromDC);
     if (ToClass->getDefinition()) {
       // Do nothing.
     } else if (ObjCInterfaceDecl *FromDef = FromClass->getDefinition()) {
-      ASTNodeImporter(*this).ImportDefinition(FromDef, ToClass,
-                                              ASTNodeImporter::IDK_Basic);
+      if (Error Err = ASTNodeImporter(*this).ImportDefinition(
+          FromDef, ToClass, ASTNodeImporter::IDK_Basic))
+        return std::move(Err);
     } else {
       CompleteDecl(ToClass);
     }
   } else if (auto *ToProto = dyn_cast<ObjCProtocolDecl>(ToDC)) {
     auto *FromProto = cast<ObjCProtocolDecl>(FromDC);
     if (ToProto->getDefinition()) {
       // Do nothing.
     } else if (ObjCProtocolDecl *FromDef = FromProto->getDefinition()) {
-      ASTNodeImporter(*this).ImportDefinition(FromDef, ToProto,
-                                              ASTNodeImporter::IDK_Basic);
+      if (Error Err = ASTNodeImporter(*this).ImportDefinition(
+          FromDef, ToProto, ASTNodeImporter::IDK_Basic))
+        return std::move(Err);
     } else {
       CompleteDecl(ToProto);
     }
   }
 
   return ToDC;
 }
 
 Expr *ASTImporter::Import(Expr *FromE) {
   if (!FromE)
     return nullptr;
 
   return cast_or_null<Expr>(Import(cast<Stmt>(FromE)));
 }
 
 Stmt *ASTImporter::Import(Stmt *FromS) {
   if (!FromS)
     return nullptr;
 
   // Check whether we've already imported this declaration.
   llvm::DenseMap<Stmt *, Stmt *>::iterator Pos = ImportedStmts.find(FromS);
   if (Pos != ImportedStmts.end())
     return Pos->second;
 
-  // Import the type
+  // Import the statement.
   ASTNodeImporter Importer(*this);
-  Stmt *ToS = Importer.Visit(FromS);
-  if (!ToS)
+  ExpectedStmt ToSOrErr = Importer.Visit(FromS);
+  if (!ToSOrErr) {
+    llvm::consumeError(ToSOrErr.takeError());
     return nullptr;
+  }
 
-  if (auto *ToE = dyn_cast<Expr>(ToS)) {
+  if (auto *ToE = dyn_cast<Expr>(*ToSOrErr)) {
     auto *FromE = cast<Expr>(FromS);
     // Copy ExprBitfields, which may not be handled in Expr subclasses
     // constructors.
     ToE->setValueKind(FromE->getValueKind());
     ToE->setObjectKind(FromE->getObjectKind());
     ToE->setTypeDependent(FromE->isTypeDependent());
     ToE->setValueDependent(FromE->isValueDependent());
     ToE->setInstantiationDependent(FromE->isInstantiationDependent());
     ToE->setContainsUnexpandedParameterPack(
         FromE->containsUnexpandedParameterPack());
   }
 
   // Record the imported declaration.
-  ImportedStmts[FromS] = ToS;
-  return ToS;
+  ImportedStmts[FromS] = *ToSOrErr;
+  return *ToSOrErr;
 }
 
 NestedNameSpecifier *ASTImporter::Import(NestedNameSpecifier *FromNNS) {
   if (!FromNNS)
     return nullptr;
 
   NestedNameSpecifier *prefix = Import(FromNNS->getPrefix());
 
SubstTemplateTemplateParmPackStorage *SubstPack
       = From.getAsSubstTemplateTemplateParmPack();
     auto *Param =
         cast_or_null<TemplateTemplateParmDecl>(
             Import(SubstPack->getParameterPack()));
     if (!Param)
       return {};
 
     ASTNodeImporter Importer(*this);
-    TemplateArgument ArgPack
+    Expected<TemplateArgument> ArgPack
       = Importer.ImportTemplateArgument(SubstPack->getArgumentPack());
-    if (ArgPack.isNull())
+    if (!ArgPack) {
+      llvm::consumeError(ArgPack.takeError());
       return {};
+    }
 
-    return ToContext.getSubstTemplateTemplateParmPack(Param, ArgPack);
+    return ToContext.getSubstTemplateTemplateParmPack(Param, *ArgPack);
   }
   }
 
   llvm_unreachable("Invalid template name kind");
 }
 
 SourceLocation ASTImporter::Import(SourceLocation FromLoc) {
   if (FromLoc.isInvalid())
CXXBaseSpecifier *Imported = new (ToContext) CXXBaseSpecifier(
         BaseSpec->isVirtual(), BaseSpec->isBaseOfClass(),
         BaseSpec->getAccessSpecifierAsWritten(),
         Import(BaseSpec->getTypeSourceInfo()),
         Import(BaseSpec->getEllipsisLoc()));
   ImportedCXXBaseSpecifiers[BaseSpec] = Imported;
   return Imported;
 }
 
-void ASTImporter::ImportDefinition(Decl *From) {
+Error ASTImporter::ImportDefinition_New(Decl *From) {
   Decl *To = Import(From);
   if (!To)
-    return;
+    return llvm::make_error<ImportError>();
 
   if (auto *FromDC = cast<DeclContext>(From)) {
     ASTNodeImporter Importer(*this);
 
     if (auto *ToRecord = dyn_cast<RecordDecl>(To)) {
       if (!ToRecord->getDefinition()) {
-        Importer.ImportDefinition(cast<RecordDecl>(FromDC), ToRecord,
-                                  ASTNodeImporter::IDK_Everything);
-        return;
+        return Importer.ImportDefinition(
+            cast<RecordDecl>(FromDC), ToRecord,
       }
     }
 
     if (auto *ToEnum = dyn_cast<EnumDecl>(To)) {
       if (!ToEnum->getDefinition()) {
-        Importer.ImportDefinition(cast<EnumDecl>(FromDC), ToEnum,
-                                  ASTNodeImporter::IDK_Everything);
-        return;
+        return Importer.ImportDefinition(
+            cast<EnumDecl>(FromDC), ToEnum, ASTNodeImporter::IDK_Everything);
       }
     }
 
     if (auto *ToIFace = dyn_cast<ObjCInterfaceDecl>(To)) {
       if (!ToIFace->getDefinition()) {
-        Importer.ImportDefinition(cast<ObjCInterfaceDecl>(FromDC), ToIFace,
-                                  ASTNodeImporter::IDK_Everything);
-        return;
+        return Importer.ImportDefinition(
+            cast<ObjCInterfaceDecl>(FromDC), ToIFace,
       }
     }
 
     if (auto *ToProto = dyn_cast<ObjCProtocolDecl>(To)) {
       if (!ToProto->getDefinition()) {
-        Importer.ImportDefinition(cast<ObjCProtocolDecl>(FromDC), ToProto,
-                                  ASTNodeImporter::IDK_Everything);
-        return;
+        return Importer.ImportDefinition(
+            cast<ObjCProtocolDecl>(FromDC), ToProto,
       }
     }
 
-    Importer.ImportDeclContext(FromDC, true);
-  }
+    return Importer.ImportDeclContext(FromDC, true);
+  }
+
+  return Error::success();
+
+void ASTImporter::ImportDefinition(Decl *From) {
+  Error Err = ImportDefinition_New(From);
+  llvm::consumeError(std::move(Err));
 }
 
 DeclarationName ASTImporter::Import(DeclarationName FromName) {
   if (!FromName)
     return {};
 
   switch (FromName.getNameKind()) {
   case DeclarationName::Identifier:

cfe/trunk/lib/AST/ExternalASTMerger.cpp

void ExternalASTMerger::CompleteType(TagDecl *Tag) {
   ForEachMatchingDC(Tag, [&](ASTImporter &Forward, ASTImporter &Reverse,
                              Source<const DeclContext *> SourceDC) -> bool {
     auto *SourceTag = const_cast<TagDecl *>(cast<TagDecl>(SourceDC.get()));
     if (SourceTag->hasExternalLexicalStorage())
       SourceTag->getASTContext().getExternalSource()->CompleteType(SourceTag);
     if (!SourceTag->getDefinition())
       return false;
     Forward.MapImported(SourceTag, Tag);
-    Forward.ImportDefinition(SourceTag);
+    if (llvm::Error Err = Forward.ImportDefinition_New(SourceTag))
+      llvm::consumeError(std::move(Err));
     Tag->setCompleteDefinition(SourceTag->isCompleteDefinition());
     return true;
   });
 }
 
 void ExternalASTMerger::CompleteType(ObjCInterfaceDecl *Interface) {
   assert(Interface->hasExternalLexicalStorage());
   ForEachMatchingDC(
       Interface, [&](ASTImporter &Forward, ASTImporter &Reverse,
                      Source<const DeclContext *> SourceDC) -> bool {
         auto *SourceInterface = const_cast<ObjCInterfaceDecl *>(
             cast<ObjCInterfaceDecl>(SourceDC.get()));
         if (SourceInterface->hasExternalLexicalStorage())
           SourceInterface->getASTContext().getExternalSource()->CompleteType(
               SourceInterface);
         if (!SourceInterface->getDefinition())
           return false;
         Forward.MapImported(SourceInterface, Interface);
-        Forward.ImportDefinition(SourceInterface);
+        if (llvm::Error Err = Forward.ImportDefinition_New(SourceInterface))
+          llvm::consumeError(std::move(Err));
         return true;
       });
 }
 
 bool ExternalASTMerger::CanComplete(DeclContext *Interface) {
   assert(Interface->hasExternalLexicalStorage() ||
          Interface->hasExternalVisibleStorage());
   bool FoundMatchingDC = false;

cfe/trunk/unittests/AST/ASTImporterTest.cpp

TEST_P(ASTImporterTestBase, ImportUnnamedFieldsInCorrectOrder) {
   EXPECT_TRUE(ToLambda);
 
   // Check if the fields of the lambda class are imported in correct order.
   unsigned FromIndex = 0u;
   for (auto *FromField : FromLambda->fields()) {
     ASSERT_FALSE(FromField->getDeclName());
     auto *ToField = cast_or_null<FieldDecl>(Import(FromField, Lang_CXX11));
     EXPECT_TRUE(ToField);
-    unsigned ToIndex = ASTImporter::getFieldIndex(ToField);
-    EXPECT_EQ(ToIndex, FromIndex + 1);
+    Optional<unsigned> ToIndex = ASTImporter::getFieldIndex(ToField);
+    EXPECT_TRUE(ToIndex);
+    EXPECT_EQ(*ToIndex, FromIndex);
     ++FromIndex;
   }
 
   EXPECT_EQ(FromIndex, 3u);
 }
 
 TEST_P(
     ASTImporterTestBase,