[Concepts] Placeholder constraints and abbreviated templates

This patch implements P1141R2 "Yet another approach for constrained declarations".

General strategy for this patch was:

- Expand AutoType to include optional type-constraint, reflecting the wording and easing the integration of constraints.
- Replace autos in parameter type specifiers with invented parameters in GetTypeSpecTypeForDeclarator, using the same logic
  previously used for generic lambdas, now unified with abbreviated templates, 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 list to append invented parameters
    to 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).

Differential Revision: https://reviews.llvm.org/D65042

clang/include/clang/AST/ASTContext.h

@@ -88,6 +88,7 @@ class AtomicExpr;
 class BlockExpr;
 class BuiltinTemplateDecl;
 class CharUnits;
+class ConceptDecl;
 class CXXABI;
 class CXXConstructorDecl;
 class CXXMethodDecl;
@@ -211,7 +212,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   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;
@@ -1542,7 +1543,9 @@ public:
 
   /// 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;

clang/include/clang/AST/ASTNodeTraverser.h

@@ -548,8 +548,8 @@ public:
   }
 
   void VisitNonTypeTemplateParmDecl(const NonTypeTemplateParmDecl *D) {
-    if (const auto *TC = D->getPlaceholderTypeConstraint())
+    if (const auto *E = D->getPlaceholderTypeConstraint())
-      Visit(TC->getImmediatelyDeclaredConstraint());
+      Visit(E);
     if (D->hasDefaultArgument())
       Visit(D->getDefaultArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),

clang/include/clang/AST/DeclTemplate.h

@@ -1102,6 +1102,17 @@ public:
   /// 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);
 
@@ -1215,7 +1226,6 @@ public:
                                       bool ParameterPack,
                                       bool HasTypeConstraint = false,
                                       Optional<unsigned> NumExpanded = None);
-
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
                                                   unsigned ID);
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
@@ -1374,7 +1384,8 @@ 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;
 
@@ -1429,10 +1440,12 @@ public:
          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;
@@ -1543,20 +1556,22 @@ public:
     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 {
+  Expr *getPlaceholderTypeConstraint() const {
-    // TODO: Concepts: Implement once we have actual placeholders with type
+    return hasPlaceholderTypeConstraint() ? *getTrailingObjects<Expr *>() :
-    //                 constraints.
+        nullptr;
-    return 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
+    auto *AT = getType()->getContainedAutoType();
-    //                 constraints.
+    return AT && AT->isConstrained();
-    return false;
   }
 
   /// \brief Get the associated-constraints of this template parameter.
@@ -1566,8 +1581,8 @@ public:
   /// 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())
+    if (Expr *E = getPlaceholderTypeConstraint())
-      AC.push_back(TC->getImmediatelyDeclaredConstraint());
+      AC.push_back(E);
   }
 
   // Implement isa/cast/dyncast/etc.

clang/include/clang/AST/PropertiesBase.td

@@ -99,6 +99,8 @@ def DeclRef : RefPropertyType<"Decl"> { let ConstWhenWriting = 1; }
     SubclassPropertyType<"TagDecl", DeclRef>;
   def TemplateDeclRef :
     SubclassPropertyType<"TemplateDecl", DeclRef>;
+  def ConceptDeclRef :
+    SubclassPropertyType<"ConceptDecl", DeclRef>;
   def TemplateTypeParmDeclRef :
     SubclassPropertyType<"TemplateTypeParmDecl", DeclRef>;
   def TemplateTemplateParmDeclRef :

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -1040,7 +1040,13 @@ DEF_TRAVERSE_TYPE(UnaryTransformType, {
   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()));
@@ -1287,6 +1293,12 @@ DEF_TRAVERSE_TYPELOC(UnaryTransformType, {
 
 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, {

clang/include/clang/AST/TemplateBase.h

@@ -637,7 +637,7 @@ public:
   }
 
   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.,
@@ -702,6 +702,11 @@ inline const TemplateArgument &
   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

@@ -58,6 +58,7 @@ namespace clang {
 
 class ExtQuals;
 class QualType;
+class ConceptDecl;
 class TagDecl;
 class Type;
 
@@ -1683,6 +1684,15 @@ protected:
     /// 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 {
@@ -4814,8 +4824,7 @@ public:
 
 /// 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++
+/// class template types, and constrained type names.
-/// Concepts TS.
 ///
 /// These types are usually a placeholder for a deduced type. However, before
 /// the initializer is attached, or (usually) if the initializer is
@@ -4860,18 +4869,50 @@ public:
   }
 };
 
-/// Represents a C++11 auto or C++14 decltype(auto) type.
+/// Represents a C++11 auto or C++14 decltype(auto) type, possibly constrained
-class AutoType : public DeducedType, public llvm::FoldingSetNode {
+/// 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)
+           bool IsDeducedAsDependent, bool IsDeducedAsPack, ConceptDecl *CD,
-      : DeducedType(Auto, DeducedAsType, IsDeducedAsDependent,
+           ArrayRef<TemplateArgument> TypeConstraintArgs);
-                    IsDeducedAsDependent, IsDeducedAsPack) {
+
-    AutoTypeBits.Keyword = (unsigned)Keyword;
+  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;
   }
@@ -4880,18 +4921,15 @@ public:
     return (AutoTypeKeyword)AutoTypeBits.Keyword;
   }
 
-  void Profile(llvm::FoldingSetNodeID &ID) {
+  void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context) {
-    Profile(ID, getDeducedType(), getKeyword(), isDependentType(),
+    Profile(ID, Context, getDeducedType(), getKeyword(), isDependentType(),
-            containsUnexpandedParameterPack());
+            getTypeConstraintConcept(), getTypeConstraintArguments());
   }
 
-  static void Profile(llvm::FoldingSetNodeID &ID, QualType Deduced,
+  static void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
-                      AutoTypeKeyword Keyword, bool IsDependent, bool IsPack) {
+                      QualType Deduced, AutoTypeKeyword Keyword,
-    ID.AddPointer(Deduced.getAsOpaquePtr());
+                      bool IsDependent, ConceptDecl *CD,
-    ID.AddInteger((unsigned)Keyword);
+                      ArrayRef<TemplateArgument> Arguments);
-    ID.AddBoolean(IsDependent);
-    ID.AddBoolean(IsPack);
-  }
 
   static bool classof(const Type *T) {
     return T->getTypeClass() == Auto;

clang/include/clang/AST/TypeLoc.h

@@ -14,6 +14,7 @@
 #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"
@@ -34,6 +35,7 @@ namespace clang {
 class Attr;
 class ASTContext;
 class CXXRecordDecl;
+class ConceptDecl;
 class Expr;
 class ObjCInterfaceDecl;
 class ObjCProtocolDecl;
@@ -181,6 +183,11 @@ public:
   ///   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.
   ///
@@ -1923,8 +1930,137 @@ 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

clang/include/clang/AST/TypeProperties.td

@@ -395,6 +395,13 @@ let Class = AutoType in {
   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> {
@@ -406,7 +413,9 @@ let Class = AutoType in {
   def : Creator<[{
     return ctx.getAutoType(makeNullableFromOptional(deducedType), keyword,
                            /*isDependentWithoutDeducedType*/ dependence > 0,
-                           /*isPackWithoutDeducedType*/ dependence > 1);
+                           /*isPackWithoutDeducedType*/ dependence > 1,
+                           makePointerFromOptional(typeConstraintConcept),
+                           typeConstraintArguments);
   }]>;
 }
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1371,6 +1371,8 @@ def err_concept_definition_not_identifier : Error<
 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

@@ -2102,11 +2102,8 @@ def err_auto_not_allowed : Error<
   "|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 template parameter|in friend declaration|in function prototype that is "
-  "|in requires expression parameter}1">;
+  "not a function 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">;
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"
@@ -2655,6 +2652,9 @@ def note_ambiguous_atomic_constraints : Note<
   "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">;
@@ -2669,6 +2669,8 @@ def err_type_constraint_non_type_concept : Error<
 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<

clang/include/clang/Sema/DeclSpec.h

@@ -349,6 +349,7 @@ private:
   unsigned TypeSpecOwned : 1;
   unsigned TypeSpecPipe : 1;
   unsigned TypeSpecSat : 1;
+  unsigned ConstrainedAuto : 1;
 
   // type-qualifiers
   unsigned TypeQualifiers : 5;  // Bitwise OR of TQ.
@@ -369,6 +370,7 @@ private:
     UnionParsedType TypeRep;
     Decl *DeclRep;
     Expr *ExprRep;
+    TemplateIdAnnotation *TemplateIdRep;
   };
 
   /// ExplicitSpecifier - Store information about explicit spicifer.
@@ -413,6 +415,9 @@ private:
   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;
@@ -430,7 +435,8 @@ public:
         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),
@@ -478,6 +484,7 @@ public:
   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");
@@ -491,6 +498,11 @@ public:
     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; }
 
@@ -666,6 +678,9 @@ public:
                        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,
@@ -1831,6 +1846,14 @@ private:
   /// 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
@@ -1861,7 +1884,8 @@ public:
         Redeclaration(false), Extension(false), ObjCIvar(false),
         ObjCWeakProperty(false), InlineStorageUsed(false),
         Attrs(ds.getAttributePool().getFactory()), AsmLabel(nullptr),
-        TrailingRequiresClause(nullptr) {}
+        TrailingRequiresClause(nullptr),
+        InventedTemplateParameterList(nullptr) {}
 
   ~Declarator() {
     clear();
@@ -2429,6 +2453,30 @@ public:
     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.
   ///
@@ -2629,6 +2677,26 @@ struct LambdaIntroducer {
   }
 };
 
+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

@@ -385,6 +385,12 @@ public:
     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;

clang/include/clang/Sema/ScopeInfo.h

@@ -22,6 +22,7 @@
 #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"
@@ -789,7 +790,8 @@ public:
   }
 };
 
-class LambdaScopeInfo final : public CapturingScopeInfo {
+class LambdaScopeInfo final :
+    public CapturingScopeInfo, public InventedTemplateParameterInfo {
 public:
   /// The class that describes the lambda.
   CXXRecordDecl *Lambda = nullptr;
@@ -823,25 +825,9 @@ public:
   /// 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).

clang/include/clang/Sema/Sema.h

@@ -620,6 +620,13 @@ public:
   /// 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;
@@ -1425,6 +1432,11 @@ public:
   /// 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 {
@@ -1519,6 +1531,15 @@ public:
   /// 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);
 
   //===--------------------------------------------------------------------===//
@@ -1922,6 +1943,8 @@ public:
     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 {
@@ -1986,6 +2009,12 @@ public:
       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;
@@ -2011,7 +2040,8 @@ public:
 
     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;
     }
 
@@ -2023,6 +2053,8 @@ public:
         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:
@@ -6890,6 +6922,10 @@ public:
                             TemplateTypeParmDecl *ConstrainedParameter,
                             SourceLocation EllipsisLoc);
 
+  bool AttachTypeConstraint(AutoTypeLoc TL,
+                            NonTypeTemplateParmDecl *ConstrainedParameter,
+                            SourceLocation EllipsisLoc);
+
   QualType CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
                                              SourceLocation Loc);
   QualType CheckNonTypeTemplateParameterType(QualType T, SourceLocation Loc);
@@ -6939,7 +6975,8 @@ public:
       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,
@@ -7851,10 +7888,12 @@ public:
 
   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);

clang/lib/AST/ASTContext.cpp

@@ -716,6 +716,61 @@ ASTContext::CanonicalTemplateTemplateParm::Profile(llvm::FoldingSetNodeID &ID,
     RequiresClause->Profile(ID, C, /*Canonical=*/true);
 }
 
+static Expr *
+canonicalizeImmediatelyDeclaredConstraint(const ASTContext &C, Expr *IDC,
+                                          QualType ConstrainedType) {
+  // 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.
+  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());
+  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(ConstrainedType);
+    for (auto &Arg : OldConverted.front().pack_elements().drop_front(1))
+      NewConverted.push_back(Arg);
+    TemplateArgument NewPack(NewConverted);
+
+    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(ConstrainedType);
+    for (auto &Arg : OldConverted.drop_front(1))
+      NewConverted.push_back(Arg);
+  }
+  Expr *NewIDC = ConceptSpecializationExpr::Create(
+      C, 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);
+
+  if (auto *OrigFold = dyn_cast<CXXFoldExpr>(IDC))
+    NewIDC = new (C) CXXFoldExpr(OrigFold->getType(), SourceLocation(), NewIDC,
+                                 BinaryOperatorKind::BO_LAnd,
+                                 SourceLocation(), /*RHS=*/nullptr,
+                                 SourceLocation(), /*NumExpansions=*/None);
+  return NewIDC;
+}
+
 TemplateTemplateParmDecl *
 ASTContext::getCanonicalTemplateTemplateParmDecl(
                                           TemplateTemplateParmDecl *TTP) const {
@@ -743,68 +798,23 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
           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) {
+        Expr *NewIDC = canonicalizeImmediatelyDeclaredConstraint(
-          // The case:
+                *this, TC->getImmediatelyDeclaredConstraint(),
-          // template<typename... T> concept C = true;
+                ParamAsArgument);
-          // template<C<int> T> struct S; -> constraint is C<{T, int}>
+        TemplateArgumentListInfo CanonArgsAsWritten;
-          NewConverted.push_back(ParamAsArgument);
+        if (auto *Args = TC->getTemplateArgsAsWritten())
-          for (auto &Arg : OldConverted.front().pack_elements().drop_front(1))
+          for (const auto &ArgLoc : Args->arguments())
-            NewConverted.push_back(Arg);
+            CanonArgsAsWritten.addArgument(
-          TemplateArgument NewPack(NewConverted);
+                TemplateArgumentLoc(ArgLoc.getArgument(),
-
+                                    TemplateArgumentLocInfo()));
-          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);
-
-        if (auto *OrigFold = dyn_cast<CXXFoldExpr>(IDC))
-          NewIDC = new (*this) CXXFoldExpr(OrigFold->getType(),
-                                           SourceLocation(), NewIDC,
-                                           BinaryOperatorKind::BO_LAnd,
-                                           SourceLocation(), /*RHS=*/nullptr,
-                                           SourceLocation(),
-                                           /*NumExpansions=*/None);
-
         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)) {
@@ -839,6 +849,13 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
                                                 NTTP->isParameterPack(),
                                                 TInfo);
       }
+      if (AutoType *AT = T->getContainedAutoType()) {
+        if (AT->isConstrained()) {
+          Param->setPlaceholderTypeConstraint(
+              canonicalizeImmediatelyDeclaredConstraint(
+                  *this, NTTP->getPlaceholderTypeConstraint(), T));
+        }
+      }
       CanonParams.push_back(Param);
 
     } else
@@ -943,7 +960,7 @@ ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
                        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)),
@@ -5124,21 +5141,29 @@ QualType ASTContext::getUnaryTransformType(QualType BaseType,
 /// 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,
+QualType
-                                 bool IsDependent, bool IsPack) const {
+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)
+  void *Mem = Allocate(sizeof(AutoType) +
-      AutoType(DeducedType, Keyword, IsDependent, IsPack);
+                       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);
@@ -5200,7 +5225,8 @@ 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;
 }

clang/lib/AST/ASTImporter.cpp

@@ -1366,9 +1366,21 @@ ExpectedType ASTNodeImporter::VisitAutoType(const AutoType *T) {
   if (!ToDeducedTypeOrErr)
     return ToDeducedTypeOrErr.takeError();
 
-  return Importer.getToContext().getAutoType(*ToDeducedTypeOrErr,
+  ExpectedDecl ToTypeConstraintConcept = import(T->getTypeConstraintConcept());
-                                             T->getKeyword(),
+  if (!ToTypeConstraintConcept)
-                                             /*IsDependent*/false);
+    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(

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -729,11 +729,31 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
       return false;
     break;
 
-  case Type::Auto:
+  case Type::Auto: {
-    if (!IsStructurallyEquivalent(Context, cast<AutoType>(T1)->getDeducedType(),
+    auto *Auto1 = cast<AutoType>(T1);
-                                  cast<AutoType>(T2)->getDeducedType()))
+    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);

clang/lib/AST/DeclTemplate.cpp

@@ -164,10 +164,15 @@ static void AdoptTemplateParameterList(TemplateParameterList *Params,
 void TemplateParameterList::
 getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
   if (HasConstrainedParameters)
-    for (const NamedDecl *Param : *this)
+    for (const NamedDecl *Param : *this) {
-      if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
+      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());
 }
@@ -684,8 +689,13 @@ NonTypeTemplateParmDecl::Create(const ASTContext &C, DeclContext *DC,
                                 unsigned D, unsigned P, IdentifierInfo *Id,
                                 QualType T, bool ParameterPack,
                                 TypeSourceInfo *TInfo) {
-  return new (C, DC) NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id,
+  AutoType *AT = TInfo->getType()->getContainedAutoType();
-                                             T, ParameterPack, TInfo);
+  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(
@@ -693,26 +703,34 @@ NonTypeTemplateParmDecl *NonTypeTemplateParmDecl::Create(
     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 *>>(
+              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
-                  ExpandedTypes.size()))
+                                    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) {
+NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID,
-  return new (C, ID) NonTypeTemplateParmDecl(nullptr, SourceLocation(),
+                                            bool HasTypeConstraint) {
-                                             SourceLocation(), 0, 0, nullptr,
+  return new (C, ID, additionalSizeToAlloc<std::pair<QualType,
-                                             QualType(), false, nullptr);
+                                                     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 *>>(
+      new (C, ID, additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
-                      NumExpandedTypes))
+                                        Expr *>(
+                      NumExpandedTypes, HasTypeConstraint ? 1 : 0))
           NonTypeTemplateParmDecl(nullptr, SourceLocation(), SourceLocation(),
                                   0, 0, nullptr, QualType(), nullptr, None,
                                   None);

clang/lib/AST/ODRHash.cpp

@@ -857,6 +857,13 @@ public:
 
   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);
   }
 

clang/lib/AST/TemplateBase.cpp

@@ -561,7 +561,7 @@ const DiagnosticBuilder &clang::operator<<(const DiagnosticBuilder &DB,
 }
 
 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));

clang/lib/AST/TextNodeDumper.cpp

@@ -1201,6 +1201,11 @@ void TextNodeDumper::VisitAutoType(const AutoType *T) {
     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(

clang/lib/AST/Type.cpp

@@ -1114,7 +1114,9 @@ public:
       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++
@@ -4158,3 +4160,35 @@ void clang::FixedPointValueToString(SmallVectorImpl<char> &Str,
                              /*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

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #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"
@@ -589,3 +590,97 @@ void TemplateSpecializationTypeLoc::initializeArgLocs(ASTContext &Context,
     }
   }
 }
+
+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

@@ -1046,6 +1046,13 @@ void TypePrinter::printAutoBefore(const AutoType *T, raw_ostream &OS) {
   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;
@@ -1234,20 +1241,18 @@ void TypePrinter::printEnumAfter(const EnumType *T, raw_ostream &OS) {}
 
 void TypePrinter::printTemplateTypeParmBefore(const TemplateTypeParmType *T,
                                               raw_ostream &OS) {
-  if (IdentifierInfo *Id = T->getIdentifier())
+  TemplateTypeParmDecl *D = T->getDecl();
-    OS << Id->getName();
+  if (D && D->isImplicit()) {
-  else {
+    if (auto *TC = D->getTypeConstraint()) {
-    bool IsLambdaAutoParam = false;
+      TC->print(OS, Policy);
-    if (auto D = T->getDecl()) {
+      OS << ' ';
-      if (auto M = dyn_cast_or_null<CXXMethodDecl>(D->getDeclContext()))
-        IsLambdaAutoParam = D->isImplicit() && M->getParent()->isLambda();
     }
-
-    if (IsLambdaAutoParam)
     OS << "auto";
+  } else if (IdentifierInfo *Id = T->getIdentifier())
+    OS << Id->getName();
   else
     OS << "type-parameter-" << T->getDepth() << '-' << T->getIndex();
-  }
+
   spaceBeforePlaceHolder(OS);
 }
 

clang/lib/Parse/ParseCXXInlineMethods.cpp

@@ -133,7 +133,9 @@ NamedDecl *Parser::ParseCXXInlineMethodDef(
 
   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();

clang/lib/Parse/ParseDecl.cpp

@@ -2962,6 +2962,7 @@ Parser::DiagnoseMissingSemiAfterTagDefinition(DeclSpec &DS, AccessSpecifier AS,
       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;
       }
@@ -3193,6 +3194,18 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
         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.
@@ -3235,6 +3248,10 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // 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);
@@ -3384,6 +3401,10 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // 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()) {
@@ -3433,9 +3454,51 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       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
@@ -6027,11 +6090,12 @@ void Parser::ParseDirectDeclarator(Declarator &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);".
@@ -6050,7 +6114,12 @@ void Parser::ParseDirectDeclarator(Declarator &D) {
       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);

clang/lib/Parse/ParseDeclCXX.cpp

@@ -2642,6 +2642,8 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
   }
 
   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.

clang/lib/Parse/ParseTemplate.cpp

@@ -240,6 +240,8 @@ Decl *Parser::ParseSingleDeclarationAfterTemplate(
 
   // 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()) {
@@ -601,6 +603,7 @@ Parser::TPResult Parser::isStartOfTemplateTypeParameter() {
 ///         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
@@ -618,7 +621,6 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
     }
 
     return ParseTypeParameter(Depth, Position);
-
   case TPResult::False:
     break;
 
@@ -678,7 +680,6 @@ bool Parser::isTypeConstraintAnnotation() {
 bool Parser::TryAnnotateTypeConstraint() {
   if (!getLangOpts().ConceptsTS)
     return false;
-
   CXXScopeSpec SS;
   bool WasScopeAnnotation = Tok.is(tok::annot_cxxscope);
   if (ParseOptionalCXXScopeSpecifier(

clang/lib/Parse/ParseTentative.cpp

@@ -1313,6 +1313,18 @@ public:
 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
@@ -1516,6 +1528,8 @@ Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
       *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;
@@ -1529,6 +1543,13 @@ Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
     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)) {

clang/lib/Parse/Parser.cpp

@@ -1136,6 +1136,7 @@ Decl *Parser::ParseFunctionDefinition(ParsingDeclarator &D,
   // 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
@@ -1262,6 +1263,15 @@ Decl *Parser::ParseFunctionDefinition(ParsingDeclarator &D,
   // 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 '='");
 
@@ -1732,6 +1742,20 @@ Parser::TryAnnotateName(CorrectionCandidateCallback *CCC) {
       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.

clang/lib/Sema/DeclSpec.cpp

@@ -784,6 +784,15 @@ bool DeclSpec::SetTypeSpecType(TST T, SourceLocation TagKwLoc,
   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,

clang/lib/Sema/Sema.cpp

@@ -52,6 +52,21 @@ SourceLocation Sema::getLocForEndOfToken(SourceLocation Loc, unsigned Offset) {
 
 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();

clang/lib/Sema/SemaDecl.cpp

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "TreeTransform.h"
 #include "TypeLocBuilder.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
@@ -1153,6 +1154,10 @@ Corrected:
     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))
@@ -8656,11 +8661,21 @@ static Scope *getTagInjectionScope(Scope *S, const LangOptions &LangOpts) {
 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);
@@ -8740,7 +8755,7 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     // 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 =
+    TemplateParameterList *TemplateParams =
         MatchTemplateParametersToScopeSpecifier(
             D.getDeclSpec().getBeginLoc(), D.getIdentifierLoc(),
             D.getCXXScopeSpec(),
@@ -8748,7 +8763,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
                 ? D.getName().TemplateId
                 : nullptr,
             TemplateParamLists, isFriend, isMemberSpecialization,
-                Invalid)) {
+            Invalid);
+    if (TemplateParams) {
       if (TemplateParams->size() > 0) {
         // This is a function template
 
@@ -8781,7 +8797,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
         // 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.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -17386,3 +17386,50 @@ MSPropertyDecl *Sema::HandleMSProperty(Scope *S, RecordDecl *Record,
 
   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

@@ -791,7 +791,8 @@ QualType Sema::buildLambdaInitCaptureInitialization(
   // 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");

clang/lib/Sema/SemaTemplate.cpp

@@ -1088,6 +1088,50 @@ bool Sema::ActOnTypeConstraint(const CXXScopeSpec &SS,
       ConstrainedParameter, EllipsisLoc);
 }
 
+template<typename ArgumentLocAppender>
+static ExprResult formImmediatelyDeclaredConstraint(
+    Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
+    ConceptDecl *NamedConcept, SourceLocation LAngleLoc,
+    SourceLocation RAngleLoc, QualType ConstrainedType,
+    SourceLocation ParamNameLoc, ArgumentLocAppender Appender,
+    SourceLocation EllipsisLoc) {
+
+  TemplateArgumentListInfo ConstraintArgs;
+  ConstraintArgs.addArgument(
+    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 = S.CheckConceptTemplateId(
+      SS, /*TemplateKWLoc=*/SourceLocation(), NameInfo,
+      /*FoundDecl=*/NamedConcept, NamedConcept, &ConstraintArgs);
+  if (ImmediatelyDeclaredConstraint.isInvalid() || !EllipsisLoc.isValid())
+    return ImmediatelyDeclaredConstraint;
+
+  // 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> && ...)
+  return S.BuildCXXFoldExpr(/*LParenLoc=*/SourceLocation(),
+                            ImmediatelyDeclaredConstraint.get(), BO_LAnd,
+                            EllipsisLoc, /*RHS=*/nullptr,
+                            /*RParenLoc=*/SourceLocation(),
+                            /*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
@@ -1106,51 +1150,21 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
                                                        *TemplateArgs) : nullptr;
 
   QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
-  TemplateArgumentListInfo ConstraintArgs;
+
-  ConstraintArgs.addArgument(
+  ExprResult ImmediatelyDeclaredConstraint =
-    TemplateArgumentLoc(
+      formImmediatelyDeclaredConstraint(
-        TemplateArgument(ParamAsArgument),
+          *this, NS, NameInfo, NamedConcept,
-        TemplateArgumentLocInfo(
+          TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
-            Context.getTrivialTypeSourceInfo(ParamAsArgument,
+          TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
-                ConstrainedParameter->getLocation()))));
+          ParamAsArgument, ConstrainedParameter->getLocation(),
-  if (TemplateArgs) {
+          [&] (TemplateArgumentListInfo &ConstraintArgs) {
-    ConstraintArgs.setRAngleLoc(TemplateArgs->getRAngleLoc());
+            if (TemplateArgs)
-    ConstraintArgs.setLAngleLoc(TemplateArgs->getLAngleLoc());
+              for (const auto &ArgLoc : TemplateArgs->arguments())
-    for (const TemplateArgumentLoc &ArgLoc : TemplateArgs->arguments())
                 ConstraintArgs.addArgument(ArgLoc);
-  }
+          }, EllipsisLoc);
-
-  // 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;
 
-  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(),
-                         ImmediatelyDeclaredConstraint.get(), BO_LAnd,
-                         EllipsisLoc, /*RHS=*/nullptr,
-                         /*RParenLoc=*/SourceLocation(),
-                         /*NumExpansions=*/None).get();
-    if (ImmediatelyDeclaredConstraint.isInvalid())
-      return true;
-  }
-
   ConstrainedParameter->setTypeConstraint(NS, NameInfo,
                                           /*FoundDecl=*/NamedConcept,
                                           NamedConcept, ArgsAsWritten,
@@ -1158,6 +1172,38 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
   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.
 ///
@@ -1319,6 +1365,11 @@ NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
       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();
 
@@ -2762,7 +2813,7 @@ 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;
 
@@ -2870,6 +2921,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
 
   auto CheckExplicitSpecialization = [&](SourceRange Range, bool Recovery) {
     if (SawNonEmptyTemplateParameterList) {
+      if (!SuppressDiagnostic)
         Diag(DeclLoc, diag::err_specialize_member_of_template)
           << !Recovery << Range;
       Invalid = true;
@@ -2892,6 +2944,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
     else
       ExpectedTemplateLoc = DeclStartLoc;
 
+    if (!SuppressDiagnostic)
       Diag(DeclLoc, diag::err_template_spec_needs_header)
         << Range
         << FixItHint::CreateInsertion(ExpectedTemplateLoc, "template<> ");
@@ -2984,6 +3037,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
       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
@@ -3025,7 +3079,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
         if (ExpectedTemplateParams &&
             !TemplateParameterListsAreEqual(ParamLists[ParamIdx],
                                             ExpectedTemplateParams,
-                                            true, TPL_TemplateMatch))
+                                            !SuppressDiagnostic, TPL_TemplateMatch))
           Invalid = true;
 
         if (!Invalid &&
@@ -3037,6 +3091,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
         continue;
       }
 
+      if (!SuppressDiagnostic)
         Diag(DeclLoc, diag::err_template_spec_needs_template_parameters)
           << T
           << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
@@ -3075,6 +3130,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
         AllExplicitSpecHeaders = false;
     }
 
+    if (!SuppressDiagnostic)
       Diag(ParamLists[ParamIdx]->getTemplateLoc(),
            AllExplicitSpecHeaders ? diag::warn_template_spec_extra_headers
                                   : diag::err_template_spec_extra_headers)
@@ -3084,7 +3140,8 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
     // 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();
@@ -6530,7 +6587,12 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
       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()

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -4414,9 +4414,10 @@ namespace {
 
       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;
     }
 
@@ -4451,9 +4452,10 @@ 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
@@ -4481,6 +4483,49 @@ static bool diagnoseAutoDeductionFailure(Sema &S,
   }
 }
 
+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
@@ -4495,9 +4540,12 @@ static bool diagnoseAutoDeductionFailure(Sema &S,
 ///        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())
@@ -4538,6 +4586,14 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
         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;
@@ -4645,6 +4701,17 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
       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;

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2685,6 +2685,16 @@ Decl *TemplateDeclInstantiator::VisitNonTypeTemplateParmDecl(
         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)

clang/lib/Sema/SemaType.cpp

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "TypeLocBuilder.h"
+#include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTMutationListener.h"
@@ -27,6 +28,7 @@
 #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"
@@ -1251,6 +1253,26 @@ getImageAccess(const ParsedAttributesView &Attrs) {
   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
@@ -1595,6 +1617,11 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
     break;
 
   case DeclSpec::TST_auto:
+    if (DS.isConstrainedAuto()) {
+      Result = ConvertConstrainedAutoDeclSpecToType(S, DS,
+                                                    AutoTypeKeyword::Auto);
+      break;
+    }
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::Auto, false);
     break;
 
@@ -1603,6 +1630,12 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
     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;
@@ -2921,6 +2954,87 @@ static void diagnoseRedundantReturnTypeQualifiers(Sema &S, QualType RetTy,
                               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();
@@ -2991,46 +3105,43 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
       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()) {
-      // In C++14, generic lambdas allow 'auto' in their parameters.
+          // This might be OK, but we'll need to convert the trailing return
-      if (!SemaRef.getLangOpts().CPlusPlus14 ||
+          // type later.
-          !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.InventedParameterInfos.back();
+      } else {
+        // In C++14, generic lambdas allow 'auto' in their parameters.
+        if (!SemaRef.getLangOpts().CPlusPlus14 || !Auto ||
+            Auto->getKeyword() != AutoTypeKeyword::Auto) {
+          Error = 16;
+          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())
@@ -4032,10 +4143,6 @@ static bool DiagnoseMultipleAddrSpaceAttributes(Sema &S, LangAS ASOld,
   return false;
 }
 
-static TypeSourceInfo *
-GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
-                               QualType T, TypeSourceInfo *ReturnTypeInfo);
-
 static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
                                                 QualType declSpecType,
                                                 TypeSourceInfo *TInfo) {
@@ -4611,7 +4718,8 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
           } 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();
@@ -4622,7 +4730,12 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
             // 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.
@@ -5242,7 +5355,8 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
       //
       // 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();
@@ -5450,14 +5564,15 @@ static void fillAttributedTypeLoc(AttributedTypeLoc TL,
 
 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());
@@ -5585,6 +5700,32 @@ namespace {
       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());
     }
@@ -5854,7 +5995,7 @@ GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
     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;

clang/lib/Sema/TreeTransform.h

@@ -951,12 +951,16 @@ public:
   /// 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
@@ -4500,7 +4504,10 @@ QualType TreeTransform<Derived>::RebuildQualifiedType(QualType T,
         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.
@@ -5233,13 +5240,13 @@ bool TreeTransform<Derived>::TransformFunctionTypeParams(
         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 =
+        Optional<unsigned> OrigNumExpansions;
-            ExpansionTL.getTypePtr()->getNumExpansions();
+        if (Unexpanded.size() > 0) {
+          OrigNumExpansions = ExpansionTL.getTypePtr()->getNumExpansions();
           NumExpansions = OrigNumExpansions;
           if (getDerived().TryExpandParameterPacks(ExpansionTL.getEllipsisLoc(),
                                                    Pattern.getSourceRange(),
@@ -5249,6 +5256,14 @@ bool TreeTransform<Derived>::TransformFunctionTypeParams(
                                                    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
@@ -5307,6 +5322,9 @@ bool TreeTransform<Derived>::TransformFunctionTypeParams(
                                                           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);
@@ -5811,32 +5829,6 @@ QualType TreeTransform<Derived>::TransformUnaryTransformType(
   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) {
@@ -6098,6 +6090,71 @@ QualType TreeTransform<Derived>::TransformPipeType(TypeLocBuilder &TLB,
     }
   };
 
+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(

clang/lib/Serialization/ASTReader.cpp

@@ -6576,6 +6576,17 @@ void TypeLocReader::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
 
 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(

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2317,12 +2317,12 @@ void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *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,
@@ -2340,6 +2340,8 @@ void ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *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 *>>();
@@ -3823,13 +3825,19 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
                                                  HasTypeConstraint);
     break;
   }
-  case DECL_NON_TYPE_TEMPLATE_PARM:
+  case DECL_NON_TYPE_TEMPLATE_PARM: {
-    D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID);
+    bool HasTypeConstraint = Record.readInt();
-    break;
-  case DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK:
     D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID,
-                                                    Record.readInt());
+                                                    HasTypeConstraint);
     break;
+  }
+  case DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK: {
+    bool HasTypeConstraint = Record.readInt();
+    D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID,
+                                                    Record.readInt(),
+                                                    HasTypeConstraint);
+    break;
+  }
   case DECL_TEMPLATE_TEMPLATE_PARM:
     D = TemplateTemplateParmDecl::CreateDeserialized(Context, ID);
     break;

clang/lib/Serialization/ASTWriter.cpp

@@ -349,6 +349,18 @@ void TypeLocWriter::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
 
 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(

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1675,6 +1675,8 @@ 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());
 
@@ -1682,6 +1684,8 @@ void ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *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) {

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

@@ -417,15 +417,24 @@ struct DoesNotAllowConstDefaultInit {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "TemplateTypeParmDecl",
 // CHECK-NEXT:      "loc": {
+// 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:      "range": {
+// CHECK-NEXT:        "end": {
-// CHECK-NEXT:       "begin": {},
+// CHECK-NEXT:          "offset": 197,
-// CHECK-NEXT:       "end": {}
+// 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
@@ -524,15 +533,24 @@ struct DoesNotAllowConstDefaultInit {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "TemplateTypeParmDecl",
 // CHECK-NEXT:      "loc": {
+// 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:      "range": {
+// CHECK-NEXT:        "end": {
-// CHECK-NEXT:       "begin": {},
+// CHECK-NEXT:          "offset": 197,
-// CHECK-NEXT:       "end": {}
+// 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
@@ -590,15 +608,24 @@ struct DoesNotAllowConstDefaultInit {
 // CHECK-NEXT:      "id": "0x{{.*}}",
 // CHECK-NEXT:      "kind": "TemplateTypeParmDecl",
 // CHECK-NEXT:      "loc": {
+// 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:      "range": {
+// CHECK-NEXT:        "end": {
-// CHECK-NEXT:       "begin": {},
+// CHECK-NEXT:          "offset": 197,
-// CHECK-NEXT:       "end": {}
+// 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

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

@@ -0,0 +1,260 @@
+// 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

@@ -0,0 +1,44 @@
+// 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; }

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

@@ -1,7 +1,7 @@
 // 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');
@@ -9,8 +9,8 @@ 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@-1{{candidate template ignored: constraints not satisfied [with x:auto = <char>]}}
-// expected-note@-2{{candidate template ignored: constraints not satisfied [with $0 = <int, 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');

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

@@ -94,6 +94,8 @@ concept OneOf = (is_same_v<T, Ts> || ...);
 // 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}}
@@ -115,3 +117,24 @@ using h1 = H<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]>;
+
+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

@@ -0,0 +1,26 @@
+// 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}}
+}

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

@@ -181,7 +181,7 @@ int test() {
     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}}
 

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

@@ -93,7 +93,8 @@ namespace test_undeclared_nontype_parm_arg {
 // 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;