[Concept] Placeholder constraints and abbreviated templates

[saar.raz]

Summary

This patch implements P1141R2 "Yet another approach for constrained declarations", and is the last in the (initial) series for implementing C++2a concepts.

General strategy for this patch was:

• Expand AutoType to include optional type-constraint, reflecting the wording and easing the integration of constraints.
• Recognize AutoTypes used in functions parameters in ActOnFunctionDeclarator (the first place where we have the required information to fetch the correct template parameter list to append the invented parameters to) and fix the function and parameter types there.

Based on D60939.

[martong]

ASTImporter.cpp and ASTStructuralEquivalence.cpp looks good to me!

lib/AST/ASTImporter.cpp

1286 ↗

(On Diff #210977)

LGTM!

lib/AST/ASTStructuralEquivalence.cpp

732 ↗

(On Diff #210977)

This looks good to me!

[saar.raz]

Greatly simplified abbreviated template parsing, unify with generic lambdas.
Instead of manually replacing autos with invented parameters after the fact, we now replace autos in parameter type specifiers with invented parameters in GetTypeSpecTypeForDeclarator, by:

• Tracking the template parameter lists in the Declarator object
• Tracking the template parameter depth before parsing function declarators (at which point we can match template parameters against scope specifiers to know if we have an explicit template parameter or not).

When encountering an AutoType in a parameter context we check a stack of InventedTemplateParameterInfo structures that contain the info required to create and accumulate
invented template parameters (fields that were already present in LambdaScopeInfo, which now inherits from this class and is looked up when an auto is encountered in a
lambda context).

[rsmith]

I think this might work out more cleanly if we represented a constrained auto type as a ConstrainedType node wrapping an AutoType node rather than putting both things into the same object. (This will become more pressing if/when C++ starts allowing, for example, constrained placeholders for deduced class template specializations, or constrained typename types, which are likely future directions.) But let's go with this approach for now.

I really like the unification of generic lambdas with abbreviated functions here.

.gitignore

57–60 ↗

(On Diff #237679)

Please revert this change =)

clang/include/clang/AST/RecursiveASTVisitor.h

1298

Should we traverse a DeclarationNameInfo for the concept name here? (Rewriting tools will want to know where the concept name was written.)

clang/include/clang/Basic/DiagnosticParseKinds.td

1375

We usually only use this "ISO C++xy requires [...]" formulation in diagnostics for something we support as a language extension (effectively, when distinguishing between "ISO C++ requires this" and "Clang requires this").

1377–1378

I'm not sure what the "for placeholders" means in these diagnostics. Maybe just "expected 'auto' or 'decltype(auto)' after concept name"?

clang/include/clang/Basic/DiagnosticSemaKinds.td

2655–2657

"simple 'auto'" isn't a constrained placeholder type, so should this just say "constrained placeholder type as type of non-type template parameter not supported yet"?

clang/include/clang/Sema/ScopeInfo.h

38

Please fully-qualify this file name ("clang/Sema/Sema.h") and order it with the other clang/Sema includes above.

That said... I think this is backwards from the layering I'd expect. How about this: move InventedTemplateParameterInfo into DeclSpec.h, and include that here.

clang/lib/AST/ASTStructuralEquivalence.cpp

742

!= on the previous line, please.

744–745

Please don't use auto here; the type is not obvious from the context, and it matters (are we copying a vector here?).

clang/lib/AST/DeclTemplate.cpp

172

Using auto rather than Expr here doesn't improve readability; please just spell out the type.

clang/lib/AST/TypeLoc.cpp

681

Please name this consistently with the corresponding function on Type (getContainedAutoTypeLoc).

clang/lib/Parse/ParseDecl.cpp

3197–3206

This seems surprising. It looks like we're just throwing away the scope specifier in this case? (Should we be storing SS as DS.getTypeSpecScope() or similar?)

3462

Consider adding a FixItHint here to insert the auto.

3462–3466

This block is over-indented.

3473–3491

Please use BalancedDelimiterTracker here.

3476

It seems unlikely that this fixit hint will often be right. (Leaving out the (auto) in decltype(auto) doesn't seem likely to be a common error.) Remove it for now? We can add it back if it turns out this is a mistake people make frequently, and this is the usual cause of this diagnostic.

3477

This looks wrong: you didn't consume Tok yet, so unconsuming it would duplicate it.

clang/lib/Parse/ParseTemplate.cpp

679–682

You should either take a scope specifier or parse one here, not both. I would suggest that you move the code to parse and annotate a scope specifier from isStartOfTemplateParameter to here. (We should change ParseTemplateParameterList to always recover from a parse error by creating a placeholder parameter and remove the ScopeError flag so that doesn't need to be plumbed through here.)

683–684

If NextToken() is annot_template_id, this might still be a type constraint.

683–685

This check is redundant.

clang/lib/Parse/ParseTentative.cpp

1546–1548

Hmm, can this really happen? I assume this would be for Namespace::ConceptName, where we might replace an annot_cxxscope for Namespace:: followed by an identifier for ConceptName with a single annot_template_id for the whole thing? Only... do we actually do that? I thought we only formed annot_template_ids for concept names in TryAnnotateTypeConstraint.

clang/lib/Sema/SemaDeclCXX.cpp

17396–17403

Can we avoid doing this again later in the case where we do it while parsing? (If it's awkward to do so, it's probably not too big a deal; it's not a very expensive operation and we only do it once per top-level templated function declaration.)

clang/lib/Sema/SemaType.cpp

2982

Stray trailing \

[saar.raz]

Address CR comments.

[saar.raz]

Updated with correct diff

[rsmith]

I've left some comments suggesting how to rebase this on rGa42fd84cff265b7e9faa3fe42885ee171393e4db; otherwise, some minor changes then this looks good to me. Thanks!

clang/include/clang/AST/Type.h

4883–4889

Can you use llvm::TrailingArguments here?

clang/include/clang/Basic/DiagnosticParseKinds.td

1375

This is still using an improper formulation; we don't permit this as an extension, so we shouldn't be saying "ISO C++2a requires". Also, this is not quite accurate: we expect 'auto' or 'decltype(auto)' here.

Let's just use the same diagnostic for this case and the one below. I'd also put 'auto' first because it's going to be the more common choice. I don't think it's problematic that we'll say "expected 'auto' or 'decltype(auto)' [...]" for ConceptName decltype(x); that still seems clear enough to me.

clang/lib/AST/ASTContext.cpp

852–853

Add braces here please.

clang/lib/Basic/CMakeLists.txt

24 ↗

(On Diff #238479)

This change doesn't look like it should be part of this commit; please revert.

clang/lib/Parse/ParseDecl.cpp

3251

You no longer need to pass in SS here; TryAnnotateTypeConstraint will read it from the token stream itself.

clang/lib/Parse/ParseTemplate.cpp

679–733

I've pushed a change rearranging this function; the changes here shouldn't be necessary any more.

clang/lib/Parse/ParseTentative.cpp

1546–1548

This should now be unreachable. You should instead handle the case of an annot_cxxscope followed by an annot_template_id.

clang/lib/Parse/Parser.cpp

1746–1748

This doesn't look right: AnnotateTemplateIdToken will consume the < for itself, and uses that to determine whether it has a template argument list or not. It looks like this will mishandle Concept<args> and require instead Concept< <args.> in the contexts where we call TryAnnotateName. Please check you have test coverage for those cases.

2012–2014

Remove this; we should put back the scope token in this case too.

[JDevlieghere]

Hey Saar, I have temporarily reverted this because it broke the LLDB bots. Please run the LLDB test suite when you make changes to the AST importer & keep an eye on the bots when you re-land this. Thanks!

commit 62e4b501ab3bc4c5815a179fdd2c4b49574506c1 (HEAD -> master, origin/master)
Author: Jonas Devlieghere <[e-mail]>
Date:   Tue Jan 21 19:01:38 2020 -0800

    Revert "[Concepts] Placeholder constraints and abbreviated templates"

    This temporarily reverts commit e03ead6771fc97b11cb0c94b7f023142184ad25f
    because it breaks LLDB.

    http://lab.llvm.org:8011/builders/lldb-x86_64-debian/builds/3356
    http://lab.llvm.org:8011/builders/lldb-x64-windows-ninja/builds/12872
    http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/6407/

clang/include/clang/AST/ASTContext.h

 class APFixedPoint;
 class APValue;
 class ASTMutationListener;
 class ASTRecordLayout;
 class AtomicExpr;
 class BlockExpr;
 class BuiltinTemplateDecl;
 class CharUnits;
+class ConceptDecl;
 class CXXABI;
 class CXXConstructorDecl;
 class CXXMethodDecl;
 class CXXRecordDecl;
 class DiagnosticsEngine;
 class Expr;
 class FixedPointSemantics;
 class GlobalDecl;
class ASTContext : public RefCountedBase<ASTContext> {
   mutable llvm::ContextualFoldingSet<DependentTemplateSpecializationType,
                                      ASTContext&>
     DependentTemplateSpecializationTypes;
   llvm::FoldingSet<PackExpansionType> PackExpansionTypes;
   mutable llvm::FoldingSet<ObjCObjectTypeImpl> ObjCObjectTypes;
   mutable llvm::FoldingSet<ObjCObjectPointerType> ObjCObjectPointerTypes;
   mutable llvm::FoldingSet<DependentUnaryTransformType>
     DependentUnaryTransformTypes;
-  mutable llvm::FoldingSet<AutoType> AutoTypes;
+  mutable llvm::ContextualFoldingSet<AutoType, ASTContext&> AutoTypes;
   mutable llvm::FoldingSet<DeducedTemplateSpecializationType>
     DeducedTemplateSpecializationTypes;
   mutable llvm::FoldingSet<AtomicType> AtomicTypes;
   llvm::FoldingSet<AttributedType> AttributedTypes;
   mutable llvm::FoldingSet<PipeType> PipeTypes;
 
   mutable llvm::FoldingSet<QualifiedTemplateName> QualifiedTemplateNames;
   mutable llvm::FoldingSet<DependentTemplateName> DependentTemplateNames;
public:
   QualType getDecltypeType(Expr *e, QualType UnderlyingType) const;
 
   /// Unary type transforms
   QualType getUnaryTransformType(QualType BaseType, QualType UnderlyingType,
                                  UnaryTransformType::UTTKind UKind) const;
 
   /// C++11 deduced auto type.
   QualType getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
-                       bool IsDependent, bool IsPack = false) const;
+                       bool IsDependent, bool IsPack = false,
+                       ConceptDecl *TypeConstraintConcept = nullptr,
+                       ArrayRef<TemplateArgument> TypeConstraintArgs ={}) const;
 
   /// C++11 deduction pattern for 'auto' type.
   QualType getAutoDeductType() const;
 
   /// C++11 deduction pattern for 'auto &&' type.
   QualType getAutoRRefDeductType() const;
 
   /// C++17 deduced class template specialization type.

clang/include/clang/AST/ASTNodeTraverser.h

if (const auto *TC = D->getTypeConstraint())
           dumpTemplateArgumentLoc(ArgLoc);
     if (D->hasDefaultArgument())
       Visit(D->getDefaultArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
             D->defaultArgumentWasInherited() ? "inherited from" : "previous");
   }
 
   void VisitNonTypeTemplateParmDecl(const NonTypeTemplateParmDecl *D) {
-    if (const auto *TC = D->getPlaceholderTypeConstraint())
-      Visit(TC->getImmediatelyDeclaredConstraint());
+    if (const auto *E = D->getPlaceholderTypeConstraint())
+      Visit(E);
     if (D->hasDefaultArgument())
       Visit(D->getDefaultArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
             D->defaultArgumentWasInherited() ? "inherited from" : "previous");
   }
 
   void VisitTemplateTemplateParmDecl(const TemplateTemplateParmDecl *D) {
     dumpTemplateParameters(D->getTemplateParameters());

clang/include/clang/AST/DeclTemplate.h

public:
   /// template parameters of this function template.
   ///
   /// Although the C++ standard has no notion of the "injected" template
   /// arguments for a function template, the notion is convenient when
   /// we need to perform substitutions inside the definition of a function
   /// template.
   ArrayRef<TemplateArgument> getInjectedTemplateArgs();
 
+  /// Return whether this function template is an abbreviated function template,
+  /// e.g. `void foo(auto x)` or `template<typename T> void foo(auto x)`
+  bool isAbbreviated() const {
+    // Since the invented template parameters generated from 'auto' parameters
+    // are either appended to the end of the explicit template parameter list or
+    // form a new template paramter list, we can simply observe the last
+    // parameter to determine if such a thing happened.
+    const TemplateParameterList *TPL = getTemplateParameters();
+    return TPL->getParam(TPL->size() - 1)->isImplicit();
+  }
+
   /// Merge \p Prev with our RedeclarableTemplateDecl::Common.
   void mergePrevDecl(FunctionTemplateDecl *Prev);
 
   /// Create a function template node.
   static FunctionTemplateDecl *Create(ASTContext &C, DeclContext *DC,
                                       SourceLocation L,
                                       DeclarationName Name,
                                       TemplateParameterList *Params,
public:
   static TemplateTypeParmDecl *Create(const ASTContext &C, DeclContext *DC,
                                       SourceLocation KeyLoc,
                                       SourceLocation NameLoc,
                                       unsigned D, unsigned P,
                                       IdentifierInfo *Id, bool Typename,
                                       bool ParameterPack,
                                       bool HasTypeConstraint = false,
                                       Optional<unsigned> NumExpanded = None);
-
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
                                                   unsigned ID);
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
                                                   unsigned ID,
                                                   bool HasTypeConstraint);
 
   /// Whether this template type parameter was declared with
   /// the 'typename' keyword.

 /// e.g., "Size" in
 /// @code
 /// template<int Size> class array { };
 /// @endcode
 class NonTypeTemplateParmDecl final
     : public DeclaratorDecl,
       protected TemplateParmPosition,
       private llvm::TrailingObjects<NonTypeTemplateParmDecl,
-                                    std::pair<QualType, TypeSourceInfo *>> {
+                                    std::pair<QualType, TypeSourceInfo *>,
+                                    Expr *> {
   friend class ASTDeclReader;
   friend TrailingObjects;
 
   /// The default template argument, if any, and whether or not
   /// it was inherited.
   using DefArgStorage = DefaultArgStorage<NonTypeTemplateParmDecl, Expr *>;
   DefArgStorage DefaultArgument;
 
public:
 
   static NonTypeTemplateParmDecl *
   Create(const ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
          SourceLocation IdLoc, unsigned D, unsigned P, IdentifierInfo *Id,
          QualType T, TypeSourceInfo *TInfo, ArrayRef<QualType> ExpandedTypes,
          ArrayRef<TypeSourceInfo *> ExpandedTInfos);
 
   static NonTypeTemplateParmDecl *CreateDeserialized(ASTContext &C,
-                                                     unsigned ID);
+                                                     unsigned ID,
+                                                     bool HasTypeConstraint);
   static NonTypeTemplateParmDecl *CreateDeserialized(ASTContext &C,
                                                      unsigned ID,
-                                                     unsigned NumExpandedTypes);
+                                                     unsigned NumExpandedTypes,
+                                                     bool HasTypeConstraint);
 
   using TemplateParmPosition::getDepth;
   using TemplateParmPosition::setDepth;
   using TemplateParmPosition::getPosition;
   using TemplateParmPosition::setPosition;
   using TemplateParmPosition::getIndex;
 
   SourceRange getSourceRange() const override LLVM_READONLY;
public:
   /// expanded parameter pack.
   TypeSourceInfo *getExpansionTypeSourceInfo(unsigned I) const {
     assert(I < NumExpandedTypes && "Out-of-range expansion type index");
     auto TypesAndInfos =
         getTrailingObjects<std::pair<QualType, TypeSourceInfo *>>();
     return TypesAndInfos[I].second;
   }
 
-  /// Return the type-constraint in the placeholder type of this non-type
+  /// Return the constraint introduced by the placeholder type of this non-type
   /// template parameter (if any).
-  TypeConstraint *getPlaceholderTypeConstraint() const {
-    // TODO: Concepts: Implement once we have actual placeholders with type
-    //                 constraints.
-    return nullptr;
+  Expr *getPlaceholderTypeConstraint() const {
+    return hasPlaceholderTypeConstraint() ? *getTrailingObjects<Expr *>() :
+        nullptr;
+  }
+
+  void setPlaceholderTypeConstraint(Expr *E) {
+    *getTrailingObjects<Expr *>() = E;
   }
 
   /// Determine whether this non-type template parameter's type has a
   /// placeholder with a type-constraint.
   bool hasPlaceholderTypeConstraint() const {
-    // TODO: Concepts: Implement once we have actual placeholders with type
-    //                 constraints.
-    return false;
+    auto *AT = getType()->getContainedAutoType();
+    return AT && AT->isConstrained();
   }
 
   /// \brief Get the associated-constraints of this template parameter.
   /// This will either be a vector of size 1 containing the immediately-declared
   /// constraint introduced by the placeholder type, or an empty vector.
   ///
   /// Use this instead of getPlaceholderImmediatelyDeclaredConstraint for
   /// concepts APIs that accept an ArrayRef of constraint expressions.
   void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
-    if (TypeConstraint *TC = getPlaceholderTypeConstraint())
-      AC.push_back(TC->getImmediatelyDeclaredConstraint());
+    if (Expr *E = getPlaceholderTypeConstraint())
+      AC.push_back(E);
   }
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == NonTypeTemplateParm; }
 };
 
 /// TemplateTemplateParmDecl - Declares a template template parameter,

clang/include/clang/AST/PropertiesBase.td

def DeclRef : RefPropertyType<"Decl"> { let ConstWhenWriting = 1; }
   def ObjCProtocolDeclRef :
     SubclassPropertyType<"ObjCProtocolDecl", DeclRef>;
   def ObjCTypeParamDeclRef :
     SubclassPropertyType<"ObjCTypeParamDecl", DeclRef>;
   def TagDeclRef :
     SubclassPropertyType<"TagDecl", DeclRef>;
   def TemplateDeclRef :
     SubclassPropertyType<"TemplateDecl", DeclRef>;
+  def ConceptDeclRef :
+    SubclassPropertyType<"ConceptDecl", DeclRef>;
   def TemplateTypeParmDeclRef :
     SubclassPropertyType<"TemplateTypeParmDecl", DeclRef>;
   def TemplateTemplateParmDeclRef :
     SubclassPropertyType<"TemplateTemplateParmDecl", DeclRef>;
   def ValueDeclRef :
     SubclassPropertyType<"ValueDecl", DeclRef>;
 def ElaboratedTypeKeyword : EnumPropertyType;
 def ExtParameterInfo : PropertyType<"FunctionProtoType::ExtParameterInfo">;

clang/include/clang/AST/RecursiveASTVisitor.h

 DEF_TRAVERSE_TYPE(DecltypeType,
                   { TRY_TO(TraverseStmt(T->getUnderlyingExpr())); })
 
 DEF_TRAVERSE_TYPE(UnaryTransformType, {
   TRY_TO(TraverseType(T->getBaseType()));
   TRY_TO(TraverseType(T->getUnderlyingType()));
 })
 
-DEF_TRAVERSE_TYPE(AutoType, { TRY_TO(TraverseType(T->getDeducedType())); })
+DEF_TRAVERSE_TYPE(AutoType, {
+  TRY_TO(TraverseType(T->getDeducedType()));
+  if (T->isConstrained()) {
+    TRY_TO(TraverseDecl(T->getTypeConstraintConcept()));
+    TRY_TO(TraverseTemplateArguments(T->getArgs(), T->getNumArgs()));
+  }
+})
 DEF_TRAVERSE_TYPE(DeducedTemplateSpecializationType, {
   TRY_TO(TraverseTemplateName(T->getTemplateName()));
   TRY_TO(TraverseType(T->getDeducedType()));
 })
 
 DEF_TRAVERSE_TYPE(RecordType, {})
 DEF_TRAVERSE_TYPE(EnumType, {})
 DEF_TRAVERSE_TYPE(TemplateTypeParmType, {})

 })
 
 DEF_TRAVERSE_TYPELOC(UnaryTransformType, {
   TRY_TO(TraverseTypeLoc(TL.getUnderlyingTInfo()->getTypeLoc()));
 })
 
 DEF_TRAVERSE_TYPELOC(AutoType, {
   TRY_TO(TraverseType(TL.getTypePtr()->getDeducedType()));
+  if (TL.isConstrained()) {
+    TRY_TO(TraverseNestedNameSpecifierLoc(TL.getNestedNameSpecifierLoc()));
+    TRY_TO(TraverseDeclarationNameInfo(TL.getConceptNameInfo()));
+    for (unsigned I = 0, E = TL.getNumArgs(); I != E; ++I)
+      TRY_TO(TraverseTemplateArgumentLoc(TL.getArgLoc(I)));
+  }
 })
 
 DEF_TRAVERSE_TYPELOC(DeducedTemplateSpecializationType, {
   TRY_TO(TraverseTemplateName(TL.getTypePtr()->getTemplateName()));
   TRY_TO(TraverseType(TL.getTypePtr()->getDeducedType()));
 })
 
 DEF_TRAVERSE_TYPELOC(RecordType, {})

clang/include/clang/AST/TemplateBase.h

llvm::ArrayRef<TemplateArgumentLoc> arguments() const {
     return llvm::makeArrayRef(getTemplateArgs(), getNumTemplateArgs());
   }
 
   const TemplateArgumentLoc &operator[](unsigned I) const {
     return getTemplateArgs()[I];
   }
 
   static const ASTTemplateArgumentListInfo *
-  Create(ASTContext &C, const TemplateArgumentListInfo &List);
+  Create(const ASTContext &C, const TemplateArgumentListInfo &List);
 };
 
 /// Represents an explicit template argument list in C++, e.g.,
 /// the "<int>" in "sort<int>".
 ///
 /// It is intended to be used as a trailing object on AST nodes, and
 /// as such, doesn't contain the array of TemplateArgumentLoc itself,
 /// but expects the containing object to also provide storage for

 }
 
 inline const TemplateArgument &
     DependentTemplateSpecializationType::getArg(unsigned Idx) const {
   assert(Idx < getNumArgs() && "Template argument out of range");
   return getArgs()[Idx];
 }
 
+inline const TemplateArgument &AutoType::getArg(unsigned Idx) const {
+  assert(Idx < getNumArgs() && "Template argument out of range");
+  return getArgs()[Idx];
+}
+
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_TEMPLATEBASE_H

clang/include/clang/AST/Type.h

 #include <string>
 #include <type_traits>
 #include <utility>
 
 namespace clang {
 
 class ExtQuals;
 class QualType;
+class ConceptDecl;
 class TagDecl;
 class Type;
 
 enum {
   TypeAlignmentInBits = 4,
   TypeAlignment = 1 << TypeAlignmentInBits
 };
 
protected:
   class AutoTypeBitfields {
     friend class AutoType;
 
     unsigned : NumTypeBits;
 
     /// Was this placeholder type spelled as 'auto', 'decltype(auto)',
     /// or '__auto_type'?  AutoTypeKeyword value.
     unsigned Keyword : 2;
+
+    /// The number of template arguments in the type-constraints, which is
+    /// expected to be able to hold at least 1024 according to [implimits].
+    /// However as this limit is somewhat easy to hit with template
+    /// metaprogramming we'd prefer to keep it as large as possible.
+    /// At the moment it has been left as a non-bitfield since this type
+    /// safely fits in 64 bits as an unsigned, so there is no reason to
+    /// introduce the performance impact of a bitfield.
+    unsigned NumArgs;
   };
 
   class SubstTemplateTypeParmPackTypeBitfields {
     friend class SubstTemplateTypeParmPackType;
 
     unsigned : NumTypeBits;
 
     /// The number of template arguments in \c Arguments, which is
public:
 
   static bool classof(const Type *T) {
     return T->getTypeClass() == SubstTemplateTypeParmPack;
   }
 };
 
 /// Common base class for placeholders for types that get replaced by
 /// placeholder type deduction: C++11 auto, C++14 decltype(auto), C++17 deduced
-/// class template types, and (eventually) constrained type names from the C++
-/// Concepts TS.
+/// class template types, and constrained type names.
 ///
 /// These types are usually a placeholder for a deduced type. However, before
 /// the initializer is attached, or (usually) if the initializer is
 /// type-dependent, there is no deduced type and the type is canonical. In
 /// the latter case, it is also a dependent type.
 class DeducedType : public Type {
 protected:
   DeducedType(TypeClass TC, QualType DeducedAsType, bool IsDependent,
public:
   }
 
   static bool classof(const Type *T) {
     return T->getTypeClass() == Auto ||
            T->getTypeClass() == DeducedTemplateSpecialization;
   }
 };
 
-/// Represents a C++11 auto or C++14 decltype(auto) type.
-class AutoType : public DeducedType, public llvm::FoldingSetNode {
+/// Represents a C++11 auto or C++14 decltype(auto) type, possibly constrained
+/// by a type-constraint.
+class alignas(8) AutoType : public DeducedType, public llvm::FoldingSetNode {
   friend class ASTContext; // ASTContext creates these
 
+  ConceptDecl *TypeConstraintConcept;
+
   AutoType(QualType DeducedAsType, AutoTypeKeyword Keyword,
-           bool IsDeducedAsDependent, bool IsDeducedAsPack)
-      : DeducedType(Auto, DeducedAsType, IsDeducedAsDependent,
-                    IsDeducedAsDependent, IsDeducedAsPack) {
-    AutoTypeBits.Keyword = (unsigned)Keyword;
+           bool IsDeducedAsDependent, bool IsDeducedAsPack, ConceptDecl *CD,
+           ArrayRef<TemplateArgument> TypeConstraintArgs);
+
+  const TemplateArgument *getArgBuffer() const {
+    return reinterpret_cast<const TemplateArgument*>(this+1);
+  }
+
+  TemplateArgument *getArgBuffer() {
+    return reinterpret_cast<TemplateArgument*>(this+1);
   }
 
 public:
+  /// Retrieve the template arguments.
+  const TemplateArgument *getArgs() const {
+    return getArgBuffer();
+  }
+
+  /// Retrieve the number of template arguments.
+  unsigned getNumArgs() const {
+    return AutoTypeBits.NumArgs;
+  }
+
+  const TemplateArgument &getArg(unsigned Idx) const; // in TemplateBase.h
+
+  ArrayRef<TemplateArgument> getTypeConstraintArguments() const {
+    return {getArgs(), getNumArgs()};
+  }
+
+  ConceptDecl *getTypeConstraintConcept() const {
+    return TypeConstraintConcept;
+  }
+
+  bool isConstrained() const {
+    return TypeConstraintConcept != nullptr;
+  }
+
   bool isDecltypeAuto() const {
     return getKeyword() == AutoTypeKeyword::DecltypeAuto;
   }
 
   AutoTypeKeyword getKeyword() const {
     return (AutoTypeKeyword)AutoTypeBits.Keyword;
   }
 
-  void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, getDeducedType(), getKeyword(), isDependentType(),
-            containsUnexpandedParameterPack());
+  void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context) {
+    Profile(ID, Context, getDeducedType(), getKeyword(), isDependentType(),
+            getTypeConstraintConcept(), getTypeConstraintArguments());
   }
 
-  static void Profile(llvm::FoldingSetNodeID &ID, QualType Deduced,
-                      AutoTypeKeyword Keyword, bool IsDependent, bool IsPack) {
-    ID.AddPointer(Deduced.getAsOpaquePtr());
-    ID.AddInteger((unsigned)Keyword);
-    ID.AddBoolean(IsDependent);
-    ID.AddBoolean(IsPack);
-  }
+  static void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
+                      QualType Deduced, AutoTypeKeyword Keyword,
+                      bool IsDependent, ConceptDecl *CD,
+                      ArrayRef<TemplateArgument> Arguments);
 
   static bool classof(const Type *T) {
     return T->getTypeClass() == Auto;
   }
 };
 
 /// Represents a C++17 deduced template specialization type.
 class DeducedTemplateSpecializationType : public DeducedType,

clang/include/clang/AST/TypeLoc.h

 //===- TypeLoc.h - Type Source Info Wrapper ---------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 /// \file
 /// Defines the clang::TypeLoc interface and its subclasses.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_AST_TYPELOC_H
 #define LLVM_CLANG_AST_TYPELOC_H
 
+#include "clang/AST/DeclarationName.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/Specifiers.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/MathExtras.h"
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
 #include <cstring>
 
 namespace clang {
 
 class Attr;
 class ASTContext;
 class CXXRecordDecl;
+class ConceptDecl;
 class Expr;
 class ObjCInterfaceDecl;
 class ObjCProtocolDecl;
 class ObjCTypeParamDecl;
 class ParmVarDecl;
 class TemplateTypeParmDecl;
 class UnqualTypeLoc;
 class UnresolvedUsingTypenameDecl;
#include "clang/AST/TypeNodes.inc"
   /// Find a type with the location of an explicit type qualifier.
   ///
   /// The result, if non-null, will be one of:
   ///   QualifiedTypeLoc
   ///   AtomicTypeLoc
   ///   AttributedTypeLoc, for those type attributes that behave as qualifiers
   TypeLoc findExplicitQualifierLoc() const;
 
+  /// Get the typeloc of an AutoType whose type will be deduced for a variable
+  /// with an initializer of this type. This looks through declarators like
+  /// pointer types, but not through decltype or typedefs.
+  AutoTypeLoc getContainedAutoTypeLoc() const;
+
   /// Initializes this to state that every location in this
   /// type is the given location.
   ///
   /// This method exists to provide a simple transition for code that
   /// relies on location-less types.
   void initialize(ASTContext &Context, SourceLocation Loc) const {
     initializeImpl(Context, *this, Loc);
   }
public:
 
   void initializeLocal(ASTContext &Context, SourceLocation Loc);
 };
 
 class DeducedTypeLoc
     : public InheritingConcreteTypeLoc<TypeSpecTypeLoc, DeducedTypeLoc,
                                        DeducedType> {};
 
+struct AutoTypeLocInfo : TypeSpecLocInfo {
+  NestedNameSpecifierLoc NestedNameSpec;
+  SourceLocation TemplateKWLoc;
+  SourceLocation ConceptNameLoc;
+  NamedDecl *FoundDecl;
+  SourceLocation LAngleLoc;
+  SourceLocation RAngleLoc;
+};
+
 class AutoTypeLoc
-    : public InheritingConcreteTypeLoc<DeducedTypeLoc, AutoTypeLoc, AutoType> {
+    : public ConcreteTypeLoc<DeducedTypeLoc,
+                             AutoTypeLoc,
+                             AutoType,
+                             AutoTypeLocInfo> {
+public:
+  AutoTypeKeyword getAutoKeyword() const {
+    return getTypePtr()->getKeyword();
+  }
+
+  bool isConstrained() const {
+    return getTypePtr()->isConstrained();
+  }
+
+  const NestedNameSpecifierLoc &getNestedNameSpecifierLoc() const {
+    return getLocalData()->NestedNameSpec;
+  }
+
+  void setNestedNameSpecifierLoc(NestedNameSpecifierLoc NNS) {
+    getLocalData()->NestedNameSpec = NNS;
+  }
+
+  SourceLocation getTemplateKWLoc() const {
+    return getLocalData()->TemplateKWLoc;
+  }
+
+  void setTemplateKWLoc(SourceLocation Loc) {
+    getLocalData()->TemplateKWLoc = Loc;
+  }
+
+  SourceLocation getConceptNameLoc() const {
+    return getLocalData()->ConceptNameLoc;
+  }
+
+  void setConceptNameLoc(SourceLocation Loc) {
+    getLocalData()->ConceptNameLoc = Loc;
+  }
+
+  NamedDecl *getFoundDecl() const {
+    return getLocalData()->FoundDecl;
+  }
+
+  void setFoundDecl(NamedDecl *D) {
+    getLocalData()->FoundDecl = D;
+  }
+
+  ConceptDecl *getNamedConcept() const {
+    return getTypePtr()->getTypeConstraintConcept();
+  }
+
+  DeclarationNameInfo getConceptNameInfo() const;
+
+  bool hasExplicitTemplateArgs() const {
+    return getLocalData()->LAngleLoc.isValid();
+  }
+
+  SourceLocation getLAngleLoc() const {
+    return this->getLocalData()->LAngleLoc;
+  }
+
+  void setLAngleLoc(SourceLocation Loc) {
+    this->getLocalData()->LAngleLoc = Loc;
+  }
+
+  SourceLocation getRAngleLoc() const {
+    return this->getLocalData()->RAngleLoc;
+  }
+
+  void setRAngleLoc(SourceLocation Loc) {
+    this->getLocalData()->RAngleLoc = Loc;
+  }
+
+  unsigned getNumArgs() const {
+    return getTypePtr()->getNumArgs();
+  }
+
+  void setArgLocInfo(unsigned i, TemplateArgumentLocInfo AI) {
+    getArgInfos()[i] = AI;
+  }
+
+  TemplateArgumentLocInfo getArgLocInfo(unsigned i) const {
+    return getArgInfos()[i];
+  }
+
+  TemplateArgumentLoc getArgLoc(unsigned i) const {
+    return TemplateArgumentLoc(getTypePtr()->getTypeConstraintArguments()[i],
+                               getArgLocInfo(i));
+  }
+
+  SourceRange getLocalSourceRange() const {
+    return{
+        isConstrained()
+          ? (getNestedNameSpecifierLoc()
+               ? getNestedNameSpecifierLoc().getBeginLoc()
+               : (getTemplateKWLoc().isValid()
+                  ? getTemplateKWLoc()
+                  : getConceptNameLoc()))
+          : getNameLoc(),
+        getNameLoc()
+    };
+  }
+
+  void copy(AutoTypeLoc Loc) {
+    unsigned size = getFullDataSize();
+    assert(size == Loc.getFullDataSize());
+    memcpy(Data, Loc.Data, size);
+  }
+
+  void initializeLocal(ASTContext &Context, SourceLocation Loc);
+
+  unsigned getExtraLocalDataSize() const {
+    return getNumArgs() * sizeof(TemplateArgumentLocInfo);
+  }
+
+  unsigned getExtraLocalDataAlignment() const {
+    return alignof(TemplateArgumentLocInfo);
+  }
+
+private:
+  TemplateArgumentLocInfo *getArgInfos() const {
+    return static_cast<TemplateArgumentLocInfo*>(getExtraLocalData());
+  }
 };
 
 class DeducedTemplateSpecializationTypeLoc
     : public InheritingConcreteTypeLoc<DeducedTypeLoc,
                                        DeducedTemplateSpecializationTypeLoc,
                                        DeducedTemplateSpecializationType> {
 public:
   SourceLocation getTemplateNameLoc() const {

clang/include/clang/AST/TypeProperties.td

 
 let Class = AutoType in {
   def : Property<"deducedType", Optional<QualType>> {
     let Read = [{ makeOptionalFromNullable(node->getDeducedType()) }];
   }
   def : Property<"keyword", AutoTypeKeyword> {
     let Read = [{ node->getKeyword() }];
   }
+  def : Property<"typeConstraintConcept", Optional<ConceptDeclRef>> {
+    let Read = [{ makeOptionalFromPointer(
+        const_cast<const ConceptDecl*>(node->getTypeConstraintConcept())) }];
+  }
+  def : Property<"typeConstraintArguments", Array<TemplateArgument>> {
+    let Read = [{ node->getTypeConstraintArguments() }];
+  }
   // FIXME: better enumerated value
   // Only really required when the deduced type is null
   def : Property<"dependence", UInt32> {
     let Read = [{ !node->getDeducedType().isNull() ? 0 :
                   node->containsUnexpandedParameterPack() ? 2 :
                   node->isDependentType() ? 1 : 0 }];
   }
 
   def : Creator<[{
     return ctx.getAutoType(makeNullableFromOptional(deducedType), keyword,
                            /*isDependentWithoutDeducedType*/ dependence > 0,
-                           /*isPackWithoutDeducedType*/ dependence > 1);
+                           /*isPackWithoutDeducedType*/ dependence > 1,
+                           makePointerFromOptional(typeConstraintConcept),
+                           typeConstraintArguments);
   }]>;
 }
 
 let Class = DeducedTemplateSpecializationType in {
   def : Property<"templateName", Optional<TemplateName>> {
     let Read = [{ makeOptionalFromNullable(node->getTemplateName()) }];
   }
   def : Property<"deducedType", QualType> {

clang/include/clang/Basic/DiagnosticParseKinds.td

 }
 
 let CategoryName = "Concepts Issue" in {
 def err_concept_definition_not_identifier : Error<
   "name defined in concept definition must be an identifier">;
 def ext_concept_legacy_bool_keyword : ExtWarn<
   "ISO C++2a does not permit the 'bool' keyword after 'concept'">,
   InGroup<DiagGroup<"concepts-ts-compat">>;
+def err_placeholder_expected_auto_or_decltype_auto : Error<
+  "expected 'auto' or 'decltype(auto)' after concept name">;
 }
 
 } // end of Parser diagnostics

clang/include/clang/Basic/DiagnosticSemaKinds.td

def err_auto_not_allowed : Error<
   "%select{in function prototype"
   "|in non-static struct member|in struct member"
   "|in non-static union member|in union member"
   "|in non-static class member|in interface member"
   "|in exception declaration|in template parameter until C++17|in block literal"
   "|in template argument|in typedef|in type alias|in function return type"
   "|in conversion function type|here|in lambda parameter"
   "|in type allocated by 'new'|in K&R-style function parameter"
-  "|in template parameter|in friend declaration"
-  "|in requires expression parameter}1">;
-def err_auto_not_allowed_in_return_type_requirement : Error<
-  "%select{'auto'|'decltype(auto)'|'__auto_type'}0 not allowed in expression "
-  "type requirement">;
+  "|in template parameter|in friend declaration|in function prototype that is "
+  "not a function declaration|in requires expression parameter}1">;
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"
   "template template parameter|template}0 member in %1; "
   "argument deduction not allowed here">;
 def err_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"

 def note_nested_requirement_unknown_substitution_error : Note<
   "%select{and|because}0 '%1' would be invalid">;
 def note_ambiguous_atomic_constraints : Note<
   "similar constraint expressions not considered equivalent; constraint "
   "expressions cannot be considered equivalent unless they originate from the "
   "same concept">;
 def note_ambiguous_atomic_constraints_similar_expression : Note<
   "similar constraint expression here">;
+def err_unsupported_placeholder_constraint : Error<
+  "constrained placeholder types other than simple 'auto' on non-type template "
+  "parameters not supported yet">;
 
 def err_template_different_requires_clause : Error<
   "requires clause differs in template redeclaration">;
 def err_template_different_type_constraint : Error<
   "type constraint differs in template redeclaration">;
 def err_template_template_parameter_not_at_least_as_constrained : Error<
   "template template argument %0 is more constrained than template template "
   "parameter %1">;
 
 def err_type_constraint_non_type_concept : Error<
   "concept named in type constraint is not a type concept">;
 def err_type_constraint_missing_arguments : Error<
   "%0 requires more than 1 template argument; provide the remaining arguments "
   "explicitly to use it here">;
+def err_placeholder_constraints_not_satisfied : Error<
+  "deduced type %0 does not satisfy %1">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
   "'%0' type specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 def warn_cxx17_compat_unicode_type : Warning<
   "'char8_t' type specifier is incompatible with C++ standards before C++20">,
   InGroup<CXXPre2aCompat>, DefaultIgnore;

clang/include/clang/Sema/DeclSpec.h

private:
   /*TSS*/unsigned TypeSpecSign : 2;
   /*TST*/unsigned TypeSpecType : 6;
   unsigned TypeAltiVecVector : 1;
   unsigned TypeAltiVecPixel : 1;
   unsigned TypeAltiVecBool : 1;
   unsigned TypeSpecOwned : 1;
   unsigned TypeSpecPipe : 1;
   unsigned TypeSpecSat : 1;
+  unsigned ConstrainedAuto : 1;
 
   // type-qualifiers
   unsigned TypeQualifiers : 5;  // Bitwise OR of TQ.
 
   // function-specifier
   unsigned FS_inline_specified : 1;
   unsigned FS_forceinline_specified: 1;
   unsigned FS_virtual_specified : 1;
   unsigned FS_noreturn_specified : 1;
 
   // friend-specifier
   unsigned Friend_specified : 1;
 
   // constexpr-specifier
   unsigned ConstexprSpecifier : 2;
 
   union {
     UnionParsedType TypeRep;
     Decl *DeclRep;
     Expr *ExprRep;
+    TemplateIdAnnotation *TemplateIdRep;
   };
 
   /// ExplicitSpecifier - Store information about explicit spicifer.
   ExplicitSpecifier FS_explicit_specifier;
 
   // attributes.
   ParsedAttributes Attrs;
 
private:
 
   static bool isTypeRep(TST T) {
     return (T == TST_typename || T == TST_typeofType ||
             T == TST_underlyingType || T == TST_atomic);
   }
   static bool isExprRep(TST T) {
     return (T == TST_typeofExpr || T == TST_decltype);
   }
+  static bool isTemplateIdRep(TST T) {
+    return (T == TST_auto || T == TST_decltype_auto);
+  }
 
   DeclSpec(const DeclSpec &) = delete;
   void operator=(const DeclSpec &) = delete;
 public:
   static bool isDeclRep(TST T) {
     return (T == TST_enum || T == TST_struct ||
             T == TST_interface || T == TST_union ||
             T == TST_class);
   }
 
   DeclSpec(AttributeFactory &attrFactory)
       : StorageClassSpec(SCS_unspecified),
         ThreadStorageClassSpec(TSCS_unspecified),
         SCS_extern_in_linkage_spec(false), TypeSpecWidth(TSW_unspecified),
         TypeSpecComplex(TSC_unspecified), TypeSpecSign(TSS_unspecified),
         TypeSpecType(TST_unspecified), TypeAltiVecVector(false),
         TypeAltiVecPixel(false), TypeAltiVecBool(false), TypeSpecOwned(false),
-        TypeSpecPipe(false), TypeSpecSat(false), TypeQualifiers(TQ_unspecified),
+        TypeSpecPipe(false), TypeSpecSat(false), ConstrainedAuto(false),
+        TypeQualifiers(TQ_unspecified),
         FS_inline_specified(false), FS_forceinline_specified(false),
         FS_virtual_specified(false), FS_noreturn_specified(false),
         Friend_specified(false), ConstexprSpecifier(CSK_unspecified),
         FS_explicit_specifier(), Attrs(attrFactory), writtenBS(),
         ObjCQualifiers(nullptr) {}
 
   // storage-class-specifier
   SCS getStorageClassSpec() const { return (SCS)StorageClassSpec; }
public:
   TST getTypeSpecType() const { return (TST)TypeSpecType; }
   bool isTypeAltiVecVector() const { return TypeAltiVecVector; }
   bool isTypeAltiVecPixel() const { return TypeAltiVecPixel; }
   bool isTypeAltiVecBool() const { return TypeAltiVecBool; }
   bool isTypeSpecOwned() const { return TypeSpecOwned; }
   bool isTypeRep() const { return isTypeRep((TST) TypeSpecType); }
   bool isTypeSpecPipe() const { return TypeSpecPipe; }
   bool isTypeSpecSat() const { return TypeSpecSat; }
+  bool isConstrainedAuto() const { return ConstrainedAuto; }
 
   ParsedType getRepAsType() const {
     assert(isTypeRep((TST) TypeSpecType) && "DeclSpec does not store a type");
     return TypeRep;
   }
   Decl *getRepAsDecl() const {
     assert(isDeclRep((TST) TypeSpecType) && "DeclSpec does not store a decl");
     return DeclRep;
   }
   Expr *getRepAsExpr() const {
     assert(isExprRep((TST) TypeSpecType) && "DeclSpec does not store an expr");
     return ExprRep;
   }
+  TemplateIdAnnotation *getRepAsTemplateId() const {
+    assert(isTemplateIdRep((TST) TypeSpecType) &&
+           "DeclSpec does not store a template id");
+    return TemplateIdRep;
+  }
   CXXScopeSpec &getTypeSpecScope() { return TypeScope; }
   const CXXScopeSpec &getTypeSpecScope() const { return TypeScope; }
 
   SourceRange getSourceRange() const LLVM_READONLY { return Range; }
   SourceLocation getBeginLoc() const LLVM_READONLY { return Range.getBegin(); }
   SourceLocation getEndLoc() const LLVM_READONLY { return Range.getEnd(); }
 
   SourceLocation getTypeSpecWidthLoc() const { return TSWRange.getBegin(); }
public:
   bool SetTypeSpecType(TST T, SourceLocation TagKwLoc,
                        SourceLocation TagNameLoc, const char *&PrevSpec,
                        unsigned &DiagID, ParsedType Rep,
                        const PrintingPolicy &Policy);
   bool SetTypeSpecType(TST T, SourceLocation TagKwLoc,
                        SourceLocation TagNameLoc, const char *&PrevSpec,
                        unsigned &DiagID, Decl *Rep, bool Owned,
                        const PrintingPolicy &Policy);
+  bool SetTypeSpecType(TST T, SourceLocation Loc, const char *&PrevSpec,
+                       unsigned &DiagID, TemplateIdAnnotation *Rep,
+                       const PrintingPolicy &Policy);
 
   bool SetTypeSpecType(TST T, SourceLocation Loc, const char *&PrevSpec,
                        unsigned &DiagID, Expr *Rep,
                        const PrintingPolicy &policy);
   bool SetTypeAltiVecVector(bool isAltiVecVector, SourceLocation Loc,
                        const char *&PrevSpec, unsigned &DiagID,
                        const PrintingPolicy &Policy);
   bool SetTypeAltiVecPixel(bool isAltiVecPixel, SourceLocation Loc,
private:
 
   /// The asm label, if specified.
   Expr *AsmLabel;
 
   /// \brief The constraint-expression specified by the trailing
   /// requires-clause, or null if no such clause was specified.
   Expr *TrailingRequiresClause;
 
+  /// If this declarator declares a template, its template parameter lists.
+  ArrayRef<TemplateParameterList *> TemplateParameterLists;
+
+  /// If the declarator declares an abbreviated function template, the innermost
+  /// template parameter list containing the invented and explicit template
+  /// parameters (if any).
+  TemplateParameterList *InventedTemplateParameterList;
+
 #ifndef _MSC_VER
   union {
 #endif
     /// InlineParams - This is a local array used for the first function decl
     /// chunk to avoid going to the heap for the common case when we have one
     /// function chunk in the declarator.
     DeclaratorChunk::ParamInfo InlineParams[16];
     DecompositionDeclarator::Binding InlineBindings[16];

 public:
   Declarator(const DeclSpec &ds, DeclaratorContext C)
       : DS(ds), Range(ds.getSourceRange()), Context(C),
         InvalidType(DS.getTypeSpecType() == DeclSpec::TST_error),
         GroupingParens(false), FunctionDefinition(FDK_Declaration),
         Redeclaration(false), Extension(false), ObjCIvar(false),
         ObjCWeakProperty(false), InlineStorageUsed(false),
         Attrs(ds.getAttributePool().getFactory()), AsmLabel(nullptr),
-        TrailingRequiresClause(nullptr) {}
+        TrailingRequiresClause(nullptr),
+        InventedTemplateParameterList(nullptr) {}
 
   ~Declarator() {
     clear();
   }
   /// getDeclSpec - Return the declaration-specifier that this declarator was
   /// declared with.
   const DeclSpec &getDeclSpec() const { return DS; }
 
public:
   }
 
   /// \brief Determine whether a trailing requires clause was written in this
   /// declarator.
   bool hasTrailingRequiresClause() const {
     return TrailingRequiresClause != nullptr;
   }
 
+  /// Sets the template parameter lists that preceded the declarator.
+  void setTemplateParameterLists(ArrayRef<TemplateParameterList *> TPLs) {
+    TemplateParameterLists = TPLs;
+  }
+
+  /// The template parameter lists that preceded the declarator.
+  ArrayRef<TemplateParameterList *> getTemplateParameterLists() const {
+    return TemplateParameterLists;
+  }
+
+  /// Sets the template parameter list generated from the explicit template
+  /// parameters along with any invented template parameters from
+  /// placeholder-typed parameters.
+  void setInventedTemplateParameterList(TemplateParameterList *Invented) {
+    InventedTemplateParameterList = Invented;
+  }
+
+  /// The template parameter list generated from the explicit template
+  /// parameters along with any invented template parameters from
+  /// placeholder-typed parameters, if there were any such parameters.
+  TemplateParameterList * getInventedTemplateParameterList() const {
+    return InventedTemplateParameterList;
+  }
+
   /// takeAttributes - Takes attributes from the given parsed-attributes
   /// set and add them to this declarator.
   ///
   /// These examples both add 3 attributes to "var":
   ///  short int var __attribute__((aligned(16),common,deprecated));
   ///  short int x, __attribute__((aligned(16)) var
   ///                                 __attribute__((common,deprecated));
   ///
void addCapture(LambdaCaptureKind Kind,
                   ExprResult Init,
                   ParsedType InitCaptureType,
                   SourceRange ExplicitRange) {
     Captures.push_back(LambdaCapture(Kind, Loc, Id, EllipsisLoc, InitKind, Init,
                                      InitCaptureType, ExplicitRange));
   }
 };
 
+struct InventedTemplateParameterInfo {
+  /// The number of parameters in the template parameter list that were
+  /// explicitly specified by the user, as opposed to being invented by use
+  /// of an auto parameter.
+  unsigned NumExplicitTemplateParams = 0;
+
+  /// If this is a generic lambda or abbreviated function template, use this
+  /// as the depth of each 'auto' parameter, during initial AST construction.
+  unsigned AutoTemplateParameterDepth = 0;
+
+  /// Store the list of the template parameters for a generic lambda or an
+  /// abbreviated function template.
+  /// If this is a generic lambda or abbreviated function template, this holds
+  /// the explicit template parameters followed by the auto parameters
+  /// converted into TemplateTypeParmDecls.
+  /// It can be used to construct the generic lambda or abbreviated template's
+  /// template parameter list during initial AST construction.
+  SmallVector<NamedDecl*, 4> TemplateParams;
+};
+
 } // end namespace clang
 
 #endif // LLVM_CLANG_SEMA_DECLSPEC_H

clang/include/clang/Sema/Scope.h

public:
   }
 
   /// isFunctionPrototypeScope - Return true if this scope is a
   /// function prototype scope.
   bool isFunctionPrototypeScope() const {
     return getFlags() & Scope::FunctionPrototypeScope;
   }
 
+  /// isFunctionDeclarationScope - Return true if this scope is a
+  /// function prototype scope.
+  bool isFunctionDeclarationScope() const {
+    return getFlags() & Scope::FunctionDeclarationScope;
+  }
+
   /// isAtCatchScope - Return true if this scope is \@catch.
   bool isAtCatchScope() const {
     return getFlags() & Scope::AtCatchScope;
   }
 
   /// isSwitchScope - Return true if this scope is a switch scope.
   bool isSwitchScope() const {
     for (const Scope *S = this; S; S = S->getParent()) {

clang/include/clang/Sema/ScopeInfo.h

 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/CapturedStmt.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/PartialDiagnostic.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Sema/CleanupInfo.h"
+#include "clang/Sema/DeclSpec.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/TinyPtrVector.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <algorithm>
 #include <cassert>
 #include <utility>
 
 namespace clang {
 
 class BlockDecl;
 class CapturedDecl;
 class CXXMethodDecl;
StringRef getRegionName() const {
     llvm_unreachable("Invalid captured region kind!");
   }
 
   static bool classof(const FunctionScopeInfo *FSI) {
     return FSI->Kind == SK_CapturedRegion;
   }
 };
 
-class LambdaScopeInfo final : public CapturingScopeInfo {
+class LambdaScopeInfo final :
+    public CapturingScopeInfo, public InventedTemplateParameterInfo {
 public:
   /// The class that describes the lambda.
   CXXRecordDecl *Lambda = nullptr;
 
   /// The lambda's compiler-generated \c operator().
   CXXMethodDecl *CallOperator = nullptr;
 
   /// Source range covering the lambda introducer [...].
public:
   CleanupInfo Cleanup;
 
   /// Whether the lambda contains an unexpanded parameter pack.
   bool ContainsUnexpandedParameterPack = false;
 
   /// Packs introduced by this lambda, if any.
   SmallVector<NamedDecl*, 4> LocalPacks;
 
-  /// If this is a generic lambda, use this as the depth of
-  /// each 'auto' parameter, during initial AST construction.
-  unsigned AutoTemplateParameterDepth = 0;
-
-  /// The number of parameters in the template parameter list that were
-  /// explicitly specified by the user, as opposed to being invented by use
-  /// of an auto parameter.
-  unsigned NumExplicitTemplateParams = 0;
-
   /// Source range covering the explicit template parameter list (if it exists).
   SourceRange ExplicitTemplateParamsRange;
 
-  /// Store the list of the template parameters for a generic lambda.
-  /// If this is a generic lambda, this holds the explicit template parameters
-  /// followed by the auto parameters converted into TemplateTypeParmDecls.
-  /// It can be used to construct the generic lambda's template parameter list
-  /// during initial AST construction.
-  SmallVector<NamedDecl*, 4> TemplateParams;
-
   /// If this is a generic lambda, and the template parameter
   /// list has been created (from the TemplateParams) then store
   /// a reference to it (cache it to avoid reconstructing it).
   TemplateParameterList *GLTemplateParameterList = nullptr;
 
   /// Contains all variable-referring-expressions (i.e. DeclRefExprs
   ///  or MemberExprs) that refer to local variables in a generic lambda
   ///  or a lambda in a potentially-evaluated-if-used context.

clang/include/clang/Sema/Sema.h

public:
   MaybeODRUseExprSet MaybeODRUseExprs;
 
   std::unique_ptr<sema::FunctionScopeInfo> CachedFunctionScope;
 
   /// Stack containing information about each of the nested
   /// function, block, and method scopes that are currently active.
   SmallVector<sema::FunctionScopeInfo *, 4> FunctionScopes;
 
+  /// Stack containing information needed when in C++2a an 'auto' is encountered
+  /// in a function declaration parameter type specifier in order to invent a
+  /// corresponding template parameter in the enclosing abbreviated function
+  /// template. This information is also present in LambdaScopeInfo, stored in
+  /// the FunctionScopes stack.
+  SmallVector<InventedTemplateParameterInfo, 4> InventedParameterInfos;
+
   typedef LazyVector<TypedefNameDecl *, ExternalSemaSource,
                      &ExternalSemaSource::ReadExtVectorDecls, 2, 2>
     ExtVectorDeclsType;
 
   /// ExtVectorDecls - This is a list all the extended vector types. This allows
   /// us to associate a raw vector type with one of the ext_vector type names.
   /// This is only necessary for issuing pretty diagnostics.
   ExtVectorDeclsType ExtVectorDecls;
public:
   std::string getFixItZeroLiteralForType(QualType T, SourceLocation Loc) const;
 
   /// Calls \c Lexer::getLocForEndOfToken()
   SourceLocation getLocForEndOfToken(SourceLocation Loc, unsigned Offset = 0);
 
   /// Retrieve the module loader associated with the preprocessor.
   ModuleLoader &getModuleLoader() const;
 
+  /// Invent a new identifier for parameters of abbreviated templates.
+  IdentifierInfo *
+  InventAbbreviatedTemplateParameterTypeName(IdentifierInfo *ParamName,
+                                             unsigned Index);
+
   void emitAndClearUnusedLocalTypedefWarnings();
 
   enum TUFragmentKind {
     /// The global module fragment, between 'module;' and a module-declaration.
     Global,
     /// A normal translation unit fragment. For a non-module unit, this is the
     /// entire translation unit. Otherwise, it runs from the module-declaration
     /// to the private-module-fragment (if any) or the end of the TU (if not).
public:
   sema::LambdaScopeInfo *getCurGenericLambda();
 
   /// Retrieve the current captured region, if any.
   sema::CapturedRegionScopeInfo *getCurCapturedRegion();
 
   /// WeakTopLevelDeclDecls - access to \#pragma weak-generated Decls
   SmallVectorImpl<Decl *> &WeakTopLevelDecls() { return WeakTopLevelDecl; }
 
+  /// Called before parsing a function declarator belonging to a function
+  /// declaration.
+  void ActOnStartFunctionDeclarationDeclarator(Declarator &D,
+                                               unsigned TemplateParameterDepth);
+
+  /// Called after parsing a function declarator belonging to a function
+  /// declaration.
+  void ActOnFinishFunctionDeclarationDeclarator(Declarator &D);
+
   void ActOnComment(SourceRange Comment);
 
   //===--------------------------------------------------------------------===//
   // Type Analysis / Processing: SemaType.cpp.
   //
 
   QualType BuildQualifiedType(QualType T, SourceLocation Loc, Qualifiers Qs,
                               const DeclSpec *DS = nullptr);
enum NameClassificationKind {
     /// The name was classified as a template whose specializations are types.
     NC_TypeTemplate,
     /// The name was classified as a variable template name.
     NC_VarTemplate,
     /// The name was classified as a function template name.
     NC_FunctionTemplate,
     /// The name was classified as an ADL-only function template name.
     NC_UndeclaredTemplate,
+    /// The name was classified as a concept name.
+    NC_Concept,
   };
 
   class NameClassification {
     NameClassificationKind Kind;
     union {
       ExprResult Expr;
       NamedDecl *NonTypeDecl;
       TemplateName Template;
public:
     }
 
     static NameClassification FunctionTemplate(TemplateName Name) {
       NameClassification Result(NC_FunctionTemplate);
       Result.Template = Name;
       return Result;
     }
 
+    static NameClassification Concept(TemplateName Name) {
+      NameClassification Result(NC_Concept);
+      Result.Template = Name;
+      return Result;
+    }
+
     static NameClassification UndeclaredTemplate(TemplateName Name) {
       NameClassification Result(NC_UndeclaredTemplate);
       Result.Template = Name;
       return Result;
     }
 
     NameClassificationKind getKind() const { return Kind; }
 
public:
 
     NamedDecl *getNonTypeDecl() const {
       assert(Kind == NC_NonType);
       return NonTypeDecl;
     }
 
     TemplateName getTemplateName() const {
       assert(Kind == NC_TypeTemplate || Kind == NC_FunctionTemplate ||
-             Kind == NC_VarTemplate || Kind == NC_UndeclaredTemplate);
+             Kind == NC_VarTemplate || Kind == NC_Concept ||
+             Kind == NC_UndeclaredTemplate);
       return Template;
     }
 
     TemplateNameKind getTemplateNameKind() const {
       switch (Kind) {
       case NC_TypeTemplate:
         return TNK_Type_template;
       case NC_FunctionTemplate:
         return TNK_Function_template;
       case NC_VarTemplate:
         return TNK_Var_template;
+      case NC_Concept:
+        return TNK_Concept_template;
       case NC_UndeclaredTemplate:
         return TNK_Undeclared_template;
       default:
         llvm_unreachable("unsupported name classification.");
       }
     }
   };
 
public:
 
   bool AttachTypeConstraint(NestedNameSpecifierLoc NS,
                             DeclarationNameInfo NameInfo,
                             ConceptDecl *NamedConcept,
                             const TemplateArgumentListInfo *TemplateArgs,
                             TemplateTypeParmDecl *ConstrainedParameter,
                             SourceLocation EllipsisLoc);
 
+  bool AttachTypeConstraint(AutoTypeLoc TL,
+                            NonTypeTemplateParmDecl *ConstrainedParameter,
+                            SourceLocation EllipsisLoc);
+
   QualType CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
                                              SourceLocation Loc);
   QualType CheckNonTypeTemplateParameterType(QualType T, SourceLocation Loc);
 
   NamedDecl *ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
                                       unsigned Depth,
                                       unsigned Position,
                                       SourceLocation EqualLoc,
public:
   bool CheckTemplateParameterList(TemplateParameterList *NewParams,
                                   TemplateParameterList *OldParams,
                                   TemplateParamListContext TPC,
                                   SkipBodyInfo *SkipBody = nullptr);
   TemplateParameterList *MatchTemplateParametersToScopeSpecifier(
       SourceLocation DeclStartLoc, SourceLocation DeclLoc,
       const CXXScopeSpec &SS, TemplateIdAnnotation *TemplateId,
       ArrayRef<TemplateParameterList *> ParamLists,
-      bool IsFriend, bool &IsMemberSpecialization, bool &Invalid);
+      bool IsFriend, bool &IsMemberSpecialization, bool &Invalid,
+      bool SuppressDiagnostic = false);
 
   DeclResult CheckClassTemplate(
       Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
       CXXScopeSpec &SS, IdentifierInfo *Name, SourceLocation NameLoc,
       const ParsedAttributesView &Attr, TemplateParameterList *TemplateParams,
       AccessSpecifier AS, SourceLocation ModulePrivateLoc,
       SourceLocation FriendLoc, unsigned NumOuterTemplateParamLists,
       TemplateParameterList **OuterTemplateParamLists,

   enum DeduceAutoResult {
     DAR_Succeeded,
     DAR_Failed,
     DAR_FailedAlreadyDiagnosed
   };
 
   DeduceAutoResult
   DeduceAutoType(TypeSourceInfo *AutoType, Expr *&Initializer, QualType &Result,
-                 Optional<unsigned> DependentDeductionDepth = None);
+                 Optional<unsigned> DependentDeductionDepth = None,
+                 bool IgnoreConstraints = false);
   DeduceAutoResult
   DeduceAutoType(TypeLoc AutoTypeLoc, Expr *&Initializer, QualType &Result,
-                 Optional<unsigned> DependentDeductionDepth = None);
+                 Optional<unsigned> DependentDeductionDepth = None,
+                 bool IgnoreConstraints = false);
   void DiagnoseAutoDeductionFailure(VarDecl *VDecl, Expr *Init);
   bool DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
                         bool Diagnose = true);
 
   /// Declare implicit deduction guides for a class template if we've
   /// not already done so.
   void DeclareImplicitDeductionGuides(TemplateDecl *Template,
                                       SourceLocation Loc);

clang/lib/AST/ASTContext.cpp

for (TemplateParameterList::const_iterator P = Params->begin(),
     Profile(ID, C, TTP);
   }
   Expr *RequiresClause = Parm->getTemplateParameters()->getRequiresClause();
   ID.AddBoolean(RequiresClause != nullptr);
   if (RequiresClause)
     RequiresClause->Profile(ID, C, /*Canonical=*/true);
 }
 
-TemplateTemplateParmDecl *
-ASTContext::getCanonicalTemplateTemplateParmDecl(
-                                          TemplateTemplateParmDecl *TTP) const {
-  // Check if we already have a canonical template template parameter.
-  llvm::FoldingSetNodeID ID;
-  CanonicalTemplateTemplateParm::Profile(ID, *this, TTP);
-  void *InsertPos = nullptr;
-  CanonicalTemplateTemplateParm *Canonical
-    = CanonTemplateTemplateParms.FindNodeOrInsertPos(ID, InsertPos);
-  if (Canonical)
-    return Canonical->getParam();
-
-  // Build a canonical template parameter list.
-  TemplateParameterList *Params = TTP->getTemplateParameters();
-  SmallVector<NamedDecl *, 4> CanonParams;
-  CanonParams.reserve(Params->size());
-  for (TemplateParameterList::const_iterator P = Params->begin(),
-                                          PEnd = Params->end();
-       P != PEnd; ++P) {
-    if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(*P)) {
-      TemplateTypeParmDecl *NewTTP = TemplateTypeParmDecl::Create(*this,
-          getTranslationUnitDecl(), SourceLocation(), SourceLocation(),
-          TTP->getDepth(), TTP->getIndex(), nullptr, false,
-          TTP->isParameterPack(), TTP->hasTypeConstraint(),
-          TTP->isExpandedParameterPack() ?
-          llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
-      if (const auto *TC = TTP->getTypeConstraint()) {
-        // This is a bit ugly - we need to form a new immediately-declared
-        // constraint that references the new parameter; this would ideally
-        // require semantic analysis (e.g. template<C T> struct S {}; - the
-        // converted arguments of C<T> could be an argument pack if C is
-        // declared as template<typename... T> concept C = ...).
-        // We don't have semantic analysis here so we dig deep into the
-        // ready-made constraint expr and change the thing manually.
-        Expr *IDC = TC->getImmediatelyDeclaredConstraint();
-        ConceptSpecializationExpr *CSE;
-        if (const auto *Fold = dyn_cast<CXXFoldExpr>(IDC))
-          CSE = cast<ConceptSpecializationExpr>(Fold->getLHS());
-        else
-          CSE = cast<ConceptSpecializationExpr>(IDC);
-        ArrayRef<TemplateArgument> OldConverted = CSE->getTemplateArguments();
-        SmallVector<TemplateArgument, 3> NewConverted;
-        NewConverted.reserve(OldConverted.size());
-
-        QualType ParamAsArgument(NewTTP->getTypeForDecl(), 0);
-        if (OldConverted.front().getKind() == TemplateArgument::Pack) {
-          // The case:
-          // template<typename... T> concept C = true;
-          // template<C<int> T> struct S; -> constraint is C<{T, int}>
-          NewConverted.push_back(ParamAsArgument);
-          for (auto &Arg : OldConverted.front().pack_elements().drop_front(1))
-            NewConverted.push_back(Arg);
-          TemplateArgument NewPack(NewConverted);
+static Expr *
+canonicalizeImmediatelyDeclaredConstraint(const ASTContext &C, Expr *IDC,
+                                          QualType ConstrainedType) {
+    NewConverted.push_back(ConstrainedType);
 
-          NewConverted.clear();
-          NewConverted.push_back(NewPack);
-          assert(OldConverted.size() == 1 &&
-                 "Template parameter pack should be the last parameter");
-        } else {
-          assert(OldConverted.front().getKind() == TemplateArgument::Type &&
-                 "Unexpected first argument kind for immediately-declared "
-                 "constraint");
-          NewConverted.push_back(ParamAsArgument);
-          for (auto &Arg : OldConverted.drop_front(1))
-            NewConverted.push_back(Arg);
-        }
-        Expr *NewIDC = ConceptSpecializationExpr::Create(*this,
-            NestedNameSpecifierLoc(), /*TemplateKWLoc=*/SourceLocation(),
-            CSE->getConceptNameInfo(), /*FoundDecl=*/CSE->getNamedConcept(),
-            CSE->getNamedConcept(),
-            // Actually canonicalizing a TemplateArgumentLoc is difficult so we
-            // simply omit the ArgsAsWritten
-            /*ArgsAsWritten=*/nullptr, NewConverted, nullptr);
+    NewConverted.push_back(ConstrainedType);
+  Expr *NewIDC = ConceptSpecializationExpr::Create(
+      C, NestedNameSpecifierLoc(), /*TemplateKWLoc=*/SourceLocation(),
 
-        if (auto *OrigFold = dyn_cast<CXXFoldExpr>(IDC))
-          NewIDC = new (*this) CXXFoldExpr(OrigFold->getType(),
-                                           SourceLocation(), NewIDC,
-                                           BinaryOperatorKind::BO_LAnd,
-                                           SourceLocation(), /*RHS=*/nullptr,
-                                           SourceLocation(),
-                                           /*NumExpansions=*/None);
+    NewIDC = new (C) CXXFoldExpr(OrigFold->getType(), SourceLocation(), NewIDC,
+                                 SourceLocation(), /*NumExpansions=*/None);
+  return NewIDC;
+}
 
+TemplateTemplateParmDecl *
+ASTContext::getCanonicalTemplateTemplateParmDecl(
+                                          TemplateTemplateParmDecl *TTP) const {
+  // Check if we already have a canonical template template parameter.
+  llvm::FoldingSetNodeID ID;
+  CanonicalTemplateTemplateParm::Profile(ID, *this, TTP);
+  void *InsertPos = nullptr;
+  CanonicalTemplateTemplateParm *Canonical
+    = CanonTemplateTemplateParms.FindNodeOrInsertPos(ID, InsertPos);
+  if (Canonical)
+    return Canonical->getParam();
+
+  // Build a canonical template parameter list.
+  TemplateParameterList *Params = TTP->getTemplateParameters();
+  SmallVector<NamedDecl *, 4> CanonParams;
+  CanonParams.reserve(Params->size());
+  for (TemplateParameterList::const_iterator P = Params->begin(),
+                                          PEnd = Params->end();
+       P != PEnd; ++P) {
+    if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(*P)) {
+      TemplateTypeParmDecl *NewTTP = TemplateTypeParmDecl::Create(*this,
+          getTranslationUnitDecl(), SourceLocation(), SourceLocation(),
+          TTP->getDepth(), TTP->getIndex(), nullptr, false,
+          TTP->isParameterPack(), TTP->hasTypeConstraint(),
+          TTP->isExpandedParameterPack() ?
+          llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
+      if (const auto *TC = TTP->getTypeConstraint()) {
+        QualType ParamAsArgument(NewTTP->getTypeForDecl(), 0);
+        Expr *NewIDC = canonicalizeImmediatelyDeclaredConstraint(
+                *this, TC->getImmediatelyDeclaredConstraint(),
+                ParamAsArgument);
+        TemplateArgumentListInfo CanonArgsAsWritten;
+        if (auto *Args = TC->getTemplateArgsAsWritten())
+          for (const auto &ArgLoc : Args->arguments())
+            CanonArgsAsWritten.addArgument(
+                TemplateArgumentLoc(ArgLoc.getArgument(),
+                                    TemplateArgumentLocInfo()));
         NewTTP->setTypeConstraint(
             NestedNameSpecifierLoc(),
             DeclarationNameInfo(TC->getNamedConcept()->getDeclName(),
                                 SourceLocation()), /*FoundDecl=*/nullptr,
             // Actually canonicalizing a TemplateArgumentLoc is difficult so we
             // simply omit the ArgsAsWritten
-            CSE->getNamedConcept(), /*ArgsAsWritten=*/nullptr, NewIDC);
+            TC->getNamedConcept(), /*ArgsAsWritten=*/nullptr, NewIDC);
       }
       CanonParams.push_back(NewTTP);
     } else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(*P)) {
       QualType T = getCanonicalType(NTTP->getType());
       TypeSourceInfo *TInfo = getTrivialTypeSourceInfo(T);
       NonTypeTemplateParmDecl *Param;
       if (NTTP->isExpandedParameterPack()) {
         SmallVector<QualType, 2> ExpandedTypes;
if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(*P)) {
                                                 SourceLocation(),
                                                 SourceLocation(),
                                                 NTTP->getDepth(),
                                                 NTTP->getPosition(), nullptr,
                                                 T,
                                                 NTTP->isParameterPack(),
                                                 TInfo);
       }
+      if (AutoType *AT = T->getContainedAutoType()) {
+        if (AT->isConstrained()) {
+          Param->setPlaceholderTypeConstraint(
+              canonicalizeImmediatelyDeclaredConstraint(
+                  *this, NTTP->getPlaceholderTypeConstraint(), T));
+        }
+      }
       CanonParams.push_back(Param);
 
     } else
       CanonParams.push_back(getCanonicalTemplateTemplateParmDecl(
                                            cast<TemplateTemplateParmDecl>(*P)));
   }
 
   Expr *CanonRequiresClause = nullptr;
static bool isAddrSpaceMapManglingEnabled(const TargetInfo &TI,
   llvm_unreachable("getAddressSpaceMapMangling() doesn't cover anything.");
 }
 
 ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
                        IdentifierTable &idents, SelectorTable &sels,
                        Builtin::Context &builtins)
     : ConstantArrayTypes(this_()), FunctionProtoTypes(this_()),
       TemplateSpecializationTypes(this_()),
-      DependentTemplateSpecializationTypes(this_()),
+      DependentTemplateSpecializationTypes(this_()), AutoTypes(this_()),
       SubstTemplateTemplateParmPacks(this_()),
       CanonTemplateTemplateParms(this_()), SourceMgr(SM), LangOpts(LOpts),
       SanitizerBL(new SanitizerBlacklist(LangOpts.SanitizerBlacklistFiles, SM)),
       XRayFilter(new XRayFunctionFilter(LangOpts.XRayAlwaysInstrumentFiles,
                                         LangOpts.XRayNeverInstrumentFiles,
                                         LangOpts.XRayAttrListFiles, SM)),
       PrintingPolicy(LOpts), Idents(idents), Selectors(sels),
       BuiltinInfo(builtins), DeclarationNames(*this), Comments(SM),
QualType ASTContext::getUnaryTransformType(QualType BaseType,
   }
   Types.push_back(ut);
   return QualType(ut, 0);
 }
 
 /// getAutoType - Return the uniqued reference to the 'auto' type which has been
 /// deduced to the given type, or to the canonical undeduced 'auto' type, or the
 /// canonical deduced-but-dependent 'auto' type.
-QualType ASTContext::getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
-                                 bool IsDependent, bool IsPack) const {
+QualType
+ASTContext::getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
+                        bool IsDependent, bool IsPack,
+                        ConceptDecl *TypeConstraintConcept,
+                        ArrayRef<TemplateArgument> TypeConstraintArgs) const {
   assert((!IsPack || IsDependent) && "only use IsPack for a dependent pack");
-  if (DeducedType.isNull() && Keyword == AutoTypeKeyword::Auto && !IsDependent)
+  if (DeducedType.isNull() && Keyword == AutoTypeKeyword::Auto &&
+      !TypeConstraintConcept && !IsDependent)
     return getAutoDeductType();
 
   // Look in the folding set for an existing type.
   void *InsertPos = nullptr;
   llvm::FoldingSetNodeID ID;
-  AutoType::Profile(ID, DeducedType, Keyword, IsDependent, IsPack);
+  AutoType::Profile(ID, *this, DeducedType, Keyword, IsDependent,
+                    TypeConstraintConcept, TypeConstraintArgs);
   if (AutoType *AT = AutoTypes.FindNodeOrInsertPos(ID, InsertPos))
     return QualType(AT, 0);
 
-  auto *AT = new (*this, TypeAlignment)
-      AutoType(DeducedType, Keyword, IsDependent, IsPack);
+  void *Mem = Allocate(sizeof(AutoType) +
+                       sizeof(TemplateArgument) * TypeConstraintArgs.size(),
+                       TypeAlignment);
+  auto *AT = new (Mem) AutoType(DeducedType, Keyword, IsDependent, IsPack,
+                                TypeConstraintConcept, TypeConstraintArgs);
   Types.push_back(AT);
   if (InsertPos)
     AutoTypes.InsertNode(AT, InsertPos);
   return QualType(AT, 0);
 }
 
 /// Return the uniqued reference to the deduced template specialization type
 /// which has been deduced to the given type, or to the canonical undeduced
QualType ASTContext::getAtomicType(QualType T) const {
   return QualType(New, 0);
 }
 
 /// getAutoDeductType - Get type pattern for deducing against 'auto'.
 QualType ASTContext::getAutoDeductType() const {
   if (AutoDeductTy.isNull())
     AutoDeductTy = QualType(
       new (*this, TypeAlignment) AutoType(QualType(), AutoTypeKeyword::Auto,
-                                          /*dependent*/false, /*pack*/false),
+                                          /*dependent*/false, /*pack*/false,
+                                          /*concept*/nullptr, /*args*/{}),
       0);
   return AutoDeductTy;
 }
 
 /// getAutoRRefDeductType - Get type pattern for deducing against 'auto &&'.
 QualType ASTContext::getAutoRRefDeductType() const {
   if (AutoRRefDeductTy.isNull())
     AutoRRefDeductTy = getRValueReferenceType(getAutoDeductType());

clang/lib/AST/ASTImporter.cpp

 }
 
 ExpectedType ASTNodeImporter::VisitAutoType(const AutoType *T) {
   // FIXME: Make sure that the "to" context supports C++11!
   ExpectedType ToDeducedTypeOrErr = import(T->getDeducedType());
   if (!ToDeducedTypeOrErr)
     return ToDeducedTypeOrErr.takeError();
 
-  return Importer.getToContext().getAutoType(*ToDeducedTypeOrErr,
-                                             T->getKeyword(),
-                                             /*IsDependent*/false);
+  ExpectedDecl ToTypeConstraintConcept = import(T->getTypeConstraintConcept());
+  if (!ToTypeConstraintConcept)
+    return ToTypeConstraintConcept.takeError();
+
+  SmallVector<TemplateArgument, 2> ToTemplateArgs;
+  ArrayRef<TemplateArgument> FromTemplateArgs = T->getTypeConstraintArguments();
+  if (Error Err = ImportTemplateArguments(FromTemplateArgs.data(),
+                                          FromTemplateArgs.size(),
+                                          ToTemplateArgs))
+    return std::move(Err);
+
+  return Importer.getToContext().getAutoType(
+      *ToDeducedTypeOrErr, T->getKeyword(), /*IsDependent*/false,
+      /*IsPack=*/false, cast_or_null<ConceptDecl>(*ToTypeConstraintConcept),
+      ToTemplateArgs);
 }
 
 ExpectedType ASTNodeImporter::VisitInjectedClassNameType(
     const InjectedClassNameType *T) {
   Expected<CXXRecordDecl *> ToDeclOrErr = import(T->getDecl());
   if (!ToDeclOrErr)
     return ToDeclOrErr.takeError();
 

clang/lib/AST/ASTStructuralEquivalence.cpp

static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
 
   case Type::Decltype:
     if (!IsStructurallyEquivalent(Context,
                                   cast<DecltypeType>(T1)->getUnderlyingExpr(),
                                   cast<DecltypeType>(T2)->getUnderlyingExpr()))
       return false;
     break;
 
-  case Type::Auto:
-    if (!IsStructurallyEquivalent(Context, cast<AutoType>(T1)->getDeducedType(),
-                                  cast<AutoType>(T2)->getDeducedType()))
+  case Type::Auto: {
+    auto *Auto1 = cast<AutoType>(T1);
+    auto *Auto2 = cast<AutoType>(T2);
+    if (!IsStructurallyEquivalent(Context, Auto1->getDeducedType(),
+                                  Auto2->getDeducedType()))
+      return false;
+    if (Auto1->isConstrained() != Auto2->isConstrained())
+      return false;
+    if (Auto1->isConstrained()) {
+      if (Auto1->getTypeConstraintConcept() !=
+          Auto2->getTypeConstraintConcept())
+        return false;
+      ArrayRef<TemplateArgument> Auto1Args =
+          Auto1->getTypeConstraintArguments();
+      ArrayRef<TemplateArgument> Auto2Args =
+          Auto2->getTypeConstraintArguments();
+      if (Auto1Args.size() != Auto2Args.size())
         return false;
+      for (unsigned I = 0, N = Auto1Args.size(); I != N; ++I) {
+        if (!IsStructurallyEquivalent(Context, Auto1Args[I], Auto2Args[I]))
+          return false;
+      }
+    }
     break;
+  }
 
   case Type::DeducedTemplateSpecialization: {
     const auto *DT1 = cast<DeducedTemplateSpecializationType>(T1);
     const auto *DT2 = cast<DeducedTemplateSpecializationType>(T2);
     if (!IsStructurallyEquivalent(Context, DT1->getTemplateName(),
                                   DT2->getTemplateName()))
       return false;
     if (!IsStructurallyEquivalent(Context, DT1->getDeducedType(),

clang/lib/AST/DeclTemplate.cpp

for (NamedDecl *P : *Params) {
     if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(P))
       AdoptTemplateParameterList(TTP->getTemplateParameters(), Owner);
   }
 }
 
 void TemplateParameterList::
 getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
   if (HasConstrainedParameters)
-    for (const NamedDecl *Param : *this)
-      if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
+    for (const NamedDecl *Param : *this) {
+      if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param)) {
         if (const auto *TC = TTP->getTypeConstraint())
           AC.push_back(TC->getImmediatelyDeclaredConstraint());
+      } else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+        if (const Expr *E = NTTP->getPlaceholderTypeConstraint())
+          AC.push_back(E);
+      }
+    }
   if (HasRequiresClause)
     AC.push_back(getRequiresClause());
 }
 
 bool TemplateParameterList::hasAssociatedConstraints() const {
   return HasRequiresClause || HasConstrainedParameters;
 }
 

 }
 
 NonTypeTemplateParmDecl *
 NonTypeTemplateParmDecl::Create(const ASTContext &C, DeclContext *DC,
                                 SourceLocation StartLoc, SourceLocation IdLoc,
                                 unsigned D, unsigned P, IdentifierInfo *Id,
                                 QualType T, bool ParameterPack,
                                 TypeSourceInfo *TInfo) {
-  return new (C, DC) NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id,
-                                             T, ParameterPack, TInfo);
+  AutoType *AT = TInfo->getType()->getContainedAutoType();
+  return new (C, DC,
+              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
+                                    Expr *>(0,
+                                            AT && AT->isConstrained() ? 1 : 0))
+      NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id, T, ParameterPack,
+                              TInfo);
 }
 
 NonTypeTemplateParmDecl *NonTypeTemplateParmDecl::Create(
     const ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
     SourceLocation IdLoc, unsigned D, unsigned P, IdentifierInfo *Id,
     QualType T, TypeSourceInfo *TInfo, ArrayRef<QualType> ExpandedTypes,
     ArrayRef<TypeSourceInfo *> ExpandedTInfos) {
+  AutoType *AT = TInfo->getType()->getContainedAutoType();
   return new (C, DC,
-              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>>(
-                  ExpandedTypes.size()))
+              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
+                                    Expr *>(
+                  ExpandedTypes.size(), AT && AT->isConstrained() ? 1 : 0))
       NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id, T, TInfo,
                               ExpandedTypes, ExpandedTInfos);
 }
 
 NonTypeTemplateParmDecl *
-NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) NonTypeTemplateParmDecl(nullptr, SourceLocation(),
-                                             SourceLocation(), 0, 0, nullptr,
-                                             QualType(), false, nullptr);
+NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID,
+                                            bool HasTypeConstraint) {
+  return new (C, ID, additionalSizeToAlloc<std::pair<QualType,
+                                                     TypeSourceInfo *>,
+                                           Expr *>(0,
+                                                   HasTypeConstraint ? 1 : 0))
+          NonTypeTemplateParmDecl(nullptr, SourceLocation(), SourceLocation(),
+                                  0, 0, nullptr, QualType(), false, nullptr);
 }
 
 NonTypeTemplateParmDecl *
 NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID,
-                                            unsigned NumExpandedTypes) {
+                                            unsigned NumExpandedTypes,
+                                            bool HasTypeConstraint) {
   auto *NTTP =
-      new (C, ID, additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>>(
-                      NumExpandedTypes))
+      new (C, ID, additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
+                                        Expr *>(
+                      NumExpandedTypes, HasTypeConstraint ? 1 : 0))
           NonTypeTemplateParmDecl(nullptr, SourceLocation(), SourceLocation(),
                                   0, 0, nullptr, QualType(), nullptr, None,
                                   None);
   NTTP->NumExpandedTypes = NumExpandedTypes;
   return NTTP;
 }
 
 SourceRange NonTypeTemplateParmDecl::getSourceRange() const {
void TypeConstraint::print(llvm::raw_ostream &OS, PrintingPolicy Policy) const {
   ConceptName.printName(OS, Policy);
   if (hasExplicitTemplateArgs()) {
     OS << "<";
     for (auto &ArgLoc : ArgsAsWritten->arguments())
       ArgLoc.getArgument().print(Policy, OS);
     OS << ">";
   }
 }
 No newline at end of file

clang/lib/AST/ODRHash.cpp

public:
 
   void VisitDeducedType(const DeducedType *T) {
     AddQualType(T->getDeducedType());
     VisitType(T);
   }
 
   void VisitAutoType(const AutoType *T) {
     ID.AddInteger((unsigned)T->getKeyword());
+    ID.AddInteger(T->isConstrained());
+    if (T->isConstrained()) {
+      AddDecl(T->getTypeConstraintConcept());
+      ID.AddInteger(T->getNumArgs());
+      for (const auto &TA : T->getTypeConstraintArguments())
+        Hash.AddTemplateArgument(TA);
+    }
     VisitDeducedType(T);
   }
 
   void VisitDeducedTemplateSpecializationType(
       const DeducedTemplateSpecializationType *T) {
     Hash.AddTemplateName(T->getTemplateName());
     VisitDeducedType(T);
   }

clang/lib/AST/TemplateBase.cpp

case TemplateArgument::Pack: {
     return DB << OS.str();
   }
   }
 
   llvm_unreachable("Invalid TemplateArgument Kind!");
 }
 
 const ASTTemplateArgumentListInfo *
-ASTTemplateArgumentListInfo::Create(ASTContext &C,
+ASTTemplateArgumentListInfo::Create(const ASTContext &C,
                                     const TemplateArgumentListInfo &List) {
   std::size_t size = totalSizeToAlloc<TemplateArgumentLoc>(List.size());
   void *Mem = C.Allocate(size, alignof(ASTTemplateArgumentListInfo));
   return new (Mem) ASTTemplateArgumentListInfo(List);
 }
 
 ASTTemplateArgumentListInfo::ASTTemplateArgumentListInfo(
     const TemplateArgumentListInfo &Info) {

clang/lib/AST/TextNodeDumper.cpp

void TextNodeDumper::VisitTemplateTypeParmType(const TemplateTypeParmType *T) {
   dumpDeclRef(T->getDecl());
 }
 
 void TextNodeDumper::VisitAutoType(const AutoType *T) {
   if (T->isDecltypeAuto())
     OS << " decltype(auto)";
   if (!T->isDeduced())
     OS << " undeduced";
+  if (T->isConstrained()) {
+    dumpDeclRef(T->getTypeConstraintConcept());
+    for (const auto &Arg : T->getTypeConstraintArguments())
+      VisitTemplateArgument(Arg);
+  }
 }
 
 void TextNodeDumper::VisitTemplateSpecializationType(
     const TemplateSpecializationType *T) {
   if (T->isTypeAlias())
     OS << " alias";
   OS << " ";
   T->getTemplateName().dump(OS);

clang/lib/AST/Type.cpp

QualType VisitAutoType(const AutoType *T) {
     if (deducedType.isNull())
       return {};
 
     if (deducedType.getAsOpaquePtr()
           == T->getDeducedType().getAsOpaquePtr())
       return QualType(T, 0);
 
     return Ctx.getAutoType(deducedType, T->getKeyword(),
-                           T->isDependentType());
+                           T->isDependentType(), /*IsPack=*/false,
+                           T->getTypeConstraintConcept(),
+                           T->getTypeConstraintArguments());
   }
 
   // FIXME: Non-trivial to implement, but important for C++
   SUGARED_TYPE_CLASS(PackExpansion)
 
   QualType VisitObjCObjectType(const ObjCObjectType *T) {
     QualType baseType = recurse(T->getBaseType());
     if (baseType.isNull())

 
 void clang::FixedPointValueToString(SmallVectorImpl<char> &Str,
                                     llvm::APSInt Val, unsigned Scale) {
   FixedPointSemantics FXSema(Val.getBitWidth(), Scale, Val.isSigned(),
                              /*IsSaturated=*/false,
                              /*HasUnsignedPadding=*/false);
   APFixedPoint(Val, FXSema).toString(Str);
 }
+
+AutoType::AutoType(QualType DeducedAsType, AutoTypeKeyword Keyword,
+                   bool IsDeducedAsDependent, bool IsDeducedAsPack,
+                   ConceptDecl *TypeConstraintConcept,
+                   ArrayRef<TemplateArgument> TypeConstraintArgs)
+    : DeducedType(Auto, DeducedAsType, IsDeducedAsDependent,
+                  IsDeducedAsDependent, IsDeducedAsPack) {
+  AutoTypeBits.Keyword = (unsigned)Keyword;
+  AutoTypeBits.NumArgs = TypeConstraintArgs.size();
+  this->TypeConstraintConcept = TypeConstraintConcept;
+  if (TypeConstraintConcept) {
+    TemplateArgument *ArgBuffer = getArgBuffer();
+    for (const TemplateArgument &Arg : TypeConstraintArgs) {
+      if (Arg.containsUnexpandedParameterPack())
+        setContainsUnexpandedParameterPack();
+
+      new (ArgBuffer++) TemplateArgument(Arg);
+    }
+  }
+}
+
+void AutoType::Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
+                      QualType Deduced, AutoTypeKeyword Keyword,
+                      bool IsDependent, ConceptDecl *CD,
+                      ArrayRef<TemplateArgument> Arguments) {
+  ID.AddPointer(Deduced.getAsOpaquePtr());
+  ID.AddInteger((unsigned)Keyword);
+  ID.AddBoolean(IsDependent);
+  ID.AddPointer(CD);
+  for (const TemplateArgument &Arg : Arguments)
+    Arg.Profile(ID, Context);
+}

clang/lib/AST/TypeLoc.cpp

 //===- TypeLoc.cpp - Type Source Info Wrapper -----------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the TypeLoc subclasses implementations.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/TypeLoc.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/TemplateName.h"
 #include "clang/AST/TypeLocVisitor.h"
 #include "clang/Basic/SourceLocation.h"
for (unsigned i = 0, e = NumArgs; i != e; ++i) {
     }
 
     case TemplateArgument::Pack:
       ArgInfos[i] = TemplateArgumentLocInfo();
       break;
     }
   }
 }
+
+DeclarationNameInfo AutoTypeLoc::getConceptNameInfo() const {
+  return DeclarationNameInfo(getNamedConcept()->getDeclName(),
+                             getLocalData()->ConceptNameLoc);
+}
+
+void AutoTypeLoc::initializeLocal(ASTContext &Context, SourceLocation Loc) {
+  setNestedNameSpecifierLoc(NestedNameSpecifierLoc());
+  setTemplateKWLoc(Loc);
+  setConceptNameLoc(Loc);
+  setFoundDecl(nullptr);
+  setRAngleLoc(Loc);
+  setLAngleLoc(Loc);
+  TemplateSpecializationTypeLoc::initializeArgLocs(Context, getNumArgs(),
+                                                   getTypePtr()->getArgs(),
+                                                   getArgInfos(), Loc);
+  setNameLoc(Loc);
+}
+
+
+namespace {
+
+  class GetContainedAutoTypeLocVisitor :
+    public TypeLocVisitor<GetContainedAutoTypeLocVisitor, TypeLoc> {
+  public:
+    using TypeLocVisitor<GetContainedAutoTypeLocVisitor, TypeLoc>::Visit;
+
+    TypeLoc VisitAutoTypeLoc(AutoTypeLoc TL) {
+      return TL;
+    }
+
+    // Only these types can contain the desired 'auto' type.
+
+    TypeLoc VisitElaboratedTypeLoc(ElaboratedTypeLoc T) {
+      return Visit(T.getNamedTypeLoc());
+    }
+
+    TypeLoc VisitQualifiedTypeLoc(QualifiedTypeLoc T) {
+      return Visit(T.getUnqualifiedLoc());
+    }
+
+    TypeLoc VisitPointerTypeLoc(PointerTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitBlockPointerTypeLoc(BlockPointerTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitReferenceTypeLoc(ReferenceTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitMemberPointerTypeLoc(MemberPointerTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitArrayTypeLoc(ArrayTypeLoc T) {
+      return Visit(T.getElementLoc());
+    }
+
+    TypeLoc VisitFunctionTypeLoc(FunctionTypeLoc T) {
+      return Visit(T.getReturnLoc());
+    }
+
+    TypeLoc VisitParenTypeLoc(ParenTypeLoc T) {
+      return Visit(T.getInnerLoc());
+    }
+
+    TypeLoc VisitAttributedTypeLoc(AttributedTypeLoc T) {
+      return Visit(T.getModifiedLoc());
+    }
+
+    TypeLoc VisitMacroQualifiedTypeLoc(MacroQualifiedTypeLoc T) {
+      return Visit(T.getInnerLoc());
+    }
+
+    TypeLoc VisitAdjustedTypeLoc(AdjustedTypeLoc T) {
+      return Visit(T.getOriginalLoc());
+    }
+
+    TypeLoc VisitPackExpansionTypeLoc(PackExpansionTypeLoc T) {
+      return Visit(T.getPatternLoc());
+    }
+  };
+
+} // namespace
+
+AutoTypeLoc TypeLoc::getContainedAutoTypeLoc() const {
+  TypeLoc Res = GetContainedAutoTypeLocVisitor().Visit(*this);
+  if (Res.isNull())
+    return AutoTypeLoc();
+  return Res.getAs<AutoTypeLoc>();
+}

clang/lib/AST/TypePrinter.cpp

void TypePrinter::printUnaryTransformAfter(const UnaryTransformType *T,
   printAfter(T->getBaseType(), OS);
 }
 
 void TypePrinter::printAutoBefore(const AutoType *T, raw_ostream &OS) {
   // If the type has been deduced, do not print 'auto'.
   if (!T->getDeducedType().isNull()) {
     printBefore(T->getDeducedType(), OS);
   } else {
+    if (T->isConstrained()) {
+      OS << T->getTypeConstraintConcept()->getName();
+      auto Args = T->getTypeConstraintArguments();
+      if (!Args.empty())
+        printTemplateArgumentList(OS, Args, Policy);
+      OS << ' ';
+    }
     switch (T->getKeyword()) {
     case AutoTypeKeyword::Auto: OS << "auto"; break;
     case AutoTypeKeyword::DecltypeAuto: OS << "decltype(auto)"; break;
     case AutoTypeKeyword::GNUAutoType: OS << "__auto_type"; break;
     }
     spaceBeforePlaceHolder(OS);
   }
 }

 void TypePrinter::printEnumBefore(const EnumType *T, raw_ostream &OS) {
   printTag(T->getDecl(), OS);
 }
 
 void TypePrinter::printEnumAfter(const EnumType *T, raw_ostream &OS) {}
 
 void TypePrinter::printTemplateTypeParmBefore(const TemplateTypeParmType *T,
                                               raw_ostream &OS) {
-  if (IdentifierInfo *Id = T->getIdentifier())
-    OS << Id->getName();
-  else {
-    bool IsLambdaAutoParam = false;
-    if (auto D = T->getDecl()) {
-      if (auto M = dyn_cast_or_null<CXXMethodDecl>(D->getDeclContext()))
-        IsLambdaAutoParam = D->isImplicit() && M->getParent()->isLambda();
+  TemplateTypeParmDecl *D = T->getDecl();
+  if (D && D->isImplicit()) {
+    if (auto *TC = D->getTypeConstraint()) {
+      TC->print(OS, Policy);
+      OS << ' ';
     }
-
-    if (IsLambdaAutoParam)
-      OS << "auto";
-    else
-      OS << "type-parameter-" << T->getDepth() << '-' << T->getIndex();
-  }
+  } else if (IdentifierInfo *Id = T->getIdentifier())
+    OS << Id->getName();
+
   spaceBeforePlaceHolder(OS);
 }
 
 void TypePrinter::printTemplateTypeParmAfter(const TemplateTypeParmType *T,
                                              raw_ostream &OS) {}
 
 void TypePrinter::printSubstTemplateTypeParmBefore(
                                              const SubstTemplateTypeParmType *T,

clang/lib/Parse/ParseCXXInlineMethods.cpp

if (getLangOpts().DelayedTemplateParsing &&
 
     return FnD;
   }
 
   // Consume the tokens and store them for later parsing.
 
   LexedMethod* LM = new LexedMethod(this, FnD);
   getCurrentClass().LateParsedDeclarations.push_back(LM);
-  LM->TemplateScope = getCurScope()->isTemplateParamScope();
+  LM->TemplateScope = getCurScope()->isTemplateParamScope() ||
+      (FnD && isa<FunctionTemplateDecl>(FnD) &&
+       cast<FunctionTemplateDecl>(FnD)->isAbbreviated());
   CachedTokens &Toks = LM->Toks;
 
   tok::TokenKind kind = Tok.getKind();
   // Consume everything up to (and including) the left brace of the
   // function body.
   if (ConsumeAndStoreFunctionPrologue(Toks)) {
     // We didn't find the left-brace we expected after the
     // constructor initializer; we already printed an error, and it's likely

clang/lib/Parse/ParseDecl.cpp

if (Next.isOneOf(tok::star, tok::amp, tok::ampamp, tok::identifier,
         break;
 
       case Sema::NC_Unknown:
       case Sema::NC_NonType:
       case Sema::NC_DependentNonType:
       case Sema::NC_ContextIndependentExpr:
       case Sema::NC_VarTemplate:
       case Sema::NC_FunctionTemplate:
+      case Sema::NC_Concept:
         // Might be a redeclaration of a prior entity.
         break;
       }
     }
   }
 
   if (MightBeDeclarator)
     return false;
case tok::annot_cxxscope: {
         DS.getTypeSpecScope() = SS;
         ConsumeAnnotationToken(); // The C++ scope.
         assert(Tok.is(tok::annot_template_id) &&
                "ParseOptionalCXXScopeSpecifier not working");
         AnnotateTemplateIdTokenAsType(SS);
         continue;
       }
 
+      if (Next.is(tok::annot_template_id) &&
+          static_cast<TemplateIdAnnotation *>(Next.getAnnotationValue())
+            ->Kind == TNK_Concept_template &&
+          GetLookAheadToken(2).isOneOf(tok::kw_auto, tok::kw_decltype)) {
+        DS.getTypeSpecScope() = SS;
+        // This is a qualified placeholder-specifier, e.g., ::C<int> auto ...
+        // Consume the scope annotation and continue to consume the template-id
+        // as a placeholder-specifier.
+        ConsumeAnnotationToken();
+        continue;
+      }
+
       if (Next.is(tok::annot_typename)) {
         DS.getTypeSpecScope() = SS;
         ConsumeAnnotationToken(); // The C++ scope.
         if (Tok.getAnnotationValue()) {
           ParsedType T = getTypeAnnotation(Tok);
           isInvalid = DS.SetTypeSpecType(DeclSpec::TST_typename,
                                          Tok.getAnnotationEndLoc(),
                                          PrevSpec, DiagID, T, Policy);
case tok::annot_cxxscope: {
                               /*WantNontrivialTypeSourceInfo=*/true,
                               isClassTemplateDeductionContext(DSContext));
 
       // If the referenced identifier is not a type, then this declspec is
       // erroneous: We already checked about that it has no type specifier, and
       // C++ doesn't have implicit int.  Diagnose it as a typo w.r.t. to the
       // typename.
       if (!TypeRep) {
+        if (TryAnnotateTypeConstraint())
+          goto DoneWithDeclSpec;
+        if (isTypeConstraintAnnotation())
+          continue;
         // Eat the scope spec so the identifier is current.
         ConsumeAnnotationToken();
         ParsedAttributesWithRange Attrs(AttrFactory);
         if (ParseImplicitInt(DS, &SS, TemplateInfo, AS, DSContext, Attrs)) {
           if (!Attrs.empty()) {
             AttrsLastTime = true;
             attrs.takeAllFrom(Attrs);
           }
case tok::identifier: {
       ParsedType TypeRep = Actions.getTypeName(
           *Tok.getIdentifierInfo(), Tok.getLocation(), getCurScope(), nullptr,
           false, false, nullptr, false, false,
           isClassTemplateDeductionContext(DSContext));
 
       // If this is not a typedef name, don't parse it as part of the declspec,
       // it must be an implicit int or an error.
       if (!TypeRep) {
+        if (TryAnnotateTypeConstraint())
+          goto DoneWithDeclSpec;
+        if (isTypeConstraintAnnotation())
+          continue;
         ParsedAttributesWithRange Attrs(AttrFactory);
         if (ParseImplicitInt(DS, nullptr, TemplateInfo, AS, DSContext, Attrs)) {
           if (!Attrs.empty()) {
             AttrsLastTime = true;
             attrs.takeAllFrom(Attrs);
           }
           continue;
         }
case tok::identifier: {
         }
       }
 
       // Need to support trailing type qualifiers (e.g. "id<p> const").
       // If a type specifier follows, it will be diagnosed elsewhere.
       continue;
     }
 
-      // type-name
+      // type-name or placeholder-specifier
     case tok::annot_template_id: {
       TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
+      if (TemplateId->Kind == TNK_Concept_template) {
+        if (NextToken().is(tok::identifier)) {
+          Diag(Loc, diag::err_placeholder_expected_auto_or_decltype_auto)
+              << FixItHint::CreateInsertion(NextToken().getLocation(), "auto");
+          // Attempt to continue as if 'auto' was placed here.
+          isInvalid = DS.SetTypeSpecType(TST_auto, Loc, PrevSpec, DiagID,
+                                         TemplateId, Policy);
+          break;
+        }
+        if (!NextToken().isOneOf(tok::kw_auto, tok::kw_decltype))
+            goto DoneWithDeclSpec;
+        ConsumeAnnotationToken();
+        SourceLocation AutoLoc = Tok.getLocation();
+        if (TryConsumeToken(tok::kw_decltype)) {
+          BalancedDelimiterTracker Tracker(*this, tok::l_paren);
+          if (Tracker.consumeOpen()) {
+            // Something like `void foo(Iterator decltype i)`
+            Diag(Tok, diag::err_expected) << tok::l_paren;
+          } else {
+            if (!TryConsumeToken(tok::kw_auto)) {
+              // Something like `void foo(Iterator decltype(int) i)`
+              Tracker.skipToEnd();
+              Diag(Tok, diag::err_placeholder_expected_auto_or_decltype_auto)
+                << FixItHint::CreateReplacement(SourceRange(AutoLoc,
+                                                            Tok.getLocation()),
+                                                "auto");
+            } else {
+              Tracker.consumeClose();
+            }
+          }
+          ConsumedEnd = Tok.getLocation();
+          // Even if something went wrong above, continue as if we've seen
+          // `decltype(auto)`.
+          isInvalid = DS.SetTypeSpecType(TST_decltype_auto, Loc, PrevSpec,
+                                         DiagID, TemplateId, Policy);
+        } else {
+          isInvalid = DS.SetTypeSpecType(TST_auto, Loc, PrevSpec, DiagID,
+                                         TemplateId, Policy);
+        }
+        break;
+      }
+
       if (TemplateId->Kind != TNK_Type_template &&
           TemplateId->Kind != TNK_Undeclared_template) {
         // This template-id does not refer to a type name, so we're
         // done with the type-specifiers.
         goto DoneWithDeclSpec;
       }
 
       // If we're in a context where the template-id could be a
assert(D.isPastIdentifier() &&
          "Haven't past the location of the identifier yet?");
 
   // Don't parse attributes unless we have parsed an unparenthesized name.
   if (D.hasName() && !D.getNumTypeObjects())
     MaybeParseCXX11Attributes(D);
 
   while (1) {
     if (Tok.is(tok::l_paren)) {
+      bool IsFunctionDeclaration = D.isFunctionDeclaratorAFunctionDeclaration();
       // Enter function-declaration scope, limiting any declarators to the
       // function prototype scope, including parameter declarators.
       ParseScope PrototypeScope(this,
                                 Scope::FunctionPrototypeScope|Scope::DeclScope|
-                                (D.isFunctionDeclaratorAFunctionDeclaration()
+                                (IsFunctionDeclaration
                                    ? Scope::FunctionDeclarationScope : 0));
 
       // The paren may be part of a C++ direct initializer, eg. "int x(1);".
       // In such a case, check if we actually have a function declarator; if it
       // is not, the declarator has been fully parsed.
       bool IsAmbiguous = false;
       if (getLangOpts().CPlusPlus && D.mayBeFollowedByCXXDirectInit()) {
         // The name of the declarator, if any, is tentatively declared within
         // a possible direct initializer.
         TentativelyDeclaredIdentifiers.push_back(D.getIdentifier());
         bool IsFunctionDecl = isCXXFunctionDeclarator(&IsAmbiguous);
         TentativelyDeclaredIdentifiers.pop_back();
         if (!IsFunctionDecl)
           break;
       }
       ParsedAttributes attrs(AttrFactory);
       BalancedDelimiterTracker T(*this, tok::l_paren);
       T.consumeOpen();
+      if (IsFunctionDeclaration)
+        Actions.ActOnStartFunctionDeclarationDeclarator(D,
+                                                        TemplateParameterDepth);
       ParseFunctionDeclarator(D, attrs, T, IsAmbiguous);
+      if (IsFunctionDeclaration)
+        Actions.ActOnFinishFunctionDeclarationDeclarator(D);
       PrototypeScope.Exit();
     } else if (Tok.is(tok::l_square)) {
       ParseBracketDeclarator(D);
     } else if (Tok.is(tok::kw_requires) && D.hasGroupingParens()) {
       // This declarator is declaring a function, but the requires clause is
       // in the wrong place:
       //   void (f() requires true);
       // instead of

clang/lib/Parse/ParseDeclCXX.cpp

if (TryConsumeToken(tok::semi)) {
     if (AnonRecord) {
       Decl* decls[] = {AnonRecord, TheDecl};
       return Actions.BuildDeclaratorGroup(decls);
     }
     return Actions.ConvertDeclToDeclGroup(TheDecl);
   }
 
   ParsingDeclarator DeclaratorInfo(*this, DS, DeclaratorContext::MemberContext);
+  if (TemplateInfo.TemplateParams)
+    DeclaratorInfo.setTemplateParameterLists(TemplateParams);
   VirtSpecifiers VS;
 
   // Hold late-parsed attributes so we can attach a Decl to them later.
   LateParsedAttrList LateParsedAttrs;
 
   SourceLocation EqualLoc;
   SourceLocation PureSpecLoc;
 

clang/lib/Parse/ParseTemplate.cpp

Decl *Parser::ParseSingleDeclarationAfterTemplate(
   // Move the attributes from the prefix into the DS.
   if (TemplateInfo.Kind == ParsedTemplateInfo::ExplicitInstantiation)
     ProhibitAttributes(prefixAttrs);
   else
     DS.takeAttributesFrom(prefixAttrs);
 
   // Parse the declarator.
   ParsingDeclarator DeclaratorInfo(*this, DS, (DeclaratorContext)Context);
+  if (TemplateInfo.TemplateParams)
+    DeclaratorInfo.setTemplateParameterLists(*TemplateInfo.TemplateParams);
   ParseDeclarator(DeclaratorInfo);
   // Error parsing the declarator?
   if (!DeclaratorInfo.hasName()) {
     // If so, skip until the semi-colon or a }.
     SkipUntil(tok::r_brace, StopAtSemi | StopBeforeMatch);
     if (Tok.is(tok::semi))
       ConsumeToken();
     return nullptr;

 ///         'template' '<' template-parameter-list '>' type-parameter-key
 ///               identifier[opt] '=' id-expression
 ///
 ///       type-parameter-key:
 ///         class
 ///         typename
 ///
 NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
+
   switch (isStartOfTemplateTypeParameter()) {
   case TPResult::True:
     // Is there just a typo in the input code? ('typedef' instead of
     // 'typename')
     if (Tok.is(tok::kw_typedef)) {
       Diag(Tok.getLocation(), diag::err_expected_template_parameter);
 
       Diag(Tok.getLocation(), diag::note_meant_to_use_typename)
           << FixItHint::CreateReplacement(CharSourceRange::getCharRange(
                                               Tok.getLocation(),
                                               Tok.getEndLoc()),
                                           "typename");
 
       Tok.setKind(tok::kw_typename);
     }
 
     return ParseTypeParameter(Depth, Position);
-
   case TPResult::False:
     break;
 
   case TPResult::Error: {
     // We return an invalid parameter as opposed to null to avoid having bogus
     // diagnostics about an empty template parameter list.
     // FIXME: Fix ParseTemplateParameterList to better handle nullptr results
     //  from here.

 
 /// Try parsing a type-constraint at the current location.
 ///
 ///     type-constraint:
 ///       nested-name-specifier[opt] concept-name
 ///       nested-name-specifier[opt] concept-name
 ///           '<' template-argument-list[opt] '>'[opt]
 ///
 /// \returns true if an error occurred, and false otherwise.
 bool Parser::TryAnnotateTypeConstraint() {
   if (!getLangOpts().ConceptsTS)
     return false;
-
   CXXScopeSpec SS;
   bool WasScopeAnnotation = Tok.is(tok::annot_cxxscope);
   if (ParseOptionalCXXScopeSpecifier(
           SS, ParsedType(),
           /*EnteringContext=*/false,
           /*MayBePseudoDestructor=*/nullptr,
           // If this is not a type-constraint, then
           // this scope-spec is part of the typename
           // of a non-type template parameter
           /*IsTypename=*/true, /*LastII=*/nullptr,
           // We won't find concepts in
           // non-namespaces anyway, so might as well
           // parse this correctly for possible type
           // names.
           /*OnlyNamespace=*/false))
     return true;
 
   if (Tok.is(tok::identifier)) {
     UnqualifiedId PossibleConceptName;
     PossibleConceptName.setIdentifier(Tok.getIdentifierInfo(),
                                       Tok.getLocation());
 
     TemplateTy PossibleConcept;
     bool MemberOfUnknownSpecialization = false;
     auto TNK = Actions.isTemplateName(getCurScope(), SS,
                                       /*hasTemplateKeyword=*/false,
                                       PossibleConceptName,
                                       /*ObjectType=*/ParsedType(),
                                       /*EnteringContext=*/false,
                                       PossibleConcept,
                                       MemberOfUnknownSpecialization);
     assert(!MemberOfUnknownSpecialization
            && "Member when we only allowed namespace scope qualifiers??");
     if (!PossibleConcept || TNK != TNK_Concept_template)
       return false;
 
     // At this point we're sure we're dealing with a constrained parameter. It
     // may or may not have a template parameter list following the concept
     // name.
     if (AnnotateTemplateIdToken(PossibleConcept, TNK, SS,
                                    /*TemplateKWLoc=*/SourceLocation(),
                                    PossibleConceptName,
                                    /*AllowTypeAnnotation=*/false,
                                    /*TypeConstraint=*/true))
       return true;
   }
 
   if (SS.isNotEmpty())
     AnnotateScopeToken(SS, !WasScopeAnnotation);
   return false;
 }
 
 /// ParseTypeParameter - Parse a template type parameter (C++ [temp.param]).
 /// Other kinds of template parameters are parsed in
 /// ParseTemplateTemplateParameter and ParseNonTypeTemplateParameter.
 ///
 ///       type-parameter:     [C++ temp.param]
 ///         'class' ...[opt][C++0x] identifier[opt]
 ///         'class' identifier[opt] '=' type-id

clang/lib/Parse/ParseTentative.cpp

 ///         cv-qualifier:
 ///           'const'
 ///           'volatile'
 /// [GNU]     restrict
 ///
 Parser::TPResult
 Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
                                   bool *InvalidAsDeclSpec) {
+  auto IsPlaceholderSpecifier = [&] (TemplateIdAnnotation *TemplateId,
+                                     int Lookahead) {
+    // We have a placeholder-constraint (we check for 'auto' or 'decltype' to
+    // distinguish 'C<int>;' from 'C<int> auto c = 1;')
+    return TemplateId->Kind == TNK_Concept_template &&
+        GetLookAheadToken(Lookahead + 1).isOneOf(tok::kw_auto, tok::kw_decltype,
+            // If we have an identifier here, the user probably forgot the
+            // 'auto' in the placeholder constraint, e.g. 'C<int> x = 2;'
+            // This will be diagnosed nicely later, so disambiguate as a
+            // declaration.
+            tok::identifier);
+  };
   switch (Tok.getKind()) {
   case tok::identifier: {
     // Check for need to substitute AltiVec __vector keyword
     // for "vector" identifier.
     if (TryAltiVecVectorToken())
       return TPResult::True;
 
     const Token &Next = NextToken();
case tok::annot_template_id: {
     if (TemplateId->Kind == TNK_Undeclared_template && InvalidAsDeclSpec) {
       // 'template-id(' can be a valid expression but not a valid decl spec if
       // the template-name is not declared, but we don't consider this to be a
       // definitive disambiguation. In any other context, it's an error either
       // way.
       *InvalidAsDeclSpec = NextToken().is(tok::l_paren);
       return TPResult::Ambiguous;
     }
+    if (IsPlaceholderSpecifier(TemplateId, /*Lookahead=*/0))
+      return TPResult::True;
     if (TemplateId->Kind != TNK_Type_template)
       return TPResult::False;
     CXXScopeSpec SS;
     AnnotateTemplateIdTokenAsType(SS);
     assert(Tok.is(tok::annot_typename));
     goto case_typename;
   }
 
   case tok::annot_cxxscope: // foo::bar or ::foo::bar, but already parsed
     // We've already annotated a scope; try to annotate a type.
     if (TryAnnotateTypeOrScopeToken())
       return TPResult::Error;
     if (!Tok.is(tok::annot_typename)) {
+      if (Tok.is(tok::annot_cxxscope) &&
+          NextToken().is(tok::annot_template_id)) {
+        TemplateIdAnnotation *TemplateId =
+            takeTemplateIdAnnotation(NextToken());
+        if (IsPlaceholderSpecifier(TemplateId, /*Lookahead=*/1))
+          return TPResult::True;
+      }
       // If the next token is an identifier or a type qualifier, then this
       // can't possibly be a valid expression either.
       if (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier)) {
         CXXScopeSpec SS;
         Actions.RestoreNestedNameSpecifierAnnotation(Tok.getAnnotationValue(),
                                                      Tok.getAnnotationRange(),
                                                      SS);
         if (SS.getScopeRep() && SS.getScopeRep()->isDependent()) {

clang/lib/Parse/Parser.cpp

 ///         decl-specifier-seq[opt] declarator function-try-block
 ///
 Decl *Parser::ParseFunctionDefinition(ParsingDeclarator &D,
                                       const ParsedTemplateInfo &TemplateInfo,
                                       LateParsedAttrList *LateParsedAttrs) {
   // Poison SEH identifiers so they are flagged as illegal in function bodies.
   PoisonSEHIdentifiersRAIIObject PoisonSEHIdentifiers(*this, true);
   const DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+  TemplateParameterDepthRAII CurTemplateDepthTracker(TemplateParameterDepth);
 
   // If this is C90 and the declspecs were completely missing, fudge in an
   // implicit int.  We do this here because this is the only place where
   // declaration-specifiers are completely optional in the grammar.
   if (getLangOpts().ImplicitInt && D.getDeclSpec().isEmpty()) {
     const char *PrevSpec;
     unsigned DiagID;
     const PrintingPolicy &Policy = Actions.getASTContext().getPrintingPolicy();
Decl *Parser::ParseFunctionDefinition(ParsingDeclarator &D,
 
   // Break out of the ParsingDeclarator context before we parse the body.
   D.complete(Res);
 
   // Break out of the ParsingDeclSpec context, too.  This const_cast is
   // safe because we're always the sole owner.
   D.getMutableDeclSpec().abort();
 
+  // With abbreviated function templates - we need to explicitly add depth to
+  // account for the implicit template parameter list induced by the template.
+  if (auto *Template = dyn_cast_or_null<FunctionTemplateDecl>(Res))
+    if (Template->isAbbreviated() &&
+        Template->getTemplateParameters()->getParam(0)->isImplicit())
+      // First template parameter is implicit - meaning no explicit template
+      // parameter list was specified.
+      CurTemplateDepthTracker.addDepth(1);
+
   if (TryConsumeToken(tok::equal)) {
     assert(getLangOpts().CPlusPlus && "Only C++ function definitions have '='");
 
     bool Delete = false;
     SourceLocation KWLoc;
     if (TryConsumeToken(tok::kw_delete, KWLoc)) {
       Diag(KWLoc, getLangOpts().CPlusPlus11
                       ? diag::warn_cxx98_compat_defaulted_deleted_function
case Sema::NC_UndeclaredTemplate: {
     UnqualifiedId Id;
     Id.setIdentifier(Name, NameLoc);
     if (AnnotateTemplateIdToken(
             TemplateTy::make(Classification.getTemplateName()),
             Classification.getTemplateNameKind(), SS, SourceLocation(), Id))
       return ANK_Error;
     return ANK_Success;
   }
+  case Sema::NC_Concept: {
+    UnqualifiedId Id;
+    Id.setIdentifier(Name, NameLoc);
+    if (Next.is(tok::less))
+      // We have a concept name followed by '<'. Consume the identifier token so
+      // we reach the '<' and annotate it.
+      ConsumeToken();
+    if (AnnotateTemplateIdToken(
+            TemplateTy::make(Classification.getTemplateName()),
+            Classification.getTemplateNameKind(), SS, SourceLocation(), Id,
+            /*AllowTypeAnnotation=*/false, /*TypeConstraint=*/true))
+      return ANK_Error;
+    return ANK_Success;
+  }
   }
 
   // Unable to classify the name, but maybe we can annotate a scope specifier.
   if (SS.isNotEmpty())
     AnnotateScopeToken(SS, !WasScopeAnnotation);
   return ANK_Unresolved;
 }
 
if (Tok.is(tok::annot_template_id)) {
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     if (TemplateId->Kind == TNK_Type_template) {
       // A template-id that refers to a type was parsed into a
       // template-id annotation in a context where we weren't allowed
       // to produce a type annotation token. Update the template-id
       // annotation token to a type annotation token now.
       AnnotateTemplateIdTokenAsType(SS);
       return false;
     }
   }
 
   if (SS.isEmpty())
     return false;
 
   // A C++ scope specifier that isn't followed by a typename.
   AnnotateScopeToken(SS, IsNewScope);
   return false;
 }
 

clang/lib/Sema/DeclSpec.cpp

bool DeclSpec::SetTypeSpecType(TST T, SourceLocation TagKwLoc,
   TypeSpecType = T;
   DeclRep = Rep;
   TSTLoc = TagKwLoc;
   TSTNameLoc = TagNameLoc;
   TypeSpecOwned = Owned && Rep != nullptr;
   return false;
 }
 
+bool DeclSpec::SetTypeSpecType(TST T, SourceLocation Loc, const char *&PrevSpec,
+                               unsigned &DiagID, TemplateIdAnnotation *Rep,
+                               const PrintingPolicy &Policy) {
+  assert(T == TST_auto || T == TST_decltype_auto);
+  ConstrainedAuto = true;
+  TemplateIdRep = Rep;
+  return SetTypeSpecType(T, Loc, PrevSpec, DiagID, Policy);
+}
+
 bool DeclSpec::SetTypeSpecType(TST T, SourceLocation Loc,
                                const char *&PrevSpec,
                                unsigned &DiagID,
                                const PrintingPolicy &Policy) {
   assert(!isDeclRep(T) && !isTypeRep(T) && !isExprRep(T) &&
          "rep required for these type-spec kinds!");
   if (TypeSpecType == TST_error)
     return false;

clang/lib/Sema/Sema.cpp

 using namespace sema;
 
 SourceLocation Sema::getLocForEndOfToken(SourceLocation Loc, unsigned Offset) {
   return Lexer::getLocForEndOfToken(Loc, Offset, SourceMgr, LangOpts);
 }
 
 ModuleLoader &Sema::getModuleLoader() const { return PP.getModuleLoader(); }
 
+IdentifierInfo *
+Sema::InventAbbreviatedTemplateParameterTypeName(IdentifierInfo *ParamName,
+                                                 unsigned int Index) {
+  std::string InventedName;
+  llvm::raw_string_ostream OS(InventedName);
+
+  if (!ParamName)
+    OS << "auto:" << Index + 1;
+  else
+    OS << ParamName->getName() << ":auto";
+
+  OS.flush();
+  return &Context.Idents.get(OS.str());
+}
+
 PrintingPolicy Sema::getPrintingPolicy(const ASTContext &Context,
                                        const Preprocessor &PP) {
   PrintingPolicy Policy = Context.getPrintingPolicy();
   // In diagnostics, we print _Bool as bool if the latter is defined as the
   // former.
   Policy.Bool = Context.getLangOpts().Bool;
   if (!Policy.Bool) {
     if (const MacroInfo *BoolMacro = PP.getMacroInfo(Context.getBoolName())) {

clang/lib/Sema/SemaDecl.cpp

 //===--- SemaDecl.cpp - Semantic Analysis for Declarations ----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file implements semantic analysis for declarations.
 //
 //===----------------------------------------------------------------------===//
 
+#include "TreeTransform.h"
 #include "TypeLocBuilder.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/CommentDiagnostic.h"
 #include "clang/AST/DeclCXX.h"
if (NextToken.is(tok::period)) {
       Result.suppressDiagnostics();
       return NameClassification::Unknown();
     }
 
     QualType T = Context.getObjCInterfaceType(Class);
     return ParsedType::make(T);
   }
 
+  if (isa<ConceptDecl>(FirstDecl))
+    return NameClassification::Concept(
+        TemplateName(cast<TemplateDecl>(FirstDecl)));
+
   // We can have a type template here if we're classifying a template argument.
   if (isa<TemplateDecl>(FirstDecl) && !isa<FunctionTemplateDecl>(FirstDecl) &&
       !isa<VarTemplateDecl>(FirstDecl))
     return NameClassification::TypeTemplate(
         TemplateName(cast<TemplateDecl>(FirstDecl)));
 
   // Check for a tag type hidden by a non-type decl in a few cases where it
   // seems likely a type is wanted instead of the non-type that was found.
while (S->isClassScope() ||
          (S->getEntity() && S->getEntity()->isTransparentContext()))
     S = S->getParent();
   return S;
 }
 
 NamedDecl*
 Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
                               TypeSourceInfo *TInfo, LookupResult &Previous,
-                              MultiTemplateParamsArg TemplateParamLists,
+                              MultiTemplateParamsArg TemplateParamListsRef,
                               bool &AddToScope) {
   QualType R = TInfo->getType();
 
   assert(R->isFunctionType());
+  SmallVector<TemplateParameterList *, 4> TemplateParamLists;
+  for (TemplateParameterList *TPL : TemplateParamListsRef)
+    TemplateParamLists.push_back(TPL);
+  if (TemplateParameterList *Invented = D.getInventedTemplateParameterList()) {
+    if (!TemplateParamLists.empty() &&
+        Invented->getDepth() == TemplateParamLists.back()->getDepth())
+      TemplateParamLists.back() = Invented;
+    else
+      TemplateParamLists.push_back(Invented);
+  }
 
   // TODO: consider using NameInfo for diagnostic.
   DeclarationNameInfo NameInfo = GetNameForDeclarator(D);
   DeclarationName Name = NameInfo.getName();
   StorageClass SC = getFunctionStorageClass(*this, D);
 
   if (DeclSpec::TSCS TSCS = D.getDeclSpec().getThreadStorageClassSpec())
     Diag(D.getDeclSpec().getThreadStorageClassSpecLoc(),
if (getLangOpts().CPlusPlus) {
     isMemberSpecialization = false;
     isFunctionTemplateSpecialization = false;
     if (D.isInvalidType())
       NewFD->setInvalidDecl();
 
     // Match up the template parameter lists with the scope specifier, then
     // determine whether we have a template or a template specialization.
     bool Invalid = false;
-    if (TemplateParameterList *TemplateParams =
-            MatchTemplateParametersToScopeSpecifier(
-                D.getDeclSpec().getBeginLoc(), D.getIdentifierLoc(),
-                D.getCXXScopeSpec(),
-                D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
-                    ? D.getName().TemplateId
-                    : nullptr,
-                TemplateParamLists, isFriend, isMemberSpecialization,
-                Invalid)) {
+    TemplateParameterList *TemplateParams =
+            Invalid);
+    if (TemplateParams) {
       if (TemplateParams->size() > 0) {
         // This is a function template
 
         // Check that we can declare a template here.
         if (CheckTemplateDeclScope(S, TemplateParams))
           NewFD->setInvalidDecl();
 
         // A destructor cannot be a template.
if (TemplateParams) {
                                                         Name, TemplateParams,
                                                         NewFD);
         FunctionTemplate->setLexicalDeclContext(CurContext);
         NewFD->setDescribedFunctionTemplate(FunctionTemplate);
 
         // For source fidelity, store the other template param lists.
         if (TemplateParamLists.size() > 1) {
           NewFD->setTemplateParameterListsInfo(Context,
-                                               TemplateParamLists.drop_back(1));
+              ArrayRef<TemplateParameterList *>(TemplateParamLists)
+                  .drop_back(1));
         }
       } else {
         // This is a function template specialization.
         isFunctionTemplateSpecialization = true;
         // For source fidelity, store all the template param lists.
         if (TemplateParamLists.size() > 0)
           NewFD->setTemplateParameterListsInfo(Context, TemplateParamLists);
 

clang/lib/Sema/SemaDeclCXX.cpp

if (NewPD->isInvalidDecl() && PrevDecl) {
     // with the same name in the same scope.
   } else if (II) {
     PushOnScopeChains(NewPD, S);
   } else
     Record->addDecl(NewPD);
 
   return NewPD;
 }
+
+void Sema::ActOnStartFunctionDeclarationDeclarator(
+    Declarator &Declarator, unsigned TemplateParameterDepth) {
+  auto &Info = InventedParameterInfos.emplace_back();
+  TemplateParameterList *ExplicitParams = nullptr;
+  ArrayRef<TemplateParameterList *> ExplicitLists =
+      Declarator.getTemplateParameterLists();
+  if (!ExplicitLists.empty()) {
+    bool IsMemberSpecialization, IsInvalid;
+    ExplicitParams = MatchTemplateParametersToScopeSpecifier(
+        Declarator.getBeginLoc(), Declarator.getIdentifierLoc(),
+        Declarator.getCXXScopeSpec(), /*TemplateId=*/nullptr,
+        ExplicitLists, /*IsFriend=*/false, IsMemberSpecialization, IsInvalid,
+        /*SuppressDiagnostic=*/true);
+  }
+  if (ExplicitParams) {
+    Info.AutoTemplateParameterDepth = ExplicitParams->getDepth();
+    for (NamedDecl *Param : *ExplicitParams)
+      Info.TemplateParams.push_back(Param);
+    Info.NumExplicitTemplateParams = ExplicitParams->size();
+  } else {
+    Info.AutoTemplateParameterDepth = TemplateParameterDepth;
+    Info.NumExplicitTemplateParams = 0;
+  }
+}
+
+void Sema::ActOnFinishFunctionDeclarationDeclarator(Declarator &Declarator) {
+  auto &FSI = InventedParameterInfos.back();
+  if (FSI.TemplateParams.size() > FSI.NumExplicitTemplateParams) {
+    if (FSI.NumExplicitTemplateParams != 0) {
+      TemplateParameterList *ExplicitParams =
+          Declarator.getTemplateParameterLists().back();
+      Declarator.setInventedTemplateParameterList(
+          TemplateParameterList::Create(
+              Context, ExplicitParams->getTemplateLoc(),
+              ExplicitParams->getLAngleLoc(), FSI.TemplateParams,
+              ExplicitParams->getRAngleLoc(),
+              ExplicitParams->getRequiresClause()));
+    } else {
+      Declarator.setInventedTemplateParameterList(
+          TemplateParameterList::Create(
+              Context, SourceLocation(), SourceLocation(), FSI.TemplateParams,
+              SourceLocation(), /*RequiresClause=*/nullptr));
+    }
+  }
+  InventedParameterInfos.pop_back();
+}

clang/lib/Sema/SemaLambda.cpp

 QualType Sema::buildLambdaInitCaptureInitialization(
     SourceLocation Loc, bool ByRef, SourceLocation EllipsisLoc,
     Optional<unsigned> NumExpansions, IdentifierInfo *Id, bool IsDirectInit,
     Expr *&Init) {
   // Create an 'auto' or 'auto&' TypeSourceInfo that we can use to
   // deduce against.
   QualType DeductType = Context.getAutoDeductType();
   TypeLocBuilder TLB;
-  TLB.pushTypeSpec(DeductType).setNameLoc(Loc);
+  AutoTypeLoc TL = TLB.push<AutoTypeLoc>(DeductType);
+  TL.setNameLoc(Loc);
   if (ByRef) {
     DeductType = BuildReferenceType(DeductType, true, Loc, Id);
     assert(!DeductType.isNull() && "can't build reference to auto");
     TLB.push<ReferenceTypeLoc>(DeductType).setSigilLoc(Loc);
   }
   if (EllipsisLoc.isValid()) {
     if (Init->containsUnexpandedParameterPack()) {
       Diag(EllipsisLoc, getLangOpts().CPlusPlus2a

clang/lib/Sema/SemaTemplate.cpp

bool Sema::ActOnTypeConstraint(const CXXScopeSpec &SS,
   return AttachTypeConstraint(
       SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
       DeclarationNameInfo(DeclarationName(TypeConstr->Name),
                           TypeConstr->TemplateNameLoc), CD,
       TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
       ConstrainedParameter, EllipsisLoc);
 }
 
-/// Attach a type-constraint to a template parameter.
-/// \returns true if an error occured. This can happen if the
-/// immediately-declared constraint could not be formed (e.g. incorrect number
-/// of arguments for the named concept).
-bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
-                                DeclarationNameInfo NameInfo,
-                                ConceptDecl *NamedConcept,
-                                const TemplateArgumentListInfo *TemplateArgs,
-                                TemplateTypeParmDecl *ConstrainedParameter,
-                                SourceLocation EllipsisLoc) {
-  // C++2a [temp.param]p4:
-  //     [...] If Q is of the form C<A1, ..., An>, then let E' be
-  //     C<T, A1, ..., An>. Otherwise, let E' be C<T>. [...]
-  const ASTTemplateArgumentListInfo *ArgsAsWritten =
-    TemplateArgs ? ASTTemplateArgumentListInfo::Create(Context,
-                                                       *TemplateArgs) : nullptr;
+template<typename ArgumentLocAppender>
+static ExprResult formImmediatelyDeclaredConstraint(
+    Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
+    ConceptDecl *NamedConcept, SourceLocation LAngleLoc,
+    SourceLocation RAngleLoc, QualType ConstrainedType,
+    SourceLocation ParamNameLoc, ArgumentLocAppender Appender,
 
-  QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
   TemplateArgumentListInfo ConstraintArgs;
   ConstraintArgs.addArgument(
-    TemplateArgumentLoc(
-        TemplateArgument(ParamAsArgument),
-        TemplateArgumentLocInfo(
-            Context.getTrivialTypeSourceInfo(ParamAsArgument,
-                ConstrainedParameter->getLocation()))));
-  if (TemplateArgs) {
-    ConstraintArgs.setRAngleLoc(TemplateArgs->getRAngleLoc());
-    ConstraintArgs.setLAngleLoc(TemplateArgs->getLAngleLoc());
-    for (const TemplateArgumentLoc &ArgLoc : TemplateArgs->arguments())
-      ConstraintArgs.addArgument(ArgLoc);
-  }
+    S.getTrivialTemplateArgumentLoc(TemplateArgument(ConstrainedType),
+                                    /*NTTPType=*/QualType(), ParamNameLoc));
+
+  ConstraintArgs.setRAngleLoc(RAngleLoc);
+  ConstraintArgs.setLAngleLoc(LAngleLoc);
+  Appender(ConstraintArgs);
 
   // C++2a [temp.param]p4:
   //     [...] This constraint-expression E is called the immediately-declared
   //     constraint of T. [...]
   CXXScopeSpec SS;
   SS.Adopt(NS);
-  ExprResult ImmediatelyDeclaredConstraint = CheckConceptTemplateId(SS,
-      /*TemplateKWLoc=*/SourceLocation(), NameInfo, /*FoundDecl=*/NamedConcept,
-      NamedConcept, &ConstraintArgs);
-  if (ImmediatelyDeclaredConstraint.isInvalid())
-    return true;
+  ExprResult ImmediatelyDeclaredConstraint = S.CheckConceptTemplateId(
+      SS, /*TemplateKWLoc=*/SourceLocation(), NameInfo,
+      /*FoundDecl=*/NamedConcept, NamedConcept, &ConstraintArgs);
+  if (ImmediatelyDeclaredConstraint.isInvalid() || !EllipsisLoc.isValid())
+    return ImmediatelyDeclaredConstraint;
 
-  if (ConstrainedParameter->isParameterPack()) {
-    // C++2a [temp.param]p4:
-    //     [...] If T is not a pack, then E is E', otherwise E is (E' && ...).
-    //
-    // We have the following case:
-    //
-    // template<typename T> concept C1 = true;
-    // template<C1... T> struct s1;
-    //
-    // The constraint: (C1<T> && ...)
-    ImmediatelyDeclaredConstraint =
-        BuildCXXFoldExpr(/*LParenLoc=*/SourceLocation(),
+  return S.BuildCXXFoldExpr(/*LParenLoc=*/SourceLocation(),
                             ImmediatelyDeclaredConstraint.get(), BO_LAnd,
                             EllipsisLoc, /*RHS=*/nullptr,
                             /*RParenLoc=*/SourceLocation(),
-                         /*NumExpansions=*/None).get();
-    if (ImmediatelyDeclaredConstraint.isInvalid())
-      return true;
-  }
+                            /*NumExpansions=*/None);
+}
+
+/// Attach a type-constraint to a template parameter.
+/// \returns true if an error occured. This can happen if the
+/// immediately-declared constraint could not be formed (e.g. incorrect number
+/// of arguments for the named concept).
+bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
+                                DeclarationNameInfo NameInfo,
+                                ConceptDecl *NamedConcept,
+                                const TemplateArgumentListInfo *TemplateArgs,
+                                TemplateTypeParmDecl *ConstrainedParameter,
+                                SourceLocation EllipsisLoc) {
+  // C++2a [temp.param]p4:
+  //     [...] If Q is of the form C<A1, ..., An>, then let E' be
+  //     C<T, A1, ..., An>. Otherwise, let E' be C<T>. [...]
+  const ASTTemplateArgumentListInfo *ArgsAsWritten =
+    TemplateArgs ? ASTTemplateArgumentListInfo::Create(Context,
+                                                       *TemplateArgs) : nullptr;
+
+  QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
+
+  ExprResult ImmediatelyDeclaredConstraint =
+      formImmediatelyDeclaredConstraint(
+          *this, NS, NameInfo, NamedConcept,
+          TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
+          TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
+          ParamAsArgument, ConstrainedParameter->getLocation(),
+          [&] (TemplateArgumentListInfo &ConstraintArgs) {
+            if (TemplateArgs)
+              for (const auto &ArgLoc : TemplateArgs->arguments())
+                ConstraintArgs.addArgument(ArgLoc);
+          }, EllipsisLoc);
 
   ConstrainedParameter->setTypeConstraint(NS, NameInfo,
                                           /*FoundDecl=*/NamedConcept,
                                           NamedConcept, ArgsAsWritten,
                                           ImmediatelyDeclaredConstraint.get());
   return false;
 }
 
+bool Sema::AttachTypeConstraint(AutoTypeLoc TL, NonTypeTemplateParmDecl *NTTP,
+                                SourceLocation EllipsisLoc) {
+  if (NTTP->getType() != TL.getType() ||
+      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
+    Diag(NTTP->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
+         diag::err_unsupported_placeholder_constraint)
+       << NTTP->getTypeSourceInfo()->getTypeLoc().getSourceRange();
+    return true;
+  }
+  // FIXME: Concepts: This should be the type of the placeholder, but this is
+  // unclear in the wording right now.
+  DeclRefExpr *Ref = BuildDeclRefExpr(NTTP, NTTP->getType(), VK_RValue,
+                                      NTTP->getLocation());
+  if (!Ref)
+    return true;
+  ExprResult ImmediatelyDeclaredConstraint =
+      formImmediatelyDeclaredConstraint(
+          *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
+          TL.getNamedConcept(), TL.getLAngleLoc(), TL.getRAngleLoc(),
+          BuildDecltypeType(Ref, NTTP->getLocation()), NTTP->getLocation(),
+          [&] (TemplateArgumentListInfo &ConstraintArgs) {
+            for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
+              ConstraintArgs.addArgument(TL.getArgLoc(I));
+          }, EllipsisLoc);
+  if (ImmediatelyDeclaredConstraint.isInvalid() ||
+     !ImmediatelyDeclaredConstraint.isUsable())
+    return true;
+
+  NTTP->setPlaceholderTypeConstraint(ImmediatelyDeclaredConstraint.get());
+  return false;
+}
+
 /// Check that the type of a non-type template parameter is
 /// well-formed.
 ///
 /// \returns the (possibly-promoted) parameter type if valid;
 /// otherwise, produces a diagnostic and returns a NULL type.
 QualType Sema::CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
                                                  SourceLocation Loc) {
   if (TSI->getType()->isUndeducedType()) {
NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
   IdentifierInfo *ParamName = D.getIdentifier();
   bool IsParameterPack = D.hasEllipsis();
   NonTypeTemplateParmDecl *Param = NonTypeTemplateParmDecl::Create(
       Context, Context.getTranslationUnitDecl(), D.getBeginLoc(),
       D.getIdentifierLoc(), Depth, Position, ParamName, T, IsParameterPack,
       TInfo);
   Param->setAccess(AS_public);
 
+  if (AutoTypeLoc TL = TInfo->getTypeLoc().getContainedAutoTypeLoc())
+    if (TL.isConstrained())
+      if (AttachTypeConstraint(TL, Param, D.getEllipsisLoc()))
+        Invalid = true;
+
   if (Invalid)
     Param->setInvalidDecl();
 
   if (Param->isParameterPack())
     if (auto *LSI = getEnclosingLambda())
       LSI->LocalPacks.push_back(Param);
 
   if (ParamName) {

 /// parameter list may have template parameters (if we're declaring a
 /// template) or may have no template parameters (if we're declaring a
 /// template specialization), or may be NULL (if what we're declaring isn't
 /// itself a template).
 TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
     SourceLocation DeclStartLoc, SourceLocation DeclLoc, const CXXScopeSpec &SS,
     TemplateIdAnnotation *TemplateId,
     ArrayRef<TemplateParameterList *> ParamLists, bool IsFriend,
-    bool &IsMemberSpecialization, bool &Invalid) {
+    bool &IsMemberSpecialization, bool &Invalid, bool SuppressDiagnostic) {
   IsMemberSpecialization = false;
   Invalid = false;
 
   // The sequence of nested types to which we will match up the template
   // parameter lists. We first build this list by starting with the type named
   // by the nested-name-specifier and walking out until we run out of types.
   SmallVector<QualType, 4> NestedTypes;
   QualType T;
TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
   //   enclosing class templates. In an explicit specialization for
   //   such a member, the member declaration shall be preceded by a
   //   template<> for each enclosing class template that is
   //   explicitly specialized.
   bool SawNonEmptyTemplateParameterList = false;
 
   auto CheckExplicitSpecialization = [&](SourceRange Range, bool Recovery) {
     if (SawNonEmptyTemplateParameterList) {
+      if (!SuppressDiagnostic)
         Diag(DeclLoc, diag::err_specialize_member_of_template)
           << !Recovery << Range;
       Invalid = true;
       IsMemberSpecialization = false;
       return true;
     }
 
     return false;
   };
 
   auto DiagnoseMissingExplicitSpecialization = [&] (SourceRange Range) {
     // Check that we can have an explicit specialization here.
     if (CheckExplicitSpecialization(Range, true))
       return true;
 
     // We don't have a template header, but we should.
     SourceLocation ExpectedTemplateLoc;
     if (!ParamLists.empty())
       ExpectedTemplateLoc = ParamLists[0]->getTemplateLoc();
     else
       ExpectedTemplateLoc = DeclStartLoc;
 
+    if (!SuppressDiagnostic)
       Diag(DeclLoc, diag::err_template_spec_needs_header)
         << Range
         << FixItHint::CreateInsertion(ExpectedTemplateLoc, "template<> ");
     return false;
   };
 
   unsigned ParamIdx = 0;
   for (unsigned TypeIdx = 0, NumTypes = NestedTypes.size(); TypeIdx != NumTypes;
        ++TypeIdx) {
     T = NestedTypes[TypeIdx];
 
if (NeedEmptyTemplateHeader) {
       // If we're on the last of the types, and we need a 'template<>' header
       // here, then it's a member specialization.
       if (TypeIdx == NumTypes - 1)
         IsMemberSpecialization = true;
 
       if (ParamIdx < ParamLists.size()) {
         if (ParamLists[ParamIdx]->size() > 0) {
           // The header has template parameters when it shouldn't. Complain.
+          if (!SuppressDiagnostic)
             Diag(ParamLists[ParamIdx]->getTemplateLoc(),
                  diag::err_template_param_list_matches_nontemplate)
               << T
               << SourceRange(ParamLists[ParamIdx]->getLAngleLoc(),
                              ParamLists[ParamIdx]->getRAngleLoc())
               << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
           Invalid = true;
           return nullptr;
         }
 
         // Consume this template header.
         ++ParamIdx;
         continue;
       }
if (NeedNonemptyTemplateHeader) {
           continue;
       }
 
       if (ParamIdx < ParamLists.size()) {
         // Check the template parameter list, if we can.
         if (ExpectedTemplateParams &&
             !TemplateParameterListsAreEqual(ParamLists[ParamIdx],
                                             ExpectedTemplateParams,
-                                            true, TPL_TemplateMatch))
+                                            !SuppressDiagnostic, TPL_TemplateMatch))
           Invalid = true;
 
         if (!Invalid &&
             CheckTemplateParameterList(ParamLists[ParamIdx], nullptr,
                                        TPC_ClassTemplateMember))
           Invalid = true;
 
         ++ParamIdx;
         continue;
       }
 
+      if (!SuppressDiagnostic)
         Diag(DeclLoc, diag::err_template_spec_needs_template_parameters)
           << T
           << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
       Invalid = true;
       continue;
     }
   }
 
   // If there were at least as many template-ids as there were template
   // parameter lists, then there are no template parameter lists remaining for
   // the declaration itself.
if (ParamIdx < ParamLists.size() - 1) {
     bool AllExplicitSpecHeaders = true;
     for (unsigned I = ParamIdx, E = ParamLists.size() - 1; I != E; ++I) {
       if (ParamLists[I]->size() == 0)
         HasAnyExplicitSpecHeader = true;
       else
         AllExplicitSpecHeaders = false;
     }
 
+    if (!SuppressDiagnostic)
       Diag(ParamLists[ParamIdx]->getTemplateLoc(),
            AllExplicitSpecHeaders ? diag::warn_template_spec_extra_headers
                                   : diag::err_template_spec_extra_headers)
           << SourceRange(ParamLists[ParamIdx]->getTemplateLoc(),
                          ParamLists[ParamLists.size() - 2]->getRAngleLoc());
 
     // If there was a specialization somewhere, such that 'template<>' is
     // not required, and there were any 'template<>' headers, note where the
     // specialization occurred.
-    if (ExplicitSpecLoc.isValid() && HasAnyExplicitSpecHeader)
+    if (ExplicitSpecLoc.isValid() && HasAnyExplicitSpecHeader &&
+        !SuppressDiagnostic)
       Diag(ExplicitSpecLoc,
            diag::note_explicit_template_spec_does_not_need_header)
         << NestedTypes.back();
 
     // We have a template parameter list with no corresponding scope, which
     // means that the resulting template declaration can't be instantiated
     // properly (we'll end up with dependent nodes when we shouldn't).
     if (!AllExplicitSpecHeaders)
if (getLangOpts().CPlusPlus17 && ParamType->isUndeducedType()) {
     // the type is dependent, in order to check the types of non-type template
     // arguments line up properly in partial ordering.
     Optional<unsigned> Depth = Param->getDepth() + 1;
     Expr *DeductionArg = Arg;
     if (auto *PE = dyn_cast<PackExpansionExpr>(DeductionArg))
       DeductionArg = PE->getPattern();
     if (DeduceAutoType(
             Context.getTrivialTypeSourceInfo(ParamType, Param->getLocation()),
-            DeductionArg, ParamType, Depth) == DAR_Failed) {
+            DeductionArg, ParamType, Depth,
+            // We do not check constraints right now because the
+            // immediately-declared constraint of the auto type is also an
+            // associated constraint, and will be checked along with the other
+            // associated constraints after checking the template argument list.
+            /*IgnoreConstraints=*/true) == DAR_Failed) {
       Diag(Arg->getExprLoc(),
            diag::err_non_type_template_parm_type_deduction_failure)
         << Param->getDeclName() << Param->getType() << Arg->getType()
         << Arg->getSourceRange();
       Diag(Param->getLocation(), diag::note_template_param_here);
       return ExprError();
     }
     // CheckNonTypeTemplateParameterType will produce a diagnostic if there's

clang/lib/Sema/SemaTemplateDeduction.cpp

QualType TransformAutoType(TypeLocBuilder &TLB, AutoTypeLoc TL) {
       // auto type deduced as T" in order for [temp.deduct.call]p3 to apply.
       //
       // FIXME: Is this still necessary?
       if (!UseTypeSugar)
         return TransformDesugared(TLB, TL);
 
       QualType Result = SemaRef.Context.getAutoType(
           Replacement, TL.getTypePtr()->getKeyword(), Replacement.isNull(),
-          ReplacementIsPack);
+          ReplacementIsPack, TL.getTypePtr()->getTypeConstraintConcept(),
+          TL.getTypePtr()->getTypeConstraintArguments());
       auto NewTL = TLB.push<AutoTypeLoc>(Result);
-      NewTL.setNameLoc(TL.getNameLoc());
+      NewTL.copy(TL);
       return Result;
     }
 
     QualType TransformDeducedTemplateSpecializationType(
         TypeLocBuilder &TLB, DeducedTemplateSpecializationTypeLoc TL) {
       if (!UseTypeSugar)
         return TransformDesugared(TLB, TL);
 
QualType Apply(TypeLoc TL) {
       return TransformType(TLB, TL);
     }
   };
 
 } // namespace
 
 Sema::DeduceAutoResult
 Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result,
-                     Optional<unsigned> DependentDeductionDepth) {
+                     Optional<unsigned> DependentDeductionDepth,
+                     bool IgnoreConstraints) {
   return DeduceAutoType(Type->getTypeLoc(), Init, Result,
-                        DependentDeductionDepth);
+                        DependentDeductionDepth, IgnoreConstraints);
 }
 
 /// Attempt to produce an informative diagostic explaining why auto deduction
 /// failed.
 /// \return \c true if diagnosed, \c false if not.
 static bool diagnoseAutoDeductionFailure(Sema &S,
                                          Sema::TemplateDeductionResult TDK,
                                          TemplateDeductionInfo &Info,
static bool diagnoseAutoDeductionFailure(Sema &S,
   // FIXME: Are there other cases for which a custom diagnostic is more useful
   // than the basic "types don't match" diagnostic?
 
   default:
     return false;
   }
 }
 
+static Sema::DeduceAutoResult
+CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
+                                   AutoTypeLoc TypeLoc, QualType Deduced) {
+  ConstraintSatisfaction Satisfaction;
+  ConceptDecl *Concept = Type.getTypeConstraintConcept();
+  TemplateArgumentListInfo TemplateArgs(TypeLoc.getLAngleLoc(),
+                                        TypeLoc.getRAngleLoc());
+  TemplateArgs.addArgument(
+      TemplateArgumentLoc(TemplateArgument(Deduced),
+                          S.Context.getTrivialTypeSourceInfo(
+                              Deduced, TypeLoc.getNameLoc())));
+  for (unsigned I = 0, C = TypeLoc.getNumArgs(); I != C; ++I)
+    TemplateArgs.addArgument(TypeLoc.getArgLoc(I));
+
+  llvm::SmallVector<TemplateArgument, 4> Converted;
+  if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
+                                  /*PartialTemplateArgs=*/false, Converted))
+    return Sema::DAR_FailedAlreadyDiagnosed;
+  if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
+                                    Converted, TypeLoc.getLocalSourceRange(),
+                                    Satisfaction))
+    return Sema::DAR_FailedAlreadyDiagnosed;
+  if (!Satisfaction.IsSatisfied) {
+    std::string Buf;
+    llvm::raw_string_ostream OS(Buf);
+    OS << "'" << Concept->getName();
+    if (TypeLoc.hasExplicitTemplateArgs()) {
+      OS << "<";
+      for (const auto &Arg : Type.getTypeConstraintArguments())
+        Arg.print(S.getPrintingPolicy(), OS);
+      OS << ">";
+    }
+    OS << "'";
+    OS.flush();
+    S.Diag(TypeLoc.getConceptNameLoc(),
+           diag::err_placeholder_constraints_not_satisfied)
+         << Deduced << Buf << TypeLoc.getLocalSourceRange();
+    S.DiagnoseUnsatisfiedConstraint(Satisfaction);
+    return Sema::DAR_FailedAlreadyDiagnosed;
+  }
+  return Sema::DAR_Succeeded;
+}
+
 /// Deduce the type for an auto type-specifier (C++11 [dcl.spec.auto]p6)
 ///
 /// Note that this is done even if the initializer is dependent. (This is
 /// necessary to support partial ordering of templates using 'auto'.)
 /// A dependent type will be produced when deducing from a dependent type.
 ///
 /// \param Type the type pattern using the auto type-specifier.
 /// \param Init the initializer for the variable whose type is to be deduced.
 /// \param Result if type deduction was successful, this will be set to the
 ///        deduced type.
 /// \param DependentDeductionDepth Set if we should permit deduction in
 ///        dependent cases. This is necessary for template partial ordering with
 ///        'auto' template parameters. The value specified is the template
 ///        parameter depth at which we should perform 'auto' deduction.
+/// \param IgnoreConstraints Set if we should not fail if the deduced type does
+///                          not satisfy the type-constraint in the auto type.
 Sema::DeduceAutoResult
 Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
-                     Optional<unsigned> DependentDeductionDepth) {
+                     Optional<unsigned> DependentDeductionDepth,
+                     bool IgnoreConstraints) {
   if (Init->getType()->isNonOverloadPlaceholderType()) {
     ExprResult NonPlaceholder = CheckPlaceholderExpr(Init);
     if (NonPlaceholder.isInvalid())
       return DAR_FailedAlreadyDiagnosed;
     Init = NonPlaceholder.get();
   }
 
   DependentAuto DependentResult = {
if (AT->isDecltypeAuto()) {
       if (ER.isInvalid())
         return DAR_FailedAlreadyDiagnosed;
       Init = ER.get();
       QualType Deduced = BuildDecltypeType(Init, Init->getBeginLoc(), false);
       if (Deduced.isNull())
         return DAR_FailedAlreadyDiagnosed;
       // FIXME: Support a non-canonical deduced type for 'auto'.
       Deduced = Context.getCanonicalType(Deduced);
+      if (AT->isConstrained() && !IgnoreConstraints) {
+        auto ConstraintsResult =
+            CheckDeducedPlaceholderConstraints(*this, *AT,
+                                               Type.getContainedAutoTypeLoc(),
+                                               Deduced);
+        if (ConstraintsResult != DAR_Succeeded)
+          return ConstraintsResult;
+      }
       Result = SubstituteDeducedTypeTransform(*this, Deduced).Apply(Type);
       if (Result.isNull())
         return DAR_FailedAlreadyDiagnosed;
       return DAR_Succeeded;
     } else if (!getLangOpts().CPlusPlus) {
       if (isa<InitListExpr>(Init)) {
         Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c);
         return DAR_FailedAlreadyDiagnosed;
Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
   QualType DeducedType = Deduced[0].getAsType();
 
   if (InitList) {
     DeducedType = BuildStdInitializerList(DeducedType, Loc);
     if (DeducedType.isNull())
       return DAR_FailedAlreadyDiagnosed;
   }
 
+  if (const auto *AT = Type.getType()->getAs<AutoType>()) {
+    if (AT->isConstrained() && !IgnoreConstraints) {
+      auto ConstraintsResult =
+          CheckDeducedPlaceholderConstraints(*this, *AT,
+                                             Type.getContainedAutoTypeLoc(),
+                                             DeducedType);
+      if (ConstraintsResult != DAR_Succeeded)
+        return ConstraintsResult;
+    }
+  }
+
   Result = SubstituteDeducedTypeTransform(*this, DeducedType).Apply(Type);
   if (Result.isNull())
     return DAR_FailedAlreadyDiagnosed;
 
   // Check that the deduced argument type is compatible with the original
   // argument type per C++ [temp.deduct.call]p4.
   QualType DeducedA = InitList ? Deduced[0].getAsType() : Result;
   for (const OriginalCallArg &OriginalArg : OriginalCallArgs) {

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

Param = NonTypeTemplateParmDecl::Create(
         D->getPosition(), D->getIdentifier(), T, DI, ExpandedParameterPackTypes,
         ExpandedParameterPackTypesAsWritten);
   else
     Param = NonTypeTemplateParmDecl::Create(
         SemaRef.Context, Owner, D->getInnerLocStart(), D->getLocation(),
         D->getDepth() - TemplateArgs.getNumSubstitutedLevels(),
         D->getPosition(), D->getIdentifier(), T, D->isParameterPack(), DI);
 
+  if (AutoTypeLoc AutoLoc = DI->getTypeLoc().getContainedAutoTypeLoc())
+    if (AutoLoc.isConstrained())
+      if (SemaRef.AttachTypeConstraint(
+              AutoLoc, Param,
+              IsExpandedParameterPack
+                ? DI->getTypeLoc().getAs<PackExpansionTypeLoc>()
+                    .getEllipsisLoc()
+                : SourceLocation()))
+        Invalid = true;
+
   Param->setAccess(AS_public);
   Param->setImplicit(D->isImplicit());
   if (Invalid)
     Param->setInvalidDecl();
 
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
     EnterExpressionEvaluationContext ConstantEvaluated(
         SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);

clang/lib/Sema/SemaType.cpp

 //===--- SemaType.cpp - Semantic Analysis for Types -----------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file implements type-related semantic analysis.
 //
 //===----------------------------------------------------------------------===//
 
 #include "TypeLocBuilder.h"
+#include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/ASTStructuralEquivalence.h"
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/AST/TypeLocVisitor.h"
 #include "clang/Basic/PartialDiagnostic.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/DelayedDiagnostic.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/ParsedTemplate.h"
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/Template.h"
 #include "clang/Sema/TemplateInstCallback.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/ErrorHandling.h"

 static OpenCLAccessAttr::Spelling
 getImageAccess(const ParsedAttributesView &Attrs) {
   for (const ParsedAttr &AL : Attrs)
     if (AL.getKind() == ParsedAttr::AT_OpenCLAccess)
       return static_cast<OpenCLAccessAttr::Spelling>(AL.getSemanticSpelling());
   return OpenCLAccessAttr::Keyword_read_only;
 }
 
+static QualType ConvertConstrainedAutoDeclSpecToType(Sema &S, DeclSpec &DS,
+                                                     AutoTypeKeyword AutoKW) {
+  assert(DS.isConstrainedAuto());
+  TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId();
+  TemplateArgumentListInfo TemplateArgsInfo;
+  TemplateArgsInfo.setLAngleLoc(TemplateId->LAngleLoc);
+  TemplateArgsInfo.setRAngleLoc(TemplateId->RAngleLoc);
+  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                     TemplateId->NumArgs);
+  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+  llvm::SmallVector<TemplateArgument, 8> TemplateArgs;
+  for (auto &ArgLoc : TemplateArgsInfo.arguments())
+    TemplateArgs.push_back(ArgLoc.getArgument());
+  return S.Context.getAutoType(QualType(), AutoTypeKeyword::Auto, false,
+                               /*IsPack=*/false,
+                               cast<ConceptDecl>(TemplateId->Template.get()
+                                                 .getAsTemplateDecl()),
+                               TemplateArgs);
+}
+
 /// Convert the specified declspec to the appropriate type
 /// object.
 /// \param state Specifies the declarator containing the declaration specifier
 /// to be converted, along with other associated processing state.
 /// \returns The type described by the declaration specifiers.  This function
 /// never returns null.
 static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
   // FIXME: Should move the logic from DeclSpec::Finish to here for validity
Result = S.BuildUnaryTransformType(Result,
                                        DS.getTypeSpecTypeLoc());
     if (Result.isNull()) {
       Result = Context.IntTy;
       declarator.setInvalidType(true);
     }
     break;
 
   case DeclSpec::TST_auto:
+    if (DS.isConstrainedAuto()) {
+      Result = ConvertConstrainedAutoDeclSpecToType(S, DS,
+                                                    AutoTypeKeyword::Auto);
+      break;
+    }
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::Auto, false);
     break;
 
   case DeclSpec::TST_auto_type:
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::GNUAutoType, false);
     break;
 
   case DeclSpec::TST_decltype_auto:
+    if (DS.isConstrainedAuto()) {
+      Result =
+          ConvertConstrainedAutoDeclSpecToType(S, DS,
+                                               AutoTypeKeyword::DecltypeAuto);
+      break;
+    }
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::DecltypeAuto,
                                  /*IsDependent*/ false);
     break;
 
   case DeclSpec::TST_unknown_anytype:
     Result = Context.UnknownAnyTy;
     break;
 
S.diagnoseIgnoredQualifiers(diag::warn_qual_return_type,
                               D.getIdentifierLoc(),
                               D.getDeclSpec().getConstSpecLoc(),
                               D.getDeclSpec().getVolatileSpecLoc(),
                               D.getDeclSpec().getRestrictSpecLoc(),
                               D.getDeclSpec().getAtomicSpecLoc(),
                               D.getDeclSpec().getUnalignedSpecLoc());
 }
 
+static void CopyTypeConstraintFromAutoType(Sema &SemaRef, const AutoType *Auto,
+                                           AutoTypeLoc AutoLoc,
+                                           TemplateTypeParmDecl *TP,
+                                           SourceLocation EllipsisLoc) {
+
+  TemplateArgumentListInfo TAL(AutoLoc.getLAngleLoc(), AutoLoc.getRAngleLoc());
+  for (unsigned Idx = 0; Idx < AutoLoc.getNumArgs(); ++Idx)
+    TAL.addArgument(AutoLoc.getArgLoc(Idx));
+
+  SemaRef.AttachTypeConstraint(
+      AutoLoc.getNestedNameSpecifierLoc(), AutoLoc.getConceptNameInfo(),
+      AutoLoc.getNamedConcept(),
+      AutoLoc.hasExplicitTemplateArgs() ? &TAL : nullptr, TP, EllipsisLoc);
+}
+
+static QualType InventTemplateParameter(
+    TypeProcessingState &state, QualType T, TypeSourceInfo *TSI, AutoType *Auto,
+    InventedTemplateParameterInfo &Info) {
+  Sema &S = state.getSema();
+  Declarator &D = state.getDeclarator();
+
+  const unsigned TemplateParameterDepth = Info.AutoTemplateParameterDepth;
+  const unsigned AutoParameterPosition = Info.TemplateParams.size();
+  const bool IsParameterPack = D.hasEllipsis();
+
+  // If auto is mentioned in a lambda parameter or abbreviated function
+  // template context, convert it to a template parameter type.
+
+  // Create the TemplateTypeParmDecl here to retrieve the corresponding
+  // template parameter type. Template parameters are temporarily added
+  // to the TU until the associated TemplateDecl is created.
+  TemplateTypeParmDecl *InventedTemplateParam =
+      TemplateTypeParmDecl::Create(
+          S.Context, S.Context.getTranslationUnitDecl(),
+          /*KeyLoc=*/D.getDeclSpec().getTypeSpecTypeLoc(),
+          /*NameLoc=*/D.getIdentifierLoc(),
+          TemplateParameterDepth, AutoParameterPosition,
+          S.InventAbbreviatedTemplateParameterTypeName(
+              D.getIdentifier(), AutoParameterPosition), false,
+          IsParameterPack, /*HasTypeConstraint=*/Auto->isConstrained());
+  InventedTemplateParam->setImplicit();
+  Info.TemplateParams.push_back(InventedTemplateParam);
+  // Attach type constraints
+  if (Auto->isConstrained()) {
+    if (TSI) {
+      CopyTypeConstraintFromAutoType(
+          S, Auto, TSI->getTypeLoc().getContainedAutoTypeLoc(),
+          InventedTemplateParam, D.getEllipsisLoc());
+    } else {
+      TemplateIdAnnotation *TemplateId = D.getDeclSpec().getRepAsTemplateId();
+      TemplateArgumentListInfo TemplateArgsInfo;
+      if (TemplateId->LAngleLoc.isValid()) {
+        ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                           TemplateId->NumArgs);
+        S.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+      }
+      S.AttachTypeConstraint(
+          D.getDeclSpec().getTypeSpecScope().getWithLocInContext(S.Context),
+          DeclarationNameInfo(DeclarationName(TemplateId->Name),
+                              TemplateId->TemplateNameLoc),
+          cast<ConceptDecl>(TemplateId->Template.get().getAsTemplateDecl()),
+          TemplateId->LAngleLoc.isValid() ? &TemplateArgsInfo : nullptr,
+          InventedTemplateParam, D.getEllipsisLoc());
+    }
+  }
+
+  // If TSI is nullptr, this is a constrained declspec auto and the type
+  // constraint will be attached later in TypeSpecLocFiller
+
+  // Replace the 'auto' in the function parameter with this invented
+  // template type parameter.
+  // FIXME: Retain some type sugar to indicate that this was written
+  //  as 'auto'?
+  return state.ReplaceAutoType(
+      T, QualType(InventedTemplateParam->getTypeForDecl(), 0));
+}
+
+static TypeSourceInfo *
+GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
+                               QualType T, TypeSourceInfo *ReturnTypeInfo);
+
 static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
                                              TypeSourceInfo *&ReturnTypeInfo) {
   Sema &SemaRef = state.getSema();
   Declarator &D = state.getDeclarator();
   QualType T;
   ReturnTypeInfo = nullptr;
 
   // The TagDecl owned by the DeclSpec.
if (DeducedType *Deduced = T->getContainedDeducedType()) {
 
     switch (D.getContext()) {
     case DeclaratorContext::LambdaExprContext:
       // Declared return type of a lambda-declarator is implicit and is always
       // 'auto'.
       break;
     case DeclaratorContext::ObjCParameterContext:
     case DeclaratorContext::ObjCResultContext:
-    case DeclaratorContext::PrototypeContext:
       Error = 0;
       break;
     case DeclaratorContext::RequiresExprContext:
+      Error = 22;
+      break;
+    case DeclaratorContext::PrototypeContext:
+    case DeclaratorContext::LambdaExprParameterContext: {
+      InventedTemplateParameterInfo *Info = nullptr;
+      if (D.getContext() == DeclaratorContext::PrototypeContext) {
+        // With concepts we allow 'auto' in function parameters.
+        if (!SemaRef.getLangOpts().ConceptsTS || !Auto ||
+            Auto->getKeyword() != AutoTypeKeyword::Auto) {
+          Error = 0;
+          break;
+        } else if (!SemaRef.getCurScope()->isFunctionDeclarationScope()) {
           Error = 21;
           break;
-    case DeclaratorContext::LambdaExprParameterContext:
+        } else if (D.hasTrailingReturnType()) {
+          // This might be OK, but we'll need to convert the trailing return
+          // type later.
+          break;
+        }
+
+        Info = &SemaRef.InventedParameterInfos.back();
+      } else {
         // In C++14, generic lambdas allow 'auto' in their parameters.
-      if (!SemaRef.getLangOpts().CPlusPlus14 ||
-          !Auto || Auto->getKeyword() != AutoTypeKeyword::Auto)
+        if (!SemaRef.getLangOpts().CPlusPlus14 || !Auto ||
+            Auto->getKeyword() != AutoTypeKeyword::Auto) {
           Error = 16;
-      else {
-        // If auto is mentioned in a lambda parameter context, convert it to a
-        // template parameter type.
-        sema::LambdaScopeInfo *LSI = SemaRef.getCurLambda();
-        assert(LSI && "No LambdaScopeInfo on the stack!");
-        const unsigned TemplateParameterDepth = LSI->AutoTemplateParameterDepth;
-        const unsigned AutoParameterPosition = LSI->TemplateParams.size();
-        const bool IsParameterPack = D.hasEllipsis();
-
-        // Create the TemplateTypeParmDecl here to retrieve the corresponding
-        // template parameter type. Template parameters are temporarily added
-        // to the TU until the associated TemplateDecl is created.
-        TemplateTypeParmDecl *CorrespondingTemplateParam =
-            TemplateTypeParmDecl::Create(
-                SemaRef.Context, SemaRef.Context.getTranslationUnitDecl(),
-                /*KeyLoc*/ SourceLocation(), /*NameLoc*/ D.getBeginLoc(),
-                TemplateParameterDepth, AutoParameterPosition,
-                /*Identifier*/ nullptr, false, IsParameterPack,
-                /*HasTypeConstraint=*/false);
-        CorrespondingTemplateParam->setImplicit();
-        LSI->TemplateParams.push_back(CorrespondingTemplateParam);
-        // Replace the 'auto' in the function parameter with this invented
-        // template type parameter.
-        // FIXME: Retain some type sugar to indicate that this was written
-        // as 'auto'.
-        T = state.ReplaceAutoType(
-            T, QualType(CorrespondingTemplateParam->getTypeForDecl(), 0));
+          break;
         }
+        Info = SemaRef.getCurLambda();
+        assert(Info && "No LambdaScopeInfo on the stack!");
+      }
+      T = InventTemplateParameter(state, T, nullptr, Auto, *Info);
       break;
+    }
     case DeclaratorContext::MemberContext: {
       if (D.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_static ||
           D.isFunctionDeclarator())
         break;
       bool Cxx = SemaRef.getLangOpts().CPlusPlus;
       switch (cast<TagDecl>(SemaRef.CurContext)->getTagKind()) {
       case TTK_Enum: llvm_unreachable("unhandled tag kind");
       case TTK_Struct: Error = Cxx ? 1 : 2; /* Struct member */ break;
if (ASOld != LangAS::Default) {
     }
     // Emit a warning if they are identical; it's likely unintended.
     S.Diag(AttrLoc,
            diag::warn_attribute_address_multiple_identical_qualifiers);
   }
   return false;
 }
 
-static TypeSourceInfo *
-GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
-                               QualType T, TypeSourceInfo *ReturnTypeInfo);
-
 static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
                                                 QualType declSpecType,
                                                 TypeSourceInfo *TInfo) {
   // The TypeSourceInfo that this function returns will not be a null type.
   // If there is an error, this function will fill in a dummy type as fallback.
   QualType T = declSpecType;
   Declarator &D = state.getDeclarator();
   Sema &S = state.getSema();
case DeclaratorChunk::Function: {
               S.Diag(D.getDeclSpec().getBeginLoc(),
                      diag::err_deduction_guide_with_complex_decl)
                   << D.getSourceRange();
               D.setInvalidType(true);
             }
           } else if (D.getContext() != DeclaratorContext::LambdaExprContext &&
                      (T.hasQualifiers() || !isa<AutoType>(T) ||
                       cast<AutoType>(T)->getKeyword() !=
-                          AutoTypeKeyword::Auto)) {
+                          AutoTypeKeyword::Auto ||
+                      cast<AutoType>(T)->isConstrained())) {
             S.Diag(D.getDeclSpec().getTypeSpecTypeLoc(),
                    diag::err_trailing_return_without_auto)
                 << T << D.getDeclSpec().getSourceRange();
             D.setInvalidType(true);
           }
           T = S.GetTypeFromParser(FTI.getTrailingReturnType(), &TInfo);
           if (T.isNull()) {
             // An error occurred parsing the trailing return type.
             T = Context.IntTy;
             D.setInvalidType(true);
-          }
+          } else if (S.getLangOpts().ConceptsTS)
+            // Handle cases like: `auto f() -> auto` or `auto f() -> C auto`.
+            if (AutoType *Auto = T->getContainedAutoType())
+              if (S.getCurScope()->isFunctionDeclarationScope())
+                T = InventTemplateParameter(state, T, TInfo, Auto,
+                                            S.InventedParameterInfos.back());
         } else {
           // This function type is not the type of the entity being declared,
           // so checking the 'auto' is not the responsibility of this chunk.
         }
       }
 
       // C99 6.7.5.3p1: The return type may not be a function or array type.
       // For conversion functions, we'll diagnose this particular error later.
case DeclaratorContext::RequiresExprContext:
       //   [...] When it is part of a parameter-declaration-clause, the
       //   parameter pack is a function parameter pack (14.5.3). The type T
       //   of the declarator-id of the function parameter pack shall contain
       //   a template parameter pack; each template parameter pack in T is
       //   expanded by the function parameter pack.
       //
       // We represent function parameter packs as function parameters whose
       // type is a pack expansion.
-      if (!T->containsUnexpandedParameterPack()) {
+      if (!T->containsUnexpandedParameterPack() &&
+          (!LangOpts.ConceptsTS || !T->getContainedAutoType())) {
         S.Diag(D.getEllipsisLoc(),
              diag::err_function_parameter_pack_without_parameter_packs)
           << T <<  D.getSourceRange();
         D.setEllipsisLoc(SourceLocation());
       } else {
         T = Context.getPackExpansionType(T, None);
       }
       break;

 
 static void fillAttributedTypeLoc(AttributedTypeLoc TL,
                                   TypeProcessingState &State) {
   TL.setAttr(State.takeAttrForAttributedType(TL.getTypePtr()));
 }
 
 namespace {
   class TypeSpecLocFiller : public TypeLocVisitor<TypeSpecLocFiller> {
+    Sema &SemaRef;
     ASTContext &Context;
     TypeProcessingState &State;
     const DeclSpec &DS;
 
   public:
-    TypeSpecLocFiller(ASTContext &Context, TypeProcessingState &State,
+    TypeSpecLocFiller(Sema &S, ASTContext &Context, TypeProcessingState &State,
                       const DeclSpec &DS)
-        : Context(Context), State(State), DS(DS) {}
+        : SemaRef(S), Context(Context), State(State), DS(DS) {}
 
     void VisitAttributedTypeLoc(AttributedTypeLoc TL) {
       Visit(TL.getModifiedLoc());
       fillAttributedTypeLoc(TL, State);
     }
     void VisitMacroQualifiedTypeLoc(MacroQualifiedTypeLoc TL) {
       Visit(TL.getInnerLoc());
       TL.setExpansionLoc(
void VisitDependentTemplateSpecializationTypeLoc(
                                  DependentTemplateSpecializationTypeLoc TL) {
       assert(DS.getTypeSpecType() == TST_typename);
       TypeSourceInfo *TInfo = nullptr;
       Sema::GetTypeFromParser(DS.getRepAsType(), &TInfo);
       assert(TInfo);
       TL.copy(
           TInfo->getTypeLoc().castAs<DependentTemplateSpecializationTypeLoc>());
     }
+    void VisitAutoTypeLoc(AutoTypeLoc TL) {
+      assert(DS.getTypeSpecType() == TST_auto ||
+             DS.getTypeSpecType() == TST_decltype_auto ||
+             DS.getTypeSpecType() == TST_auto_type ||
+             DS.getTypeSpecType() == TST_unspecified);
+      TL.setNameLoc(DS.getTypeSpecTypeLoc());
+      if (!DS.isConstrainedAuto())
+        return;
+      TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId();
+      if (DS.getTypeSpecScope().isNotEmpty())
+        TL.setNestedNameSpecifierLoc(
+            DS.getTypeSpecScope().getWithLocInContext(Context));
+      else
+        TL.setNestedNameSpecifierLoc(NestedNameSpecifierLoc());
+      TL.setConceptNameLoc(TemplateId->TemplateNameLoc);
+      TL.setLAngleLoc(TemplateId->LAngleLoc);
+      TL.setRAngleLoc(TemplateId->RAngleLoc);
+      if (TemplateId->NumArgs == 0)
+        return;
+      TemplateArgumentListInfo TemplateArgsInfo;
+      ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                         TemplateId->NumArgs);
+      SemaRef.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+      for (unsigned I = 0; I < TemplateId->NumArgs; ++I)
+        TL.setArgLocInfo(I, TemplateArgsInfo.arguments()[I].getLocInfo());
+    }
     void VisitTagTypeLoc(TagTypeLoc TL) {
       TL.setNameLoc(DS.getTypeSpecTypeNameLoc());
     }
     void VisitAtomicTypeLoc(AtomicTypeLoc TL) {
       // An AtomicTypeLoc can come from either an _Atomic(...) type specifier
       // or an _Atomic qualifier.
       if (DS.getTypeSpecType() == DeclSpec::TST_atomic) {
         TL.setKWLoc(DS.getTypeSpecTypeLoc());
GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
 
   // If we have different source information for the return type, use
   // that.  This really only applies to C++ conversion functions.
   if (ReturnTypeInfo) {
     TypeLoc TL = ReturnTypeInfo->getTypeLoc();
     assert(TL.getFullDataSize() == CurrTL.getFullDataSize());
     memcpy(CurrTL.getOpaqueData(), TL.getOpaqueData(), TL.getFullDataSize());
   } else {
-    TypeSpecLocFiller(S.Context, State, D.getDeclSpec()).Visit(CurrTL);
+    TypeSpecLocFiller(S, S.Context, State, D.getDeclSpec()).Visit(CurrTL);
   }
 
   return TInfo;
 }
 
 /// Create a LocInfoType to hold the given QualType and TypeSourceInfo.
 ParsedType Sema::CreateParsedType(QualType T, TypeSourceInfo *TInfo) {
   // FIXME: LocInfoTypes are "transient", only needed for passing to/from Parser

clang/lib/Sema/TreeTransform.h

#include "clang/Basic/OpenMPKinds.def"
   ///
   /// By default, performs semantic analysis when building the decltype type.
   /// Subclasses may override this routine to provide different behavior.
   QualType RebuildDecltypeType(Expr *Underlying, SourceLocation Loc);
 
   /// Build a new C++11 auto type.
   ///
   /// By default, builds a new AutoType with the given deduced type.
-  QualType RebuildAutoType(QualType Deduced, AutoTypeKeyword Keyword) {
+  QualType RebuildAutoType(QualType Deduced, AutoTypeKeyword Keyword,
+                           ConceptDecl *TypeConstraintConcept,
+                           ArrayRef<TemplateArgument> TypeConstraintArgs) {
     // Note, IsDependent is always false here: we implicitly convert an 'auto'
     // which has been deduced to a dependent type into an undeduced 'auto', so
     // that we'll retry deduction after the transformation.
     return SemaRef.Context.getAutoType(Deduced, Keyword,
-                                       /*IsDependent*/ false);
+                                       /*IsDependent*/ false, /*IsPack=*/false,
+                                       TypeConstraintConcept,
+                                       TypeConstraintArgs);
   }
 
   /// By default, builds a new DeducedTemplateSpecializationType with the given
   /// deduced type.
   QualType RebuildDeducedTemplateSpecializationType(TemplateName Template,
       QualType Deduced) {
     return SemaRef.Context.getDeducedTemplateSpecializationType(
         Template, Deduced, /*IsDependent*/ false);
else if (T.getObjCLifetime()) {
       } else if ((AutoTy = dyn_cast<AutoType>(T)) && AutoTy->isDeduced()) {
         // 'auto' types behave the same way as template parameters.
         QualType Deduced = AutoTy->getDeducedType();
         Qualifiers Qs = Deduced.getQualifiers();
         Qs.removeObjCLifetime();
         Deduced =
             SemaRef.Context.getQualifiedType(Deduced.getUnqualifiedType(), Qs);
         T = SemaRef.Context.getAutoType(Deduced, AutoTy->getKeyword(),
-                                        AutoTy->isDependentType());
+                                        AutoTy->isDependentType(),
+                                        /*isPack=*/false,
+                                        AutoTy->getTypeConstraintConcept(),
+                                        AutoTy->getTypeConstraintArguments());
       } else {
         // Otherwise, complain about the addition of a qualifier to an
         // already-qualified type.
         // FIXME: Why is this check not in Sema::BuildQualifiedType?
         SemaRef.Diag(Loc, diag::err_attr_objc_ownership_redundant) << T;
         Quals.removeObjCLifetime();
       }
     }
if (ParmVarDecl *OldParm = Params[i]) {
         // We have a function parameter pack that may need to be expanded.
         SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 
         // Find the parameter packs that could be expanded.
         TypeLoc TL = OldParm->getTypeSourceInfo()->getTypeLoc();
         PackExpansionTypeLoc ExpansionTL = TL.castAs<PackExpansionTypeLoc>();
         TypeLoc Pattern = ExpansionTL.getPatternLoc();
         SemaRef.collectUnexpandedParameterPacks(Pattern, Unexpanded);
-        assert(Unexpanded.size() > 0 && "Could not find parameter packs!");
 
         // Determine whether we should expand the parameter packs.
         bool ShouldExpand = false;
         bool RetainExpansion = false;
-        Optional<unsigned> OrigNumExpansions =
-            ExpansionTL.getTypePtr()->getNumExpansions();
+        Optional<unsigned> OrigNumExpansions;
+        if (Unexpanded.size() > 0) {
+          OrigNumExpansions = ExpansionTL.getTypePtr()->getNumExpansions();
           NumExpansions = OrigNumExpansions;
           if (getDerived().TryExpandParameterPacks(ExpansionTL.getEllipsisLoc(),
                                                    Pattern.getSourceRange(),
                                                    Unexpanded,
                                                    ShouldExpand,
                                                    RetainExpansion,
                                                    NumExpansions)) {
             return true;
           }
+        } else {
+#ifndef NDEBUG
+          const AutoType *AT =
+              Pattern.getType().getTypePtr()->getContainedAutoType();
+          assert((AT && (!AT->isDeduced() || AT->getDeducedType().isNull())) &&
+                 "Could not find parameter packs or undeduced auto type!");
+#endif
+        }
 
         if (ShouldExpand) {
           // Expand the function parameter pack into multiple, separate
           // parameters.
           getDerived().ExpandingFunctionParameterPack(OldParm);
           for (unsigned I = 0; I != *NumExpansions; ++I) {
             Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
             ParmVarDecl *NewParm
#endif
 
         // We'll substitute the parameter now without expanding the pack
         // expansion.
         Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
         NewParm = getDerived().TransformFunctionTypeParam(OldParm,
                                                           indexAdjustment,
                                                           NumExpansions,
                                                   /*ExpectParameterPack=*/true);
+        assert(NewParm->isParameterPack() &&
+               "Parameter pack no longer a parameter pack after "
+               "transformation.");
       } else {
         NewParm = getDerived().TransformFunctionTypeParam(
             OldParm, indexAdjustment, None, /*ExpectParameterPack=*/ false);
       }
 
       if (!NewParm)
         return true;
 
QualType TreeTransform<Derived>::TransformUnaryTransformType(
   UnaryTransformTypeLoc NewTL = TLB.push<UnaryTransformTypeLoc>(Result);
   NewTL.setKWLoc(TL.getKWLoc());
   NewTL.setParensRange(TL.getParensRange());
   NewTL.setUnderlyingTInfo(TL.getUnderlyingTInfo());
   return Result;
 }
 
 template<typename Derived>
-QualType TreeTransform<Derived>::TransformAutoType(TypeLocBuilder &TLB,
-                                                   AutoTypeLoc TL) {
-  const AutoType *T = TL.getTypePtr();
-  QualType OldDeduced = T->getDeducedType();
-  QualType NewDeduced;
-  if (!OldDeduced.isNull()) {
-    NewDeduced = getDerived().TransformType(OldDeduced);
-    if (NewDeduced.isNull())
-      return QualType();
-  }
-
-  QualType Result = TL.getType();
-  if (getDerived().AlwaysRebuild() || NewDeduced != OldDeduced ||
-      T->isDependentType()) {
-    Result = getDerived().RebuildAutoType(NewDeduced, T->getKeyword());
-    if (Result.isNull())
-      return QualType();
-  }
-
-  AutoTypeLoc NewTL = TLB.push<AutoTypeLoc>(Result);
-  NewTL.setNameLoc(TL.getNameLoc());
-
-  return Result;
-}
-
-template<typename Derived>
 QualType TreeTransform<Derived>::TransformDeducedTemplateSpecializationType(
     TypeLocBuilder &TLB, DeducedTemplateSpecializationTypeLoc TL) {
   const DeducedTemplateSpecializationType *T = TL.getTypePtr();
 
   CXXScopeSpec SS;
   TemplateName TemplateName = getDerived().TransformTemplateName(
       SS, T->getTemplateName(), TL.getTemplateNameLoc());
   if (TemplateName.isNull())
public:
     }
 
     friend bool operator!=(const TemplateArgumentLocContainerIterator &X,
                            const TemplateArgumentLocContainerIterator &Y) {
       return !(X == Y);
     }
   };
 
+template<typename Derived>
+QualType TreeTransform<Derived>::TransformAutoType(TypeLocBuilder &TLB,
+                                                   AutoTypeLoc TL) {
+  const AutoType *T = TL.getTypePtr();
+  QualType OldDeduced = T->getDeducedType();
+  QualType NewDeduced;
+  if (!OldDeduced.isNull()) {
+    NewDeduced = getDerived().TransformType(OldDeduced);
+    if (NewDeduced.isNull())
+      return QualType();
+  }
+
+  ConceptDecl *NewCD = nullptr;
+  TemplateArgumentListInfo NewTemplateArgs;
+  NestedNameSpecifierLoc NewNestedNameSpec;
+  if (TL.getTypePtr()->isConstrained()) {
+    NewCD = cast_or_null<ConceptDecl>(
+        getDerived().TransformDecl(
+            TL.getConceptNameLoc(),
+            TL.getTypePtr()->getTypeConstraintConcept()));
+
+    NewTemplateArgs.setLAngleLoc(TL.getLAngleLoc());
+    NewTemplateArgs.setRAngleLoc(TL.getRAngleLoc());
+    typedef TemplateArgumentLocContainerIterator<AutoTypeLoc> ArgIterator;
+    if (getDerived().TransformTemplateArguments(ArgIterator(TL, 0),
+                                                ArgIterator(TL,
+                                                            TL.getNumArgs()),
+                                                NewTemplateArgs))
+      return QualType();
+
+    if (TL.getNestedNameSpecifierLoc()) {
+      NewNestedNameSpec
+        = getDerived().TransformNestedNameSpecifierLoc(
+            TL.getNestedNameSpecifierLoc());
+      if (!NewNestedNameSpec)
+        return QualType();
+    }
+  }
+
+  QualType Result = TL.getType();
+  if (getDerived().AlwaysRebuild() || NewDeduced != OldDeduced ||
+      T->isDependentType()) {
+    llvm::SmallVector<TemplateArgument, 4> NewArgList;
+    NewArgList.reserve(NewArgList.size());
+    for (const auto &ArgLoc : NewTemplateArgs.arguments())
+      NewArgList.push_back(ArgLoc.getArgument());
+    Result = getDerived().RebuildAutoType(NewDeduced, T->getKeyword(), NewCD,
+                                          NewArgList);
+    if (Result.isNull())
+      return QualType();
+  }
+
+  AutoTypeLoc NewTL = TLB.push<AutoTypeLoc>(Result);
+  NewTL.setNameLoc(TL.getNameLoc());
+  NewTL.setNestedNameSpecifierLoc(NewNestedNameSpec);
+  NewTL.setTemplateKWLoc(TL.getTemplateKWLoc());
+  NewTL.setConceptNameLoc(TL.getConceptNameLoc());
+  NewTL.setFoundDecl(TL.getFoundDecl());
+  NewTL.setLAngleLoc(TL.getLAngleLoc());
+  NewTL.setRAngleLoc(TL.getRAngleLoc());
+  for (unsigned I = 0; I < TL.getNumArgs(); ++I)
+    NewTL.setArgLocInfo(I, NewTemplateArgs.arguments()[I].getLocInfo());
+
+  return Result;
+}
 
 template <typename Derived>
 QualType TreeTransform<Derived>::TransformTemplateSpecializationType(
                                                         TypeLocBuilder &TLB,
                                            TemplateSpecializationTypeLoc TL,
                                                       TemplateName Template) {
   TemplateArgumentListInfo NewTemplateArgs;
   NewTemplateArgs.setLAngleLoc(TL.getLAngleLoc());

clang/lib/Serialization/ASTReader.cpp

void TypeLocReader::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
   TL.setKWLoc(readSourceLocation());
   TL.setLParenLoc(readSourceLocation());
   TL.setRParenLoc(readSourceLocation());
   TL.setUnderlyingTInfo(GetTypeSourceInfo());
 }
 
 void TypeLocReader::VisitAutoTypeLoc(AutoTypeLoc TL) {
   TL.setNameLoc(readSourceLocation());
+  if (Reader.readBool()) {
+    TL.setNestedNameSpecifierLoc(ReadNestedNameSpecifierLoc());
+    TL.setTemplateKWLoc(readSourceLocation());
+    TL.setConceptNameLoc(readSourceLocation());
+    TL.setFoundDecl(Reader.readDeclAs<NamedDecl>());
+    TL.setLAngleLoc(readSourceLocation());
+    TL.setRAngleLoc(readSourceLocation());
+    for (unsigned i = 0, e = TL.getNumArgs(); i != e; ++i)
+      TL.setArgLocInfo(i, Reader.readTemplateArgumentLocInfo(
+                              TL.getTypePtr()->getArg(i).getKind()));
+  }
 }
 
 void TypeLocReader::VisitDeducedTemplateSpecializationTypeLoc(
     DeducedTemplateSpecializationTypeLoc TL) {
   TL.setTemplateNameLoc(readSourceLocation());
 }
 
 void TypeLocReader::VisitRecordTypeLoc(RecordTypeLoc TL) {

clang/lib/Serialization/ASTReaderDecl.cpp

void ASTDeclReader::VisitVarTemplatePartialSpecializationDecl(
   }
 }
 
 void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   VisitTypeDecl(D);
 
   D->setDeclaredWithTypename(Record.readInt());
 
-  if (Record.readInt()) {
+  if (Record.readBool()) {
     NestedNameSpecifierLoc NNS = Record.readNestedNameSpecifierLoc();
     DeclarationNameInfo DN = Record.readDeclarationNameInfo();
-    ConceptDecl *NamedConcept = cast<ConceptDecl>(Record.readDecl());
+    ConceptDecl *NamedConcept = Record.readDeclAs<ConceptDecl>();
     const ASTTemplateArgumentListInfo *ArgsAsWritten = nullptr;
-    if (Record.readInt())
+    if (Record.readBool())
         ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
     Expr *ImmediatelyDeclaredConstraint = Record.readExpr();
     D->setTypeConstraint(NNS, DN, /*FoundDecl=*/nullptr, NamedConcept,
                          ArgsAsWritten, ImmediatelyDeclaredConstraint);
     if ((D->ExpandedParameterPack = Record.readInt()))
       D->NumExpanded = Record.readInt();
   }
 
   if (Record.readInt())
     D->setDefaultArgument(readTypeSourceInfo());
 }
 
 void ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   VisitDeclaratorDecl(D);
   // TemplateParmPosition.
   D->setDepth(Record.readInt());
   D->setPosition(Record.readInt());
+  if (D->hasPlaceholderTypeConstraint())
+    D->setPlaceholderTypeConstraint(Record.readExpr());
   if (D->isExpandedParameterPack()) {
     auto TypesAndInfos =
         D->getTrailingObjects<std::pair<QualType, TypeSourceInfo *>>();
     for (unsigned I = 0, N = D->getNumExpansionTypes(); I != N; ++I) {
       new (&TypesAndInfos[I].first) QualType(Record.readType());
       TypesAndInfos[I].second = readTypeSourceInfo();
     }
   } else {
case DECL_FUNCTION_TEMPLATE:
     D = FunctionTemplateDecl::CreateDeserialized(Context, ID);
     break;
   case DECL_TEMPLATE_TYPE_PARM: {
     bool HasTypeConstraint = Record.readInt();
     D = TemplateTypeParmDecl::CreateDeserialized(Context, ID,
                                                  HasTypeConstraint);
     break;
   }
-  case DECL_NON_TYPE_TEMPLATE_PARM:
-    D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID);
+  case DECL_NON_TYPE_TEMPLATE_PARM: {
+    bool HasTypeConstraint = Record.readInt();
+    D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID,
+                                                    HasTypeConstraint);
     break;
-  case DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK:
+  }
+  case DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK: {
+    bool HasTypeConstraint = Record.readInt();
     D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID,
-                                                    Record.readInt());
+                                                    Record.readInt(),
+                                                    HasTypeConstraint);
     break;
+  }
   case DECL_TEMPLATE_TEMPLATE_PARM:
     D = TemplateTemplateParmDecl::CreateDeserialized(Context, ID);
     break;
   case DECL_EXPANDED_TEMPLATE_TEMPLATE_PARM_PACK:
     D = TemplateTemplateParmDecl::CreateDeserialized(Context, ID,
                                                      Record.readInt());
     break;
   case DECL_TYPE_ALIAS_TEMPLATE:

clang/lib/Serialization/ASTWriter.cpp

void TypeLocWriter::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
   Record.AddSourceLocation(TL.getKWLoc());
   Record.AddSourceLocation(TL.getLParenLoc());
   Record.AddSourceLocation(TL.getRParenLoc());
   Record.AddTypeSourceInfo(TL.getUnderlyingTInfo());
 }
 
 void TypeLocWriter::VisitAutoTypeLoc(AutoTypeLoc TL) {
   Record.AddSourceLocation(TL.getNameLoc());
+  Record.push_back(TL.isConstrained());
+  if (TL.isConstrained()) {
+    Record.AddNestedNameSpecifierLoc(TL.getNestedNameSpecifierLoc());
+    Record.AddSourceLocation(TL.getTemplateKWLoc());
+    Record.AddSourceLocation(TL.getConceptNameLoc());
+    Record.AddDeclRef(TL.getFoundDecl());
+    Record.AddSourceLocation(TL.getLAngleLoc());
+    Record.AddSourceLocation(TL.getRAngleLoc());
+    for (unsigned I = 0; I < TL.getNumArgs(); ++I)
+      Record.AddTemplateArgumentLocInfo(TL.getTypePtr()->getArg(I).getKind(),
+                                        TL.getArgLocInfo(I));
+  }
 }
 
 void TypeLocWriter::VisitDeducedTemplateSpecializationTypeLoc(
     DeducedTemplateSpecializationTypeLoc TL) {
   Record.AddSourceLocation(TL.getTemplateNameLoc());
 }
 
 void TypeLocWriter::VisitRecordTypeLoc(RecordTypeLoc TL) {

clang/lib/Serialization/ASTWriterDecl.cpp

void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
 
   Code = serialization::DECL_TEMPLATE_TYPE_PARM;
 }
 
 void ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // For an expanded parameter pack, record the number of expansion types here
   // so that it's easier for deserialization to allocate the right amount of
   // memory.
+  Expr *TypeConstraint = D->getPlaceholderTypeConstraint();
+  Record.push_back(!!TypeConstraint);
   if (D->isExpandedParameterPack())
     Record.push_back(D->getNumExpansionTypes());
 
   VisitDeclaratorDecl(D);
   // TemplateParmPosition.
   Record.push_back(D->getDepth());
   Record.push_back(D->getPosition());
+  if (TypeConstraint)
+    Record.AddStmt(TypeConstraint);
 
   if (D->isExpandedParameterPack()) {
     for (unsigned I = 0, N = D->getNumExpansionTypes(); I != N; ++I) {
       Record.AddTypeRef(D->getExpansionType(I));
       Record.AddTypeSourceInfo(D->getExpansionTypeSourceInfo(I));
     }
 
     Code = serialization::DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK;

clang/test/AST/ast-dump-record-definition-data-json.cpp

 // CHECK-NEXT:     }
 // CHECK-NEXT:    },
 // CHECK-NEXT:    "name": "operator()",
 // CHECK-NEXT:    "inner": [
 // CHECK-NEXT:     {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "TemplateTypeParmDecl",
 // CHECK-NEXT:      "loc": {
-// CHECK-NEXT:       "offset": 193,
-// CHECK-NEXT:       "col": 29,
-// CHECK-NEXT:       "tokLen": 4
-// CHECK-NEXT:      },
-// CHECK-NEXT:      "range": {
-// CHECK-NEXT:       "begin": {},
-// CHECK-NEXT:       "end": {}
+// CHECK-NEXT:        "offset": 197,
+// CHECK-NEXT:        "col": 33,
+// CHECK-NEXT:        "tokLen": 1
+// CHECK-NEXT:      },
+// CHECK-NEXT:      "range": {
+// CHECK-NEXT:        "begin": {
+// CHECK-NEXT:          "offset": 193,
+// CHECK-NEXT:          "col": 29,
+// CHECK-NEXT:          "tokLen": 4
+// CHECK-NEXT:        },
+// CHECK-NEXT:        "end": {
+// CHECK-NEXT:          "offset": 197,
+// CHECK-NEXT:          "col": 33,
+// CHECK-NEXT:          "tokLen": 1
+// CHECK-NEXT:        }
 // CHECK-NEXT:      },
 // CHECK-NEXT:      "isImplicit": true,
+// CHECK-NEXT:      "name": "auto:1",
 // CHECK-NEXT:      "tagUsed": "class",
 // CHECK-NEXT:      "depth": 0,
 // CHECK-NEXT:      "index": 0
 // CHECK-NEXT:     },
 // CHECK-NEXT:     {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "CXXMethodDecl",
 // CHECK-NEXT:      "loc": {

 // CHECK-NEXT:    },
 // CHECK-NEXT:    "isImplicit": true,
 // CHECK-NEXT:    "name": "operator auto (*)(type-parameter-0-0)",
 // CHECK-NEXT:    "inner": [
 // CHECK-NEXT:     {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "TemplateTypeParmDecl",
 // CHECK-NEXT:      "loc": {
-// CHECK-NEXT:       "offset": 193,
-// CHECK-NEXT:       "col": 29,
-// CHECK-NEXT:       "tokLen": 4
-// CHECK-NEXT:      },
-// CHECK-NEXT:      "range": {
-// CHECK-NEXT:       "begin": {},
-// CHECK-NEXT:       "end": {}
+// CHECK-NEXT:        "offset": 197,
+// CHECK-NEXT:        "col": 33,
+// CHECK-NEXT:        "tokLen": 1
+// CHECK-NEXT:      },
+// CHECK-NEXT:      "range": {
+// CHECK-NEXT:        "begin": {
+// CHECK-NEXT:          "offset": 193,
+// CHECK-NEXT:          "col": 29,
+// CHECK-NEXT:          "tokLen": 4
+// CHECK-NEXT:        },
+// CHECK-NEXT:        "end": {
+// CHECK-NEXT:          "offset": 197,
+// CHECK-NEXT:          "col": 33,
+// CHECK-NEXT:          "tokLen": 1
+// CHECK-NEXT:        }
 // CHECK-NEXT:      },
 // CHECK-NEXT:      "isImplicit": true,
+// CHECK-NEXT:      "name": "auto:1",
 // CHECK-NEXT:      "tagUsed": "class",
 // CHECK-NEXT:      "depth": 0,
 // CHECK-NEXT:      "index": 0
 // CHECK-NEXT:     },
 // CHECK-NEXT:     {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "CXXConversionDecl",
 // CHECK-NEXT:      "loc": {

 // CHECK-NEXT:    },
 // CHECK-NEXT:    "isImplicit": true,
 // CHECK-NEXT:    "name": "__invoke",
 // CHECK-NEXT:    "inner": [
 // CHECK-NEXT:     {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "TemplateTypeParmDecl",
 // CHECK-NEXT:      "loc": {
-// CHECK-NEXT:       "offset": 193,
-// CHECK-NEXT:       "col": 29,
-// CHECK-NEXT:       "tokLen": 4
-// CHECK-NEXT:      },
-// CHECK-NEXT:      "range": {
-// CHECK-NEXT:       "begin": {},
-// CHECK-NEXT:       "end": {}
+// CHECK-NEXT:        "offset": 197,
+// CHECK-NEXT:        "col": 33,
+// CHECK-NEXT:        "tokLen": 1
+// CHECK-NEXT:      },
+// CHECK-NEXT:      "range": {
+// CHECK-NEXT:        "begin": {
+// CHECK-NEXT:          "offset": 193,
+// CHECK-NEXT:          "col": 29,
+// CHECK-NEXT:          "tokLen": 4
+// CHECK-NEXT:        },
+// CHECK-NEXT:        "end": {
+// CHECK-NEXT:          "offset": 197,
+// CHECK-NEXT:          "col": 33,
+// CHECK-NEXT:          "tokLen": 1
+// CHECK-NEXT:        }
 // CHECK-NEXT:      },
 // CHECK-NEXT:      "isImplicit": true,
+// CHECK-NEXT:      "name": "auto:1",
 // CHECK-NEXT:      "tagUsed": "class",
 // CHECK-NEXT:      "depth": 0,
 // CHECK-NEXT:      "index": 0
 // CHECK-NEXT:     },
 // CHECK-NEXT:     {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "CXXMethodDecl",
 // CHECK-NEXT:      "loc": {

clang/test/CXX/dcl/dcl.fct/p17.cpp

+// RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
+template<typename T, typename U> constexpr bool is_same_v = false;
+template<typename T> constexpr bool is_same_v<T, T> = true;
+
+template<typename... T>
+struct type_list;
+
+namespace unconstrained {
+  decltype(auto) f1(auto x) { return x; }
+  static_assert(is_same_v<decltype(f1(1)), int>);
+  static_assert(is_same_v<decltype(f1('c')), char>);
+
+  decltype(auto) f2(auto &x) { return x; }
+  // expected-note@-1{{candidate function [with x:auto = int] not viable: expects an l-value for 1st argument}}
+  // expected-note@-2{{candidate function [with x:auto = char] not viable: expects an l-value for 1st argument}}
+  static_assert(is_same_v<decltype(f2(1)), int &>); // expected-error{{no matching}}
+  static_assert(is_same_v<decltype(f2('c')), char &>); // expected-error{{no matching}}
+
+  decltype(auto) f3(const auto &x) { return x; }
+  static_assert(is_same_v<decltype(f3(1)), const int &>);
+  static_assert(is_same_v<decltype(f3('c')), const char &>);
+
+  decltype(auto) f4(auto (*x)(auto y)) { return x; } // expected-error{{'auto' not allowed in function prototype}}
+
+  decltype(auto) f5(void (*x)(decltype(auto) y)) { return x; } // expected-error{{'decltype(auto)' not allowed in function prototype}}
+
+  int return_int(); void return_void(); int foo(int);
+
+  decltype(auto) f6(auto (*x)()) { return x; }
+  // expected-note@-1{{candidate template ignored: failed template argument deduction}}
+  static_assert(is_same_v<decltype(f6(return_int)), int (*)()>);
+  static_assert(is_same_v<decltype(f6(return_void)), void (*)()>);
+  using f6c1 = decltype(f6(foo)); // expected-error{{no matching}}
+
+  decltype(auto) f7(auto (*x)() -> int) { return x; }
+  // expected-note@-1{{candidate function not viable: no known conversion from 'void ()' to 'auto (*)() -> int' for 1st argument}}
+  // expected-note@-2{{candidate function not viable: no known conversion from 'int (int)' to 'auto (*)() -> int' for 1st argument}}
+  static_assert(is_same_v<decltype(f7(return_int)), int (*)()>);
+  using f7c1 = decltype(f7(return_void)); // expected-error{{no matching}}
+  using f7c2 = decltype(f7(foo)); // expected-error{{no matching}}
+  static_assert(is_same_v<decltype(&f7), int (*(*)(int (*x)()))()>);
+
+  decltype(auto) f8(auto... x) { return (x + ...); }
+  static_assert(is_same_v<decltype(f8(1, 2, 3)), int>);
+  static_assert(is_same_v<decltype(f8('c', 'd')), int>);
+  static_assert(is_same_v<decltype(f8('c', 1)), int>);
+
+  decltype(auto) f9(auto &... x) { return (x, ...); }
+  // expected-note@-1{{candidate function [with x:auto = <int (), int>] not viable: expects an l-value for 2nd argument}}
+  using f9c1 = decltype(f9(return_int, 1)); // expected-error{{no matching}}
+
+  decltype(auto) f11(decltype(auto) x) { return x; } // expected-error{{'decltype(auto)' not allowed in function prototype}}
+
+  template<typename T>
+  auto f12(auto x, T y) -> type_list<T, decltype(x)>;
+  static_assert(is_same_v<decltype(f12(1, 'c')), type_list<char, int>>);
+  static_assert(is_same_v<decltype(f12<char>(1, 'c')), type_list<char, int>>);
+
+  template<typename T>
+  auto f13(T x, auto y) -> type_list<T, decltype(y)>;
+  static_assert(is_same_v<decltype(f13(1, 'c')), type_list<int, char>>);
+  static_assert(is_same_v<decltype(f13<char>(1, 'c')), type_list<char, char>>);
+
+  template<typename T>
+  auto f14(auto y) -> type_list<T, decltype(y)>;
+  static_assert(is_same_v<decltype(f14<int>('c')), type_list<int, char>>);
+  static_assert(is_same_v<decltype(f14<int, char>('c')), type_list<int, char>>);
+
+  template<typename T, typename U>
+  auto f15(auto y, U u) -> type_list<T, U, decltype(y)>;
+  static_assert(is_same_v<decltype(f15<int>('c', nullptr)), type_list<int, decltype(nullptr), char>>);
+  static_assert(is_same_v<decltype(f15<int, decltype(nullptr)>('c', nullptr)), type_list<int, decltype(nullptr), char>>);
+
+  auto f16(auto x, auto y) -> type_list<decltype(x), decltype(y)>;
+  static_assert(is_same_v<decltype(f16('c', 1)), type_list<char, int>>);
+  static_assert(is_same_v<decltype(f16<int>('c', 1)), type_list<int, int>>);
+  static_assert(is_same_v<decltype(f16<int, char>('c', 1)), type_list<int, char>>);
+
+  void f17(auto x, auto y) requires (sizeof(x) > 1);
+  // expected-note@-1{{candidate template ignored: constraints not satisfied [with x:auto = char, y:auto = int]}}
+  // expected-note@-2{{because 'sizeof (x) > 1' (1 > 1) evaluated to false}}
+  static_assert(is_same_v<decltype(f17('c', 1)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f17<int>('c', 1)), void>);
+  static_assert(is_same_v<decltype(f17<int, char>('c', 1)), void>);
+
+  void f18(auto... x) requires (sizeof...(x) == 2);
+  // expected-note@-1{{candidate template ignored: constraints not satisfied [with x:auto = <char, int, int>]}}
+  // expected-note@-2{{candidate template ignored: constraints not satisfied [with x:auto = <char>]}}
+  // expected-note@-3{{because 'sizeof...(x) == 2' (1 == 2) evaluated to false}}
+  // expected-note@-4{{because 'sizeof...(x) == 2' (3 == 2) evaluated to false}}
+  static_assert(is_same_v<decltype(f18('c')), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f18('c', 1)), void>);
+  static_assert(is_same_v<decltype(f18('c', 1, 2)), void>);
+  // expected-error@-1{{no matching}}
+
+  template<typename T>
+  struct S {
+    constexpr auto f1(auto x, T t) -> decltype(x + t);
+
+    template<typename U>
+    constexpr auto f2(U u, auto x, T t) -> decltype(x + u + t);
+  };
+
+  template<typename T>
+  constexpr auto S<T>::f1(auto x, T t) -> decltype(x + t) { return x + t; }
+
+  template<typename T>
+  template<typename U>
+  constexpr auto S<T>::f2(auto x, U u, T t) -> decltype(x + u + t) { return x + u + t; }
+  // expected-error@-1 {{out-of-line definition of 'f2' does not match any declaration in 'S<T>'}}
+
+  template<typename T>
+  template<typename U>
+  constexpr auto S<T>::f2(U u, auto x, T t) -> decltype(x + u + t) { return x + u + t; }
+
+  template<>
+  template<>
+  constexpr auto S<int>::f2<double>(double u, char x, int t) -> double { return 42; }
+
+  static_assert(S<char>{}.f1(1, 2) == 3);
+  static_assert(S<char>{}.f2(1, 2, '\x00') == 3);
+  static_assert(S<char>{}.f2<double>(1, 2, '\x00') == 3.);
+  static_assert(S<int>{}.f2<double>(1, '2', '\x00') == 42);
+}
+
+namespace constrained {
+  template<typename T>
+  concept C = is_same_v<T, int>;
+  // expected-note@-1 12{{because}}
+  template<typename T, typename U>
+  concept C2 = is_same_v<T, U>;
+  // expected-note@-1 12{{because}}
+
+  int i;
+  const int ci = 1;
+  char c;
+  const char cc = 'a';
+  int g(int);
+  char h(int);
+
+  void f1(C auto x);
+  // expected-note@-1 {{candidate template ignored: constraints not satisfied [with x:auto = }}
+  // expected-note@-2{{because}}
+  static_assert(is_same_v<decltype(f1(1)), void>);
+  static_assert(is_same_v<decltype(f1('a')), void>);
+  // expected-error@-1{{no matching}}
+  void f2(C auto &x);
+  // expected-note@-1 2{{candidate template ignored}} expected-note@-1 2{{because}}
+  static_assert(is_same_v<decltype(f2(i)), void>);
+  static_assert(is_same_v<decltype(f2(ci)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f2(c)), void>);
+  // expected-error@-1{{no matching}}
+  void f3(const C auto &x);
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f3(i)), void>);
+  static_assert(is_same_v<decltype(f3(ci)), void>);
+  static_assert(is_same_v<decltype(f3(c)), void>);
+  // expected-error@-1{{no matching}}
+  void f4(C auto (*x)(C auto y)); // expected-error{{'auto' not allowed}}
+  void f5(C auto (*x)(int y));
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f5(g)), void>);
+  static_assert(is_same_v<decltype(f5(h)), void>);
+  // expected-error@-1{{no matching}}
+  void f6(C auto (*x)() -> int); // expected-error{{function with trailing return type must specify return type 'auto', not 'C auto'}}
+  void f7(C auto... x);
+  // expected-note@-1 2{{candidate template ignored}} expected-note@-1 2{{because}}
+  static_assert(is_same_v<decltype(f7(1, 2)), void>);
+  static_assert(is_same_v<decltype(f7(1, 'a')), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f7('a', 2)), void>);
+  // expected-error@-1{{no matching}}
+  void f8(C auto &... x);
+  // expected-note@-1 2{{candidate template ignored}} expected-note@-1 2{{because}}
+  static_assert(is_same_v<decltype(f8(i, i)), void>);
+  static_assert(is_same_v<decltype(f8(i, c)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f8(i, i, ci)), void>);
+  // expected-error@-1{{no matching}}
+  void f9(const C auto &... x);
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f9(i, i)), void>);
+  static_assert(is_same_v<decltype(f9(i, c)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f9(i, i, ci)), void>);
+  void f10(C decltype(auto) x);
+  auto f11 = [] (C auto x) { };
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f11(1)), void>);
+  static_assert(is_same_v<decltype(f11('a')), void>);
+  // expected-error@-1{{no matching}}
+
+  void f12(C2<char> auto x);
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f12(1)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f12('a')), void>);
+  void f13(C2<char> auto &x);
+  // expected-note@-1 2{{candidate template ignored}} expected-note@-1 2{{because}}
+  static_assert(is_same_v<decltype(f13(i)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f13(cc)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f13(c)), void>);
+  void f14(const C2<char> auto &x);
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f14(i)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f14(cc)), void>);
+  static_assert(is_same_v<decltype(f14(c)), void>);
+  void f15(C2<char> auto (*x)(C2<int> auto y)); // expected-error{{'auto' not allowed}}
+  void f16(C2<char> auto (*x)(int y));
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f16(g)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f16(h)), void>);
+  void f17(C2<char> auto (*x)() -> int); // expected-error{{function with trailing return type must specify return type 'auto', not 'C2<char> auto'}}
+  void f18(C2<char> auto... x);
+  // expected-note@-1 2{{candidate template ignored}} expected-note@-1 2{{because}}
+  static_assert(is_same_v<decltype(f18('a', 'b')), void>);
+  static_assert(is_same_v<decltype(f18('a', 1)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f18(2, 'a')), void>);
+  // expected-error@-1{{no matching}}
+  void f19(C2<char> auto &... x);
+  // expected-note@-1 2{{candidate template ignored}} expected-note@-1 2{{because}}
+  static_assert(is_same_v<decltype(f19(c, c)), void>);
+  static_assert(is_same_v<decltype(f19(i, c)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f19(c, c, cc)), void>);
+  // expected-error@-1{{no matching}}
+  void f20(const C2<char> auto &... x);
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f20(c, c)), void>);
+  static_assert(is_same_v<decltype(f20(i, c)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f20(c, c, cc)), void>);
+  void f21(C2<char> decltype(auto) x);
+  auto f22 = [] (C2<char> auto x) { };
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  static_assert(is_same_v<decltype(f22(1)), void>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(f22('a')), void>);
+
+  struct S1 { S1(C auto); };
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  // expected-note@-2 2{{candidate constructor}}
+  static_assert(is_same_v<decltype(S1(1)), S1>);
+  static_assert(is_same_v<decltype(S1('a')), S1>);
+  // expected-error@-1{{no matching}}
+  struct S2 { S2(C2<char> auto); };
+  // expected-note@-1{{candidate template ignored}} expected-note@-1{{because}}
+  // expected-note@-2 2{{candidate constructor}}
+  static_assert(is_same_v<decltype(S2(1)), S2>);
+  // expected-error@-1{{no matching}}
+  static_assert(is_same_v<decltype(S2('a')), S2>);
+}

clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp

+// RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
+
+template<typename T, unsigned size>
+concept LargerThan = sizeof(T) > size;
+// expected-note@-1 2{{because 'sizeof(char) > 1U' (1 > 1) evaluated to false}}
+// expected-note@-2 {{because 'sizeof(int) > 10U' (4 > 10) evaluated to false}}
+// expected-note@-3 {{because 'sizeof(int) > 4U' (4 > 4) evaluated to false}}
+
+template<typename T>
+concept Large = LargerThan<T, 1>;
+// expected-note@-1 2{{because 'LargerThan<char, 1>' evaluated to false}}
+
+namespace X {
+  template<typename T, unsigned size>
+  concept SmallerThan = sizeof(T) < size;
+  template<typename T>
+  concept Small = SmallerThan<T, 2>;
+}
+
+Large auto test1() { // expected-error{{deduced type 'char' does not satisfy 'Large'}}
+  Large auto i = 'a';
+  // expected-error@-1{{deduced type 'char' does not satisfy 'Large'}}
+  Large auto j = 10;
+  ::Large auto k = 10;
+  LargerThan<1> auto l = 10;
+  ::LargerThan<1> auto m = 10;
+  LargerThan<10> auto n = 10;
+  // expected-error@-1{{deduced type 'int' does not satisfy 'LargerThan<10>'}}
+  X::Small auto o = 'x';
+  X::SmallerThan<5> auto p = 1;
+  return 'a';
+}
+
+::Large auto test2() { return 10; }
+LargerThan<4> auto test3() { return 10; }
+// expected-error@-1{{deduced type 'int' does not satisfy 'LargerThan<4>'}}
+::LargerThan<2> auto test4() { return 10; }
+
+Large auto test5() -> void;
+// expected-error@-1{{function with trailing return type must specify return type 'auto', not 'Large auto'}}
+auto test6() -> Large auto { return 1; }
+
+X::Small auto test7() { return 'a'; }
+X::SmallerThan<5> auto test8() { return 10; }
 No newline at end of file

clang/test/CXX/expr/expr.prim/expr.prim.lambda/expr.prim.lambda.closure/p3.cpp

 // RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
 
 auto l1 = [] (auto x) requires (sizeof(decltype(x)) == 1) { return x; };
-// expected-note@-1{{candidate template ignored: constraints not satisfied [with $0 = int]}}
+// expected-note@-1{{candidate template ignored: constraints not satisfied [with x:auto = int]}}
 // expected-note@-2{{because 'sizeof(decltype(x)) == 1' (4 == 1) evaluated to false}}
 
 auto l1t1 = l1('a');
 auto l1t2 = l1(1);
 // expected-error@-1{{no matching function for call to object of type '(lambda at}}
 
 auto l2 = [] (auto... x) requires ((sizeof(decltype(x)) >= 2) && ...) { return (x + ...); };
-// expected-note@-1{{candidate template ignored: constraints not satisfied [with $0 = <char>]}}
-// expected-note@-2{{candidate template ignored: constraints not satisfied [with $0 = <int, char>]}}
+// expected-note@-1{{candidate template ignored: constraints not satisfied [with x:auto = <char>]}}
+// expected-note@-2{{candidate template ignored: constraints not satisfied [with x:auto = <int, char>]}}
 // expected-note@-3 2{{because 'sizeof(decltype(x)) >= 2' (1 >= 2) evaluated to false}}
 
 auto l2t1 = l2('a');
 // expected-error@-1{{no matching function for call to object of type '(lambda at}}
 auto l2t2 = l2(1, 'a');
 // expected-error@-1{{no matching function for call to object of type '(lambda at}}
 auto l2t3 = l2((short)1, (short)1);
 No newline at end of file

clang/test/CXX/temp/temp.param/p10-2a.cpp

 concept OneOf = (is_same_v<T, Ts> || ...);
 // expected-note@-1 2{{because 'is_same_v<char, char [1]>' evaluated to false}}
 // expected-note@-2 2{{and 'is_same_v<char, char [2]>' evaluated to false}}
 // expected-note@-3 {{because 'is_same_v<short, int>' evaluated to false}}
 // expected-note@-4 {{and 'is_same_v<short, long>' evaluated to false}}
 // expected-note@-5 {{and 'is_same_v<short, char>' evaluated to false}}
 // expected-note@-6 3{{because 'is_same_v<int, char [1]>' evaluated to false}}
 // expected-note@-7 3{{and 'is_same_v<int, char [2]>' evaluated to false}}
+// expected-note@-8 2{{because 'is_same_v<nullptr_t, char>' evaluated to false}}
+// expected-note@-9 2{{and 'is_same_v<nullptr_t, int>' evaluated to false}}
 
 template<OneOf<char[1], char[2]> T, OneOf<int, long, char> U>
 // expected-note@-1 2{{because 'OneOf<char, char [1], char [2]>' evaluated to false}}
 // expected-note@-2 {{because 'OneOf<short, int, long, char>' evaluated to false}}
 using G = T;
 
 using g1 = G<char[1], int>;
 using g2 = G<char, int>; // expected-error{{constraints not satisfied for alias template 'G' [with T = char, U = int]}}
 using g3 = G<char[1], short>; // expected-error{{constraints not satisfied for alias template 'G' [with T = char [1], U = short]}}
 using g4 = G<char, short>; // expected-error{{constraints not satisfied for alias template 'G' [with T = char, U = short]}}
 
 template<OneOf<char[1], char[2]>... Ts>
 // expected-note@-1 2{{because 'OneOf<int, char [1], char [2]>' evaluated to false}}
 // expected-note@-2 {{and 'OneOf<int, char [1], char [2]>' evaluated to false}}
 using H = int;
 
 using h1 = H<char[1], int>;
 // expected-error@-1 {{constraints not satisfied for alias template 'H' [with Ts = <char [1], int>]}}
 using h2 = H<int, int>;
 // expected-error@-1 {{constraints not satisfied for alias template 'H' [with Ts = <int, int>]}}
-using h3 = H<char[1], char[2]>;
+using h3 = H<char[1], char[2]>;
+
+template<OneOf<char, int> auto x>
+// expected-note@-1 {{because 'OneOf<decltype(nullptr), char, int>' evaluated to false}}
+using I = int;
+
+using i1 = I<1>;
+using i2 = I<'a'>;
+using i3 = I<nullptr>;
+// expected-error@-1 {{constraints not satisfied for alias template 'I' [with x = nullptr]}}
+
+template<OneOf<char, int> auto... x>
+// expected-note@-1 {{because 'OneOf<decltype(nullptr), char, int>' evaluated to false}}
+using J = int;
+
+using j1 = J<1, 'b'>;
+using j2 = J<'a', nullptr>;
+// expected-error@-1 {{constraints not satisfied for alias template 'J' [with x = <'a', nullptr>]}}
+
+template<OneOf<char, int> auto &x>
+// expected-error@-1 {{constrained placeholder types other than simple 'auto' on non-type template parameters not supported yet}}
+using K = int;

clang/test/Parser/cxx2a-placeholder-type-constraint.cpp

+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+template<typename T, typename U=void>
+concept C = true;
+
+int foo() {
+  C auto a4 = 1;
+  C<> auto a5 = 1;
+  C<int> auto a6 = 1;
+  const C auto &a7 = 1;
+  const C<> auto &a8 = 1;
+  const C<int> auto &a9 = 1;
+  C decltype(auto) a10 = 1;
+  C<> decltype(auto) a11 = 1;
+  C<int> decltype(auto) a12 = 1;
+  const C<> decltype(auto) &a13 = 1; // expected-error{{'decltype(auto)' cannot be combined with other type specifiers}}
+  // expected-error@-1{{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
+  const C<int> decltype(auto) &a14 = 1; // expected-error{{'decltype(auto)' cannot be combined with other type specifiers}}
+  // expected-error@-1{{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
+  C a15 = 1;
+  // expected-error@-1{{expected 'auto' or 'decltype(auto)' after concept name}}
+  C decltype a19 = 1;
+  // expected-error@-1{{expected '('}}
+  C decltype(1) a20 = 1;
+  // expected-error@-1{{expected 'auto' or 'decltype(auto)' after concept name}}
+}
 No newline at end of file

clang/test/SemaCXX/cxx1y-generic-lambdas.cpp

 
 
 int test() {
   {
     int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; }; // OK
     int (*fp2)(int) = [](auto b) -> int {  return b; };
     int (*fp3)(char) = [](auto c) -> int { return c; };
     char (*fp4)(int) = [](auto d) { return d; }; //expected-error{{no viable conversion}}\
-                                                 //expected-note{{candidate function [with $0 = int]}}
+                                                 //expected-note{{candidate function [with d:auto = int]}}
     char (*fp5)(char) = [](auto e) -> int { return e; }; //expected-error{{no viable conversion}}\
                                                  //expected-note{{candidate template ignored}}
 
     fp2(3);
     fp3('\n');
     fp3('a');
     return 0;
   }

clang/test/SemaTemplate/ms-delayed-default-template-args.cpp

 template struct Foo<Yodel>; // expected-error {{use of undeclared identifier 'Yodel'}}
 }
 
 namespace test_undeclared_nontype_parm_arg {
 // Bury an undeclared type as a template argument to the type of a non-type
 // template parameter.
 template <typename T> struct Bar { T x; };
 
-template <Bar<Xylophone> *P> // expected-error {{use of undeclared identifier 'Xylophone'}} expected-note {{declared here}}
+template <Bar<Xylophone> *P> // expected-error {{use of undeclared identifier 'Xylophone'}}
+// expected-note@-1{{template parameter is declared here}}
 struct Foo { };
 
 typedef int Xylophone;
 Bar<Xylophone> g;
 template struct Foo<&g>; // expected-error {{value of type}}
 }
 
 #endif