Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -58,7 +58,7 @@
     QualType VisitExtVectorType(const ExtVectorType *T);
     QualType VisitFunctionNoProtoType(const FunctionNoProtoType *T);
     QualType VisitFunctionProtoType(const FunctionProtoType *T);
-    // FIXME: UnresolvedUsingType
+    QualType VisitUnresolvedUsingType(const UnresolvedUsingType *T);
     QualType VisitParenType(const ParenType *T);
     QualType VisitTypedefType(const TypedefType *T);
     QualType VisitTypeOfExprType(const TypeOfExprType *T);
@@ -128,8 +128,8 @@
     TemplateParameterList *ImportTemplateParameterList(
                                                  TemplateParameterList *Params);
     TemplateArgument ImportTemplateArgument(const TemplateArgument &From);
-    TemplateArgumentLoc ImportTemplateArgumentLoc(
-        const TemplateArgumentLoc &TALoc, bool &Error);
+    Optional<TemplateArgumentLoc> ImportTemplateArgumentLoc(
+        const TemplateArgumentLoc &TALoc);
     bool ImportTemplateArguments(const TemplateArgument *FromArgs,
                                  unsigned NumFromArgs,
                                SmallVectorImpl<TemplateArgument> &ToArgs);
@@ -142,10 +142,12 @@
     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);
@@ -170,6 +172,12 @@
     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);
@@ -563,6 +571,22 @@
   return Importer.getToContext().getFunctionType(ToResultType, ArgTypes, ToEPI);
 }
 
+QualType ASTNodeImporter::VisitUnresolvedUsingType(
+    const UnresolvedUsingType *T) {
+  UnresolvedUsingTypenameDecl *ToD = cast_or_null<UnresolvedUsingTypenameDecl>(
+        Importer.Import(T->getDecl()));
+  if (!ToD)
+    return QualType();
+
+  UnresolvedUsingTypenameDecl *ToPrevD =
+      cast_or_null<UnresolvedUsingTypenameDecl>(
+        Importer.Import(T->getDecl()->getPreviousDecl()));
+  if (!ToPrevD && T->getDecl()->getPreviousDecl())
+    return QualType();
+
+  return Importer.getToContext().getTypeDeclType(ToD, ToPrevD);
+}
+
 QualType ASTNodeImporter::VisitParenType(const ParenType *T) {
   QualType ToInnerType = Importer.Import(T->getInnerType());
   if (ToInnerType.isNull())
@@ -1163,9 +1187,8 @@
   llvm_unreachable("Invalid template argument kind");
 }
 
-TemplateArgumentLoc ASTNodeImporter::ImportTemplateArgumentLoc(
-    const TemplateArgumentLoc &TALoc, bool &Error) {
-  Error = false;
+Optional<TemplateArgumentLoc>
+ASTNodeImporter::ImportTemplateArgumentLoc(const TemplateArgumentLoc &TALoc) {
   TemplateArgument Arg = ImportTemplateArgument(TALoc.getArgument());
   TemplateArgumentLocInfo FromInfo = TALoc.getLocInfo();
   TemplateArgumentLocInfo ToInfo;
@@ -1173,12 +1196,12 @@
     Expr *E = Importer.Import(FromInfo.getAsExpr());
     ToInfo = TemplateArgumentLocInfo(E);
     if (!E)
-      Error = true;
+      return None;
   } else if (Arg.getKind() == TemplateArgument::Type) {
     if (TypeSourceInfo *TSI = Importer.Import(FromInfo.getAsTypeSourceInfo()))
       ToInfo = TemplateArgumentLocInfo(TSI);
     else
-      Error = true;
+      return None;
   } else {
     ToInfo = TemplateArgumentLocInfo(
           Importer.Import(FromInfo.getTemplateQualifierLoc()),
@@ -1268,6 +1291,29 @@
   return nullptr;
 }
 
+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;
+  }
+
+  // Import the location of this declaration.
+  SourceLocation Loc = Importer.Import(D->getLocation());
+
+  EmptyDecl *ToD = EmptyDecl::Create(Importer.getToContext(), DC, Loc);
+  ToD->setLexicalDeclContext(LexicalDC);
+  Importer.Imported(D, ToD);
+  LexicalDC->addDeclInternal(ToD);
+  return ToD;
+}
+
 Decl *ASTNodeImporter::VisitTranslationUnitDecl(TranslationUnitDecl *D) {
   TranslationUnitDecl *ToD = 
     Importer.getToContext().getTranslationUnitDecl();
@@ -1402,6 +1448,44 @@
   return ToNamespace;
 }
 
+Decl *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 (LookupD)
+    return LookupD;
+
+  // NOTE: No conflict resolution is done for namespace aliases now.
+
+  NamespaceDecl *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;
+
+  NamespaceAliasDecl *ToD = NamespaceAliasDecl::Create(
+        Importer.getToContext(), DC, Importer.Import(D->getNamespaceLoc()),
+        Importer.Import(D->getAliasLoc()), ToII, ToQLoc,
+        Importer.Import(D->getTargetNameLoc()), TargetDecl);
+
+  ToD->setLexicalDeclContext(LexicalDC);
+  Importer.Imported(D, ToD);
+  LexicalDC->addDeclInternal(ToD);
+
+  return ToD;
+}
+
 Decl *ASTNodeImporter::VisitTypedefNameDecl(TypedefNameDecl *D, bool IsAlias) {
   // Import the major distinguishing characteristics of this typedef.
   DeclContext *DC, *LexicalDC;
@@ -1734,6 +1818,31 @@
       }
       D2 = D2CXX;
       D2->setAccess(D->getAccess());
+
+      Importer.Imported(D, D2);
+
+      if (ClassTemplateDecl *FromDescribed =
+          DCXX->getDescribedClassTemplate()) {
+        ClassTemplateDecl *ToDescribed = cast_or_null<ClassTemplateDecl>(
+              Importer.Import(FromDescribed));
+        if (!ToDescribed)
+          return nullptr;
+        D2CXX->setDescribedClassTemplate(ToDescribed);
+
+      } else if (MemberSpecializationInfo *MemberInfo =
+                   DCXX->getMemberSpecializationInfo()) {
+        TemplateSpecializationKind SK =
+            MemberInfo->getTemplateSpecializationKind();
+        CXXRecordDecl *FromInst = DCXX->getInstantiatedFromMemberClass();
+        CXXRecordDecl *ToInst =
+            cast_or_null<CXXRecordDecl>(Importer.Import(FromInst));
+        if (FromInst && !ToInst)
+          return nullptr;
+        D2CXX->setInstantiationOfMemberClass(ToInst, SK);
+        D2CXX->getMemberSpecializationInfo()->setPointOfInstantiation(
+              Importer.Import(MemberInfo->getPointOfInstantiation()));
+      }
+
     } else {
       D2 = RecordDecl::Create(Importer.getToContext(), D->getTagKind(),
                               DC, StartLoc, Loc, Name.getAsIdentifierInfo());
@@ -2852,6 +2961,176 @@
   return ToLinkageSpec;
 }
 
+Decl *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 (ToD)
+    return ToD;
+
+  DeclarationNameInfo NameInfo(Name,
+                               Importer.Import(D->getNameInfo().getLoc()));
+  ImportDeclarationNameLoc(D->getNameInfo(), NameInfo);
+
+  UsingDecl *ToUsing = UsingDecl::Create(Importer.getToContext(), DC,
+                                         Importer.Import(D->getUsingLoc()),
+                                         Importer.Import(D->getQualifierLoc()),
+                                         NameInfo, D->hasTypename());
+  ToUsing->setLexicalDeclContext(LexicalDC);
+  LexicalDC->addDeclInternal(ToUsing);
+  Importer.Imported(D, ToUsing);
+
+  if (NamedDecl *FromPattern =
+      Importer.getFromContext().getInstantiatedFromUsingDecl(D)) {
+    if (NamedDecl *ToPattern =
+        dyn_cast_or_null<NamedDecl>(Importer.Import(FromPattern)))
+      Importer.getToContext().setInstantiatedFromUsingDecl(ToUsing, ToPattern);
+    else
+      return nullptr;
+  }
+
+  for (UsingShadowDecl *FromShadow : D->shadows()) {
+    if (UsingShadowDecl *ToShadow =
+        dyn_cast_or_null<UsingShadowDecl>(Importer.Import(FromShadow)))
+      ToUsing->addShadowDecl(ToShadow);
+    else
+      // FIXME: We return a nullptr here but the definition is already created
+      // and available with lookups. How to fix this?..
+      return nullptr;
+  }
+  return ToUsing;
+}
+
+Decl *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 (ToD)
+    return ToD;
+
+  UsingDecl *ToUsing = dyn_cast_or_null<UsingDecl>(
+        Importer.Import(D->getUsingDecl()));
+  if (!ToUsing)
+    return nullptr;
+
+  NamedDecl *ToTarget = dyn_cast_or_null<NamedDecl>(
+        Importer.Import(D->getTargetDecl()));
+  if (!ToTarget)
+    return nullptr;
+
+  UsingShadowDecl *ToShadow = UsingShadowDecl::Create(
+        Importer.getToContext(), DC, Loc, ToUsing, ToTarget);
+
+  ToShadow->setLexicalDeclContext(LexicalDC);
+  ToShadow->setAccess(D->getAccess());
+  Importer.Imported(D, ToShadow);
+
+  if (UsingShadowDecl *FromPattern =
+      Importer.getFromContext().getInstantiatedFromUsingShadowDecl(D)) {
+    if (UsingShadowDecl *ToPattern =
+        dyn_cast_or_null<UsingShadowDecl>(Importer.Import(FromPattern)))
+      Importer.getToContext().setInstantiatedFromUsingShadowDecl(ToShadow,
+                                                                 ToPattern);
+    else
+      return nullptr;
+  }
+
+  LexicalDC->addDeclInternal(ToShadow);
+
+  return ToShadow;
+}
+
+
+Decl *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 (ToD)
+    return ToD;
+
+  DeclContext *ToComAncestor = Importer.ImportContext(D->getCommonAncestor());
+  if (!ToComAncestor)
+    return nullptr;
+
+  NamespaceDecl *ToNominated = cast_or_null<NamespaceDecl>(
+        Importer.Import(D->getNominatedNamespace()));
+  if (!ToNominated)
+    return nullptr;
+
+  UsingDirectiveDecl *ToUsingDir = UsingDirectiveDecl::Create(
+        Importer.getToContext(), DC, Importer.Import(D->getUsingLoc()),
+        Importer.Import(D->getNamespaceKeyLocation()),
+        Importer.Import(D->getQualifierLoc()),
+        Importer.Import(D->getIdentLocation()), ToNominated, ToComAncestor);
+  ToUsingDir->setLexicalDeclContext(LexicalDC);
+  LexicalDC->addDeclInternal(ToUsingDir);
+  Importer.Imported(D, ToUsingDir);
+
+  return ToUsingDir;
+}
+
+Decl *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 (ToD)
+    return ToD;
+
+  DeclarationNameInfo NameInfo(Name, Importer.Import(D->getNameInfo().getLoc()));
+  ImportDeclarationNameLoc(D->getNameInfo(), NameInfo);
+
+  UnresolvedUsingValueDecl *ToUsingValue = UnresolvedUsingValueDecl::Create(
+        Importer.getToContext(), DC, Importer.Import(D->getUsingLoc()),
+        Importer.Import(D->getQualifierLoc()), NameInfo,
+        Importer.Import(D->getEllipsisLoc()));
+
+  Importer.Imported(D, ToUsingValue);
+  ToUsingValue->setAccess(D->getAccess());
+  ToUsingValue->setLexicalDeclContext(LexicalDC);
+  LexicalDC->addDeclInternal(ToUsingValue);
+
+  return ToUsingValue;
+}
+
+Decl *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 (ToD)
+    return ToD;
+
+  UnresolvedUsingTypenameDecl *ToUsing = UnresolvedUsingTypenameDecl::Create(
+        Importer.getToContext(), DC, Importer.Import(D->getUsingLoc()),
+        Importer.Import(D->getTypenameLoc()),
+        Importer.Import(D->getQualifierLoc()), Loc, Name,
+        Importer.Import(D->getEllipsisLoc()));
+
+  Importer.Imported(D, ToUsing);
+  ToUsing->setAccess(D->getAccess());
+  ToUsing->setLexicalDeclContext(LexicalDC);
+  LexicalDC->addDeclInternal(ToUsing);
+
+  return ToUsing;
+}
+
+
 bool ASTNodeImporter::ImportDefinition(ObjCInterfaceDecl *From, 
                                        ObjCInterfaceDecl *To,
                                        ImportDefinitionKind Kind) {
@@ -3536,11 +3815,10 @@
       TemplateArgumentListInfo ToTAInfo;
       auto &ASTTemplateArgs = *PartialSpec->getTemplateArgsAsWritten();
       for (unsigned I = 0, E = ASTTemplateArgs.NumTemplateArgs; I < E; ++I) {
-        bool Error = false;
-        auto ToLoc = ImportTemplateArgumentLoc(ASTTemplateArgs[I], Error);
-        if (Error)
+        if (auto ToLoc = ImportTemplateArgumentLoc(ASTTemplateArgs[I]))
+          ToTAInfo.addArgument(*ToLoc);
+        else
           return nullptr;
-        ToTAInfo.addArgument(ToLoc);
       }
 
       QualType CanonInjType = Importer.Import(
@@ -4419,11 +4697,10 @@
   TemplateArgumentListInfo *ResInfo = nullptr;
   if (E->hasExplicitTemplateArgs()) {
     for (const auto &FromLoc : E->template_arguments()) {
-      bool Error = false;
-      TemplateArgumentLoc ToTALoc = ImportTemplateArgumentLoc(FromLoc, Error);
-      if (Error)
+      if (auto ToTALoc = ImportTemplateArgumentLoc(FromLoc))
+        ToTAInfo.addArgument(*ToTALoc);
+      else
         return nullptr;
-      ToTAInfo.addArgument(ToTALoc);
     }
     ResInfo = &ToTAInfo;
   }
Index: test/ASTMerge/namespace/Inputs/namespace1.cpp
===================================================================
--- test/ASTMerge/namespace/Inputs/namespace1.cpp
+++ test/ASTMerge/namespace/Inputs/namespace1.cpp
@@ -15,3 +15,13 @@
 namespace N3 {
   extern float z;
 }
+
+namespace AliasWithSameName = N3;
+
+namespace TestUnresolvedTypenameAndValueDecls {
+template <class T> class Base {
+public:
+  typedef T foo;
+  void bar();
+};
+}
Index: test/ASTMerge/namespace/Inputs/namespace2.cpp
===================================================================
--- test/ASTMerge/namespace/Inputs/namespace2.cpp
+++ test/ASTMerge/namespace/Inputs/namespace2.cpp
@@ -15,3 +15,46 @@
 namespace N3 {
   extern double z;
 }
+
+namespace Enclosing {
+namespace Nested {
+  const int z = 4;
+}
+}
+
+namespace ContainsInline {
+  inline namespace Inline {
+    const int z = 10;
+  }
+}
+
+namespace TestAliasName = Enclosing::Nested;
+// NOTE: There is no warning on this alias.
+namespace AliasWithSameName = Enclosing::Nested;
+
+namespace TestUsingDecls {
+
+namespace A {
+void foo();
+}
+namespace B {
+using A::foo; // <- a UsingDecl creating a UsingShadow
+}
+
+}// end namespace TestUsingDecls
+
+namespace TestUnresolvedTypenameAndValueDecls {
+
+template <class T> class Base;
+template <class T> class Derived : public Base<T> {
+public:
+  using typename Base<T>::foo;
+  using Base<T>::bar;
+  typedef typename Derived::foo NewUnresolvedUsingType;
+};
+
+} // end namespace TestUnresolvedTypenameAndValueDecls
+
+namespace TestUsingNamespace {
+  using namespace Enclosing;
+}
Index: test/ASTMerge/namespace/test.cpp
===================================================================
--- test/ASTMerge/namespace/test.cpp
+++ test/ASTMerge/namespace/test.cpp
@@ -1,6 +1,17 @@
-// RUN: %clang_cc1 -emit-pch -o %t.1.ast %S/Inputs/namespace1.cpp
-// RUN: %clang_cc1 -emit-pch -o %t.2.ast %S/Inputs/namespace2.cpp
-// RUN: not %clang_cc1 -ast-merge %t.1.ast -ast-merge %t.2.ast -fsyntax-only %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -emit-pch -std=c++1z -o %t.1.ast %S/Inputs/namespace1.cpp
+// RUN: %clang_cc1 -emit-pch -std=c++1z -o %t.2.ast %S/Inputs/namespace2.cpp
+// RUN: not %clang_cc1 -std=c++1z -ast-merge %t.1.ast -ast-merge %t.2.ast -fsyntax-only %s 2>&1 | FileCheck %s
+
+static_assert(TestAliasName::z == 4);
+static_assert(ContainsInline::z == 10);
+
+void testImport() {
+  typedef TestUnresolvedTypenameAndValueDecls::Derived<int> Imported;
+  Imported a; // Successfull instantiation
+  static_assert(sizeof(Imported::foo) == sizeof(int));
+  static_assert(sizeof(TestUnresolvedTypenameAndValueDecls::Derived<double>::NewUnresolvedUsingType) == sizeof(double));
+}
+
 
 // CHECK: namespace2.cpp:16:17: error: external variable 'z' declared with incompatible types in different translation units ('double' vs. 'float')
 // CHECK: namespace1.cpp:16:16: note: declared here with type 'float'