[Concepts] Type Constraints

Add support for type-constraints in template type parameters.
Also add support for template type parameters as pack expansions (where the type constraint can now contain an unexpanded parameter pack).

Differential Revision: https://reviews.llvm.org/D44352

40 files changed, 1566 insertions, 315 deletions

diff --git a/clang/include/clang/AST/ASTConcept.h b/clang/include/clang/AST/ASTConcept.h
index 937a8a9b345..896d857d8c9 100644
--- a/clang/include/clang/AST/ASTConcept.h
+++ b/clang/include/clang/AST/ASTConcept.h
@@ -21,6 +21,7 @@
 #include <string>
 #include <utility>
 namespace clang {
+class ConceptDecl;
 
 /// \brief The result of a constraint satisfaction check, containing the
 /// necessary information to diagnose an unsatisfied constraint.
@@ -75,6 +76,102 @@ struct ASTConstraintSatisfaction final :
   Create(const ASTContext &C, const ConstraintSatisfaction &Satisfaction);
 };
 
+/// \brief Common data class for constructs that reference concepts with
+/// template arguments.
+class ConceptReference {
+protected:
+  // \brief The optional nested name specifier used when naming the concept.
+  NestedNameSpecifierLoc NestedNameSpec;
+
+  /// \brief The location of the template keyword, if specified when naming the
+  /// concept.
+  SourceLocation TemplateKWLoc;
+
+  /// \brief The concept name used.
+  DeclarationNameInfo ConceptName;
+
+  /// \brief The declaration found by name lookup when the expression was
+  /// created.
+  /// Can differ from NamedConcept when, for example, the concept was found
+  /// through a UsingShadowDecl.
+  NamedDecl *FoundDecl;
+
+  /// \brief The concept named.
+  ConceptDecl *NamedConcept;
+
+  /// \brief The template argument list source info used to specialize the
+  /// concept.
+  const ASTTemplateArgumentListInfo *ArgsAsWritten;
+
+public:
+
+  ConceptReference(NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
+                   DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
+                   ConceptDecl *NamedConcept,
+                   const ASTTemplateArgumentListInfo *ArgsAsWritten) :
+      NestedNameSpec(NNS), TemplateKWLoc(TemplateKWLoc),
+      ConceptName(ConceptNameInfo), FoundDecl(FoundDecl),
+      NamedConcept(NamedConcept), ArgsAsWritten(ArgsAsWritten) {}
+
+  ConceptReference() : NestedNameSpec(), TemplateKWLoc(), ConceptName(),
+      FoundDecl(nullptr), NamedConcept(nullptr), ArgsAsWritten(nullptr) {}
+
+  const NestedNameSpecifierLoc &getNestedNameSpecifierLoc() const {
+    return NestedNameSpec;
+  }
+
+  const DeclarationNameInfo &getConceptNameInfo() const { return ConceptName; }
+
+  SourceLocation getConceptNameLoc() const {
+    return getConceptNameInfo().getLoc();
+  }
+
+  SourceLocation getTemplateKWLoc() const { return TemplateKWLoc; }
+
+  NamedDecl *getFoundDecl() const {
+    return FoundDecl;
+  }
+
+  ConceptDecl *getNamedConcept() const {
+    return NamedConcept;
+  }
+
+  const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
+    return ArgsAsWritten;
+  }
+
+  /// \brief Whether or not template arguments were explicitly specified in the
+  /// concept reference (they might not be in type constraints, for example)
+  bool hasExplicitTemplateArgs() const {
+    return ArgsAsWritten != nullptr;
+  }
+};
+
+class TypeConstraint : public ConceptReference {
+  /// \brief The immediately-declared constraint expression introduced by this
+  /// type-constraint.
+  Expr *ImmediatelyDeclaredConstraint = nullptr;
+
+public:
+  TypeConstraint(NestedNameSpecifierLoc NNS,
+                 DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
+                 ConceptDecl *NamedConcept,
+                 const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                 Expr *ImmediatelyDeclaredConstraint) :
+      ConceptReference(NNS, /*TemplateKWLoc=*/SourceLocation(), ConceptNameInfo,
+                       FoundDecl, NamedConcept, ArgsAsWritten),
+      ImmediatelyDeclaredConstraint(ImmediatelyDeclaredConstraint) {}
+
+  /// \brief Get the immediately-declared constraint expression introduced by
+  /// this type-constraint, that is - the constraint expression that is added to
+  /// the associated constraints of the enclosing declaration in practice.
+  Expr *getImmediatelyDeclaredConstraint() const {
+    return ImmediatelyDeclaredConstraint;
+  }
+
+  void print(llvm::raw_ostream &OS, PrintingPolicy Policy) const;
+};
+
 } // clang
 
 #endif // LLVM_CLANG_AST_ASTCONCEPT_H
\ No newline at end of file
diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index d5ade9340c8..fb269cef1ce 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -283,12 +283,16 @@ class ASTContext : public RefCountedBase<ASTContext> {
 
     TemplateTemplateParmDecl *getParam() const { return Parm; }
 
-    void Profile(llvm::FoldingSetNodeID &ID) { Profile(ID, Parm); }
+    void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &C) {
+      Profile(ID, C, Parm);
+    }
 
     static void Profile(llvm::FoldingSetNodeID &ID,
+                        const ASTContext &C,
                         TemplateTemplateParmDecl *Parm);
   };
-  mutable llvm::FoldingSet<CanonicalTemplateTemplateParm>
+  mutable llvm::ContextualFoldingSet<CanonicalTemplateTemplateParm,
+                                     const ASTContext&>
     CanonTemplateTemplateParms;
 
   TemplateTemplateParmDecl *
diff --git a/clang/include/clang/AST/ASTNodeTraverser.h b/clang/include/clang/AST/ASTNodeTraverser.h
index a672d92695d..e0ebb020e69 100644
--- a/clang/include/clang/AST/ASTNodeTraverser.h
+++ b/clang/include/clang/AST/ASTNodeTraverser.h
@@ -537,6 +537,10 @@ public:
   }
 
   void VisitTemplateTypeParmDecl(const TemplateTypeParmDecl *D) {
+    if (const auto *TC = D->getTypeConstraint())
+      if (TC->hasExplicitTemplateArgs())
+        for (const auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
+          dumpTemplateArgumentLoc(ArgLoc);
     if (D->hasDefaultArgument())
       Visit(D->getDefaultArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
@@ -544,6 +548,8 @@ public:
   }
 
   void VisitNonTypeTemplateParmDecl(const NonTypeTemplateParmDecl *D) {
+    if (const auto *TC = D->getPlaceholderTypeConstraint())
+      Visit(TC->getImmediatelyDeclaredConstraint());
     if (D->hasDefaultArgument())
       Visit(D->getDefaultArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index 23905d12c8b..7a55d04a0f3 100755
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_AST_DECLTEMPLATE_H
 #define LLVM_CLANG_AST_DECLTEMPLATE_H
 
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -51,6 +52,7 @@ class NonTypeTemplateParmDecl;
 class TemplateDecl;
 class TemplateTemplateParmDecl;
 class TemplateTypeParmDecl;
+class ConceptDecl;
 class UnresolvedSetImpl;
 class VarTemplateDecl;
 class VarTemplatePartialSpecializationDecl;
@@ -81,20 +83,24 @@ class TemplateParameterList final
   /// pack.
   unsigned ContainsUnexpandedParameterPack : 1;
 
-  /// Whether this template parameter list has an associated requires-clause
+  /// Whether this template parameter list has a requires clause.
   unsigned HasRequiresClause : 1;
 
+  /// Whether any of the template parameters has constrained-parameter
+  /// constraint-expression.
+  unsigned HasConstrainedParameters : 1;
+
 protected:
-  TemplateParameterList(SourceLocation TemplateLoc, SourceLocation LAngleLoc,
-                        ArrayRef<NamedDecl *> Params, SourceLocation RAngleLoc,
-                        Expr *RequiresClause);
+  TemplateParameterList(const ASTContext& C, SourceLocation TemplateLoc,
+                        SourceLocation LAngleLoc, ArrayRef<NamedDecl *> Params,
+                        SourceLocation RAngleLoc, Expr *RequiresClause);
 
   size_t numTrailingObjects(OverloadToken<NamedDecl *>) const {
     return NumParams;
   }
 
   size_t numTrailingObjects(OverloadToken<Expr *>) const {
-    return HasRequiresClause;
+    return HasRequiresClause ? 1 : 0;
   }
 
 public:
@@ -158,14 +164,22 @@ public:
     return ContainsUnexpandedParameterPack;
   }
 
+  /// Determine whether this template parameter list contains a parameter pack.
+  bool hasParameterPack() const {
+    for (const NamedDecl *P : asArray())
+      if (P->isParameterPack())
+        return true;
+    return false;
+  }
+
   /// The constraint-expression of the associated requires-clause.
   Expr *getRequiresClause() {
-    return HasRequiresClause ? *getTrailingObjects<Expr *>() : nullptr;
+    return HasRequiresClause ? getTrailingObjects<Expr *>()[0] : nullptr;
   }
 
   /// The constraint-expression of the associated requires-clause.
   const Expr *getRequiresClause() const {
-    return HasRequiresClause ? *getTrailingObjects<Expr *>() : nullptr;
+    return HasRequiresClause ? getTrailingObjects<Expr *>()[0] : nullptr;
   }
 
   /// \brief All associated constraints derived from this template parameter
@@ -208,15 +222,16 @@ class FixedSizeTemplateParameterListStorage
       >::type storage;
 
 public:
-  FixedSizeTemplateParameterListStorage(SourceLocation TemplateLoc,
+  FixedSizeTemplateParameterListStorage(const ASTContext &C,
+                                        SourceLocation TemplateLoc,
                                         SourceLocation LAngleLoc,
                                         ArrayRef<NamedDecl *> Params,
                                         SourceLocation RAngleLoc,
                                         Expr *RequiresClause)
       : FixedSizeStorageOwner(
             (assert(N == Params.size()),
-             assert(HasRequiresClause == static_cast<bool>(RequiresClause)),
-             new (static_cast<void *>(&storage)) TemplateParameterList(
+             assert(HasRequiresClause == (RequiresClause != nullptr)),
+             new (static_cast<void *>(&storage)) TemplateParameterList(C,
                  TemplateLoc, LAngleLoc, Params, RAngleLoc, RequiresClause))) {}
 };
 
@@ -1148,9 +1163,12 @@ public:
 /// \code
 /// template<typename T> class vector;
 /// \endcode
-class TemplateTypeParmDecl : public TypeDecl {
+class TemplateTypeParmDecl final : public TypeDecl,
+    private llvm::TrailingObjects<TemplateTypeParmDecl, TypeConstraint> {
   /// Sema creates these on the stack during auto type deduction.
   friend class Sema;
+  friend TrailingObjects;
+  friend class ASTDeclReader;
 
   /// Whether this template type parameter was declaration with
   /// the 'typename' keyword.
@@ -1158,6 +1176,22 @@ class TemplateTypeParmDecl : public TypeDecl {
   /// If false, it was declared with the 'class' keyword.
   bool Typename : 1;
 
+  /// Whether this template type parameter has a type-constraint construct.
+  bool HasTypeConstraint : 1;
+
+  /// Whether the type constraint has been initialized. This can be false if the
+  /// constraint was not initialized yet or if there was an error forming the
+  /// type constriant.
+  bool TypeConstraintInitialized : 1;
+
+  /// Whether this non-type template parameter is an "expanded"
+  /// parameter pack, meaning that its type is a pack expansion and we
+  /// already know the set of types that expansion expands to.
+  bool ExpandedParameterPack : 1;
+
+  /// The number of type parameters in an expanded parameter pack.
+  unsigned NumExpanded = 0;
+
   /// The default template argument, if any.
   using DefArgStorage =
       DefaultArgStorage<TemplateTypeParmDecl, TypeSourceInfo *>;
@@ -1165,8 +1199,12 @@ class TemplateTypeParmDecl : public TypeDecl {
 
   TemplateTypeParmDecl(DeclContext *DC, SourceLocation KeyLoc,
                        SourceLocation IdLoc, IdentifierInfo *Id,
-                       bool Typename)
-      : TypeDecl(TemplateTypeParm, DC, IdLoc, Id, KeyLoc), Typename(Typename) {}
+                       bool Typename, bool HasTypeConstraint,
+                       Optional<unsigned> NumExpanded)
+      : TypeDecl(TemplateTypeParm, DC, IdLoc, Id, KeyLoc), Typename(Typename),
+      HasTypeConstraint(HasTypeConstraint), TypeConstraintInitialized(false),
+      ExpandedParameterPack(NumExpanded),
+      NumExpanded(NumExpanded ? *NumExpanded : 0) {}
 
 public:
   static TemplateTypeParmDecl *Create(const ASTContext &C, DeclContext *DC,
@@ -1174,15 +1212,24 @@ public:
                                       SourceLocation NameLoc,
                                       unsigned D, unsigned P,
                                       IdentifierInfo *Id, bool Typename,
-                                      bool ParameterPack);
+                                      bool ParameterPack,
+                                      bool HasTypeConstraint = false,
+                                      Optional<unsigned> NumExpanded = None);
+
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
                                                   unsigned ID);
+  static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
+                                                  unsigned ID,
+                                                  bool HasTypeConstraint);
 
   /// Whether this template type parameter was declared with
   /// the 'typename' keyword.
   ///
-  /// If not, it was declared with the 'class' keyword.
-  bool wasDeclaredWithTypename() const { return Typename; }
+  /// If not, it was either declared with the 'class' keyword or with a
+  /// type-constraint (see hasTypeConstraint()).
+  bool wasDeclaredWithTypename() const {
+    return Typename && !HasTypeConstraint;
+  }
 
   const DefArgStorage &getDefaultArgStorage() const { return DefaultArgument; }
 
@@ -1239,6 +1286,78 @@ public:
   /// Returns whether this is a parameter pack.
   bool isParameterPack() const;
 
+  /// Whether this parameter pack is a pack expansion.
+  ///
+  /// A template type template parameter pack can be a pack expansion if its
+  /// type-constraint contains an unexpanded parameter pack.
+  bool isPackExpansion() const {
+    if (!isParameterPack())
+      return false;
+    if (const TypeConstraint *TC = getTypeConstraint())
+      if (TC->hasExplicitTemplateArgs())
+        for (const auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
+          if (ArgLoc.getArgument().containsUnexpandedParameterPack())
+            return true;
+    return false;
+  }
+
+  /// Whether this parameter is a template type parameter pack that has a known
+  /// list of different type-constraints at different positions.
+  ///
+  /// A parameter pack is an expanded parameter pack when the original
+  /// parameter pack's type-constraint was itself a pack expansion, and that
+  /// expansion has already been expanded. For example, given:
+  ///
+  /// \code
+  /// template<typename ...Types>
+  /// struct X {
+  ///   template<convertible_to<Types> ...Convertibles>
+  ///   struct Y { /* ... */ };
+  /// };
+  /// \endcode
+  ///
+  /// The parameter pack \c Convertibles has (convertible_to<Types> && ...) as
+  /// its type-constraint. When \c Types is supplied with template arguments by
+  /// instantiating \c X, the instantiation of \c Convertibles becomes an
+  /// expanded parameter pack. For example, instantiating
+  /// \c X<int, unsigned int> results in \c Convertibles being an expanded
+  /// parameter pack of size 2 (use getNumExpansionTypes() to get this number).
+  bool isExpandedParameterPack() const { return ExpandedParameterPack; }
+
+  /// Retrieves the number of parameters in an expanded parameter pack.
+  unsigned getNumExpansionParameters() const {
+    assert(ExpandedParameterPack && "Not an expansion parameter pack");
+    return NumExpanded;
+  }
+
+  /// Returns the type constraint associated with this template parameter (if
+  /// any).
+  const TypeConstraint *getTypeConstraint() const {
+    return TypeConstraintInitialized ? getTrailingObjects<TypeConstraint>() :
+         nullptr;
+  }
+
+  void setTypeConstraint(NestedNameSpecifierLoc NNS,
+                         DeclarationNameInfo NameInfo, NamedDecl *FoundDecl,
+                         ConceptDecl *CD,
+                         const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                         Expr *ImmediatelyDeclaredConstraint);
+
+  /// Determine whether this template parameter has a type-constraint.
+  bool hasTypeConstraint() const {
+    return HasTypeConstraint;
+  }
+
+  /// \brief Get the associated-constraints of this template parameter.
+  /// This will either be the immediately-introduced constraint or empty.
+  ///
+  /// Use this instead of getConstraintExpression for concepts APIs that
+  /// accept an ArrayRef of constraint expressions.
+  void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
+    if (HasTypeConstraint)
+      AC.push_back(getTypeConstraint()->getImmediatelyDeclaredConstraint());
+  }
+
   SourceRange getSourceRange() const override LLVM_READONLY;
 
   // Implement isa/cast/dyncast/etc.
@@ -1424,6 +1543,33 @@ public:
     return TypesAndInfos[I].second;
   }
 
+  /// Return the type-constraint in the placeholder type of this non-type
+  /// template parameter (if any).
+  TypeConstraint *getPlaceholderTypeConstraint() const {
+    // TODO: Concepts: Implement once we have actual placeholders with type
+    //                 constraints.
+    return nullptr;
+  }
+
+  /// Determine whether this non-type template parameter's type has a
+  /// placeholder with a type-constraint.
+  bool hasPlaceholderTypeConstraint() const {
+    // TODO: Concepts: Implement once we have actual placeholders with type
+    //                 constraints.
+    return false;
+  }
+
+  /// \brief Get the associated-constraints of this template parameter.
+  /// This will either be a vector of size 1 containing the immediately-declared
+  /// constraint introduced by the placeholder type, or an empty vector.
+  ///
+  /// Use this instead of getPlaceholderImmediatelyDeclaredConstraint for
+  /// concepts APIs that accept an ArrayRef of constraint expressions.
+  void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
+    if (TypeConstraint *TC = getPlaceholderTypeConstraint())
+      AC.push_back(TC->getImmediatelyDeclaredConstraint());
+  }
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == NonTypeTemplateParm; }
@@ -3086,6 +3232,10 @@ public:
                        ConstraintExpr->getEndLoc());
   }
 
+  bool isTypeConcept() const {
+    return isa<TemplateTypeParmDecl>(getTemplateParameters()->getParam(0));
+  }
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == Concept; }
diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index 1a16aa7aace..2c29409e0ca 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -4841,7 +4841,7 @@ public:
 ///
 /// According to C++2a [expr.prim.id]p3 an id-expression that denotes the
 /// specialization of a concept results in a prvalue of type bool.
-class ConceptSpecializationExpr final : public Expr,
+class ConceptSpecializationExpr final : public Expr, public ConceptReference,
       private llvm::TrailingObjects<ConceptSpecializationExpr,
                                     TemplateArgument> {
   friend class ASTStmtReader;
@@ -4850,30 +4850,6 @@ public:
   using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
 
 protected:
-
-  // \brief The optional nested name specifier used when naming the concept.
-  NestedNameSpecifierLoc NestedNameSpec;
-
-  /// \brief The location of the template keyword, if specified when naming the
-  /// concept.
-  SourceLocation TemplateKWLoc;
-
-  /// \brief The location of the concept name in the expression.
-  SourceLocation ConceptNameLoc;
-
-  /// \brief The declaration found by name lookup when the expression was
-  /// created.
-  /// Can differ from NamedConcept when, for example, the concept was found
-  /// through a UsingShadowDecl.
-  NamedDecl *FoundDecl;
-
-  /// \brief The concept named.
-  ConceptDecl *NamedConcept;
-
-  /// \brief The template argument list source info used to specialize the
-  /// concept.
-  const ASTTemplateArgumentListInfo *ArgsAsWritten = nullptr;
-
   /// \brief The number of template arguments in the tail-allocated list of
   /// converted template arguments.
   unsigned NumTemplateArgs;
@@ -4883,10 +4859,10 @@ protected:
   /// ignored.
   ASTConstraintSatisfaction *Satisfaction;
 
-  ConceptSpecializationExpr(ASTContext &C, NestedNameSpecifierLoc NNS,
+  ConceptSpecializationExpr(const ASTContext &C, NestedNameSpecifierLoc NNS,
                             SourceLocation TemplateKWLoc,
-                            SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
-                            ConceptDecl *NamedConcept,
+                            DeclarationNameInfo ConceptNameInfo,
+                            NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
                             const ASTTemplateArgumentListInfo *ArgsAsWritten,
                             ArrayRef<TemplateArgument> ConvertedArgs,
                             const ConstraintSatisfaction *Satisfaction);
@@ -4896,8 +4872,8 @@ protected:
 public:
 
   static ConceptSpecializationExpr *
-  Create(ASTContext &C, NestedNameSpecifierLoc NNS,
-         SourceLocation TemplateKWLoc, SourceLocation ConceptNameLoc,
+  Create(const ASTContext &C, NestedNameSpecifierLoc NNS,
+         SourceLocation TemplateKWLoc, DeclarationNameInfo ConceptNameInfo,
          NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
          const ASTTemplateArgumentListInfo *ArgsAsWritten,
          ArrayRef<TemplateArgument> ConvertedArgs,
@@ -4906,30 +4882,13 @@ public:
   static ConceptSpecializationExpr *
   Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
 
-  const NestedNameSpecifierLoc &getNestedNameSpecifierLoc() const {
-    return NestedNameSpec;
-  }
-
-  NamedDecl *getFoundDecl() const {
-    return FoundDecl;
-  }
-
-  ConceptDecl *getNamedConcept() const {
-    return NamedConcept;
-  }
-
   ArrayRef<TemplateArgument> getTemplateArguments() const {
     return ArrayRef<TemplateArgument>(getTrailingObjects<TemplateArgument>(),
                                       NumTemplateArgs);
   }
 
-  const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
-    return ArgsAsWritten;
-  }
-
   /// \brief Set new template arguments for this concept specialization.
-  void setTemplateArguments(const ASTTemplateArgumentListInfo *ArgsAsWritten,
-                            ArrayRef<TemplateArgument> Converted);
+  void setTemplateArguments(ArrayRef<TemplateArgument> Converted);
 
   /// \brief Whether or not the concept with the given arguments was satisfied
   /// when the expression was created.
@@ -4949,15 +4908,14 @@ public:
     return *Satisfaction;
   }
 
-  SourceLocation getConceptNameLoc() const { return ConceptNameLoc; }
-
-  SourceLocation getTemplateKWLoc() const { return TemplateKWLoc; }
-
   static bool classof(const Stmt *T) {
     return T->getStmtClass() == ConceptSpecializationExprClass;
   }
 
-  SourceLocation getBeginLoc() const LLVM_READONLY { return ConceptNameLoc; }
+  SourceLocation getBeginLoc() const LLVM_READONLY {
+    return ConceptName.getBeginLoc();
+  }
+
   SourceLocation getEndLoc() const LLVM_READONLY {
     return ArgsAsWritten->RAngleLoc;
   }
diff --git a/clang/include/clang/AST/RecursiveASTVisitor.h b/clang/include/clang/AST/RecursiveASTVisitor.h
index 144ef221d69..f8ab8e451d8 100644
--- a/clang/include/clang/AST/RecursiveASTVisitor.h
+++ b/clang/include/clang/AST/RecursiveASTVisitor.h
@@ -304,6 +304,11 @@ public:
   bool TraverseSynOrSemInitListExpr(InitListExpr *S,
                                     DataRecursionQueue *Queue = nullptr);
 
+  /// Recursively visit a reference to a concept with potential arguments.
+  ///
+  /// \returns false if the visitation was terminated early, true otherwise.
+  bool TraverseConceptReference(const ConceptReference &C);
+
   // ---- Methods on Attrs ----
 
   // Visit an attribute.
@@ -1773,9 +1778,8 @@ DEF_TRAVERSE_DECL(TemplateTemplateParmDecl, {
   // D is the "T" in something like
   //   template <template <typename> class T> class container { };
   TRY_TO(TraverseDecl(D->getTemplatedDecl()));
-  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
+  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited())
     TRY_TO(TraverseTemplateArgumentLoc(D->getDefaultArgument()));
-  }
   TRY_TO(TraverseTemplateParameterListHelper(D->getTemplateParameters()));
 })
 
@@ -1787,6 +1791,8 @@ DEF_TRAVERSE_DECL(TemplateTypeParmDecl, {
   // D is the "T" in something like "template<typename T> class vector;"
   if (D->getTypeForDecl())
     TRY_TO(TraverseType(QualType(D->getTypeForDecl(), 0)));
+  if (const auto *TC = D->getTypeConstraint())
+    TRY_TO(TraverseConceptReference(*TC));
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited())
     TRY_TO(TraverseTypeLoc(D->getDefaultArgumentInfo()->getTypeLoc()));
 })
@@ -2330,6 +2336,18 @@ bool RecursiveASTVisitor<Derived>::TraverseSynOrSemInitListExpr(
   return true;
 }
 
+template<typename Derived>
+bool RecursiveASTVisitor<Derived>::TraverseConceptReference(
+    const ConceptReference &C) {
+  TRY_TO(TraverseNestedNameSpecifierLoc(C.getNestedNameSpecifierLoc()));
+  TRY_TO(TraverseDeclarationNameInfo(C.getConceptNameInfo()));
+  if (C.hasExplicitTemplateArgs())
+    TRY_TO(TraverseTemplateArgumentLocsHelper(
+        C.getTemplateArgsAsWritten()->getTemplateArgs(),
+        C.getTemplateArgsAsWritten()->NumTemplateArgs));
+  return true;
+}
+
 // If shouldVisitImplicitCode() returns false, this method traverses only the
 // syntactic form of InitListExpr.
 // If shouldVisitImplicitCode() return true, this method is called once for
@@ -2688,9 +2706,7 @@ DEF_TRAVERSE_STMT(CoyieldExpr, {
 })
 
 DEF_TRAVERSE_STMT(ConceptSpecializationExpr, {
-  TRY_TO(TraverseTemplateArgumentLocsHelper(
-          S->getTemplateArgsAsWritten()->getTemplateArgs(),
-          S->getTemplateArgsAsWritten()->NumTemplateArgs));
+  TRY_TO(TraverseConceptReference(*S));
 })
 
 // These literals (all of them) do not need any action.
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index d27691d2112..7d8231d140e 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2612,6 +2612,17 @@ def note_ambiguous_atomic_constraints_similar_expression : Note<
 
 def err_template_different_requires_clause : Error<
   "requires clause differs in template redeclaration">;
+def err_template_different_type_constraint : Error<
+  "type constraint differs in template redeclaration">;
+def err_template_template_parameter_not_at_least_as_constrained : Error<
+  "template template argument %0 is more constrained than template template "
+  "parameter %1">;
+
+def err_type_constraint_non_type_concept : Error<
+  "concept named in type constraint is not a type concept">;
+def err_type_constraint_missing_arguments : Error<
+  "%0 requires more than 1 template argument; provide the remaining arguments "
+  "explicitly to use it here">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
@@ -4819,8 +4830,8 @@ def err_unexpanded_parameter_pack : Error<
   "size|static assertion|fixed underlying type|enumerator value|"
   "using declaration|friend declaration|qualifier|initializer|default argument|"
   "non-type template parameter type|exception type|partial specialization|"
-  "__if_exists name|__if_not_exists name|lambda|block}0 contains"
-  "%plural{0: an|:}1 unexpanded parameter pack"
+  "__if_exists name|__if_not_exists name|lambda|block|type constraint}0 "
+  "contains%plural{0: an|:}1 unexpanded parameter pack"
   "%plural{0:|1: %2|2:s %2 and %3|:s %2, %3, ...}1">;
 
 def err_pack_expansion_without_parameter_packs : Error<
diff --git a/clang/include/clang/Basic/TokenKinds.def b/clang/include/clang/Basic/TokenKinds.def
index ae908bbdf3a..d58b0d987d7 100644
--- a/clang/include/clang/Basic/TokenKinds.def
+++ b/clang/include/clang/Basic/TokenKinds.def
@@ -729,7 +729,9 @@ ANNOTATION(typename)     // annotation for a C typedef name, a C++ (possibly
                          // template-id that names a type ("std::vector<int>")
 ANNOTATION(template_id)  // annotation for a C++ template-id that names a
                          // function template specialization (not a type),
-                         // e.g., "std::swap<int>"
+                         // e.g., "std::swap<int>", or a type-constraint (which
+                         // might not have explicit template arguments),
+                         // e.g. "C", "C<int>".
 ANNOTATION(non_type)     // annotation for a single non-type declaration
 ANNOTATION(non_type_undeclared) // annotation for an undeclared identifier that
                                 // was assumed to be an ADL-only function name
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index eb2f4152e72..e320c964781 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -3064,11 +3064,15 @@ private:
                                SourceLocation &RAngleLoc);
   bool ParseTemplateParameterList(unsigned Depth,
                                   SmallVectorImpl<NamedDecl*> &TemplateParams);
-  bool isStartOfTemplateTypeParameter();
+  bool isStartOfTemplateTypeParameter(bool &ScopeError);
   NamedDecl *ParseTemplateParameter(unsigned Depth, unsigned Position);
   NamedDecl *ParseTypeParameter(unsigned Depth, unsigned Position);
   NamedDecl *ParseTemplateTemplateParameter(unsigned Depth, unsigned Position);
   NamedDecl *ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position);
+  bool isTypeConstraintAnnotation();
+  bool TryAnnotateTypeConstraint(CXXScopeSpec &SS);
+  NamedDecl *
+  ParseConstrainedTemplateTypeParameter(unsigned Depth, unsigned Position);
   void DiagnoseMisplacedEllipsis(SourceLocation EllipsisLoc,
                                  SourceLocation CorrectLoc,
                                  bool AlreadyHasEllipsis,
@@ -3090,7 +3094,8 @@ private:
                                CXXScopeSpec &SS,
                                SourceLocation TemplateKWLoc,
                                UnqualifiedId &TemplateName,
-                               bool AllowTypeAnnotation = true);
+                               bool AllowTypeAnnotation = true,
+                               bool TypeConstraint = false);
   void AnnotateTemplateIdTokenAsType(bool IsClassName = false);
   bool ParseTemplateArgumentList(TemplateArgList &TemplateArgs);
   ParsedTemplateArgument ParseTemplateTemplateArgument();
diff --git a/clang/include/clang/Sema/ParsedTemplate.h b/clang/include/clang/Sema/ParsedTemplate.h
index 2eed301e8ae..0874905b38a 100644
--- a/clang/include/clang/Sema/ParsedTemplate.h
+++ b/clang/include/clang/Sema/ParsedTemplate.h
@@ -144,6 +144,10 @@ namespace clang {
   /// expressions, or template names, and the source locations for important
   /// tokens. All of the information about template arguments is allocated
   /// directly after this structure.
+  /// A template-id annotation token can also be generated by a type-constraint
+  /// construct with no explicit template arguments, e.g. "template<C T>" would
+  /// annotate C as a TemplateIdAnnotation with no template arguments (the angle
+  /// locations would be invalid in this case).
   struct TemplateIdAnnotation final
       : private llvm::TrailingObjects<TemplateIdAnnotation,
                                       ParsedTemplateArgument> {
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 5aea6102c0e..22063cbe965 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -5923,6 +5923,8 @@ public:
   ///
   /// \param OnlyNamespace If true, only considers namespaces in lookup.
   ///
+  /// \param SuppressDiagnostic If true, suppress diagnostic on error.
+  ///
   /// \returns true if an error occurred, false otherwise.
   bool ActOnCXXNestedNameSpecifier(Scope *S,
                                    NestedNameSpecInfo &IdInfo,
@@ -6263,12 +6265,6 @@ public:
   bool CheckConstraintSatisfaction(const Expr *ConstraintExpr,
                                    ConstraintSatisfaction &Satisfaction);
 
-  /// Check that the associated constraints of a template declaration match the
-  /// associated constraints of an older declaration of which it is a
-  /// redeclaration.
-  bool CheckRedeclarationConstraintMatch(TemplateParameterList *Old,
-                                         TemplateParameterList *New);
-
   /// \brief Ensure that the given template arguments satisfy the constraints
   /// associated with the given template, emitting a diagnostic if they do not.
   ///
@@ -6301,9 +6297,8 @@ public:
   void DiagnoseUnsatisfiedIllFormedConstraint(SourceLocation DiagnosticLocation,
                                               StringRef Diagnostic);
 
-  void
-  DiagnoseRedeclarationConstraintMismatch(const TemplateParameterList *Old,
-                                          const TemplateParameterList *New);
+  void DiagnoseRedeclarationConstraintMismatch(SourceLocation Old,
+                                               SourceLocation New);
 
   // ParseObjCStringLiteral - Parse Objective-C string literals.
   ExprResult ParseObjCStringLiteral(SourceLocation *AtLocs,
@@ -6872,13 +6867,24 @@ public:
   TemplateDecl *AdjustDeclIfTemplate(Decl *&Decl);
 
   NamedDecl *ActOnTypeParameter(Scope *S, bool Typename,
-                           SourceLocation EllipsisLoc,
-                           SourceLocation KeyLoc,
-                           IdentifierInfo *ParamName,
-                           SourceLocation ParamNameLoc,
-                           unsigned Depth, unsigned Position,
-                           SourceLocation EqualLoc,
-                           ParsedType DefaultArg);
+                                SourceLocation EllipsisLoc,
+                                SourceLocation KeyLoc,
+                                IdentifierInfo *ParamName,
+                                SourceLocation ParamNameLoc,
+                                unsigned Depth, unsigned Position,
+                                SourceLocation EqualLoc,
+                                ParsedType DefaultArg, bool HasTypeConstraint);
+
+  bool ActOnTypeConstraint(TemplateIdAnnotation *TypeConstraint,
+                           TemplateTypeParmDecl *ConstrainedParameter,
+                           SourceLocation EllipsisLoc);
+
+  bool AttachTypeConstraint(NestedNameSpecifierLoc NS,
+                            DeclarationNameInfo NameInfo,
+                            ConceptDecl *NamedConcept,
+                            const TemplateArgumentListInfo *TemplateArgs,
+                            TemplateTypeParmDecl *ConstrainedParameter,
+                            SourceLocation EllipsisLoc);
 
   QualType CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
                                              SourceLocation Loc);
@@ -7000,8 +7006,8 @@ public:
   ExprResult
   CheckConceptTemplateId(const CXXScopeSpec &SS,
                          SourceLocation TemplateKWLoc,
-                         SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
-                         ConceptDecl *NamedConcept,
+                         const DeclarationNameInfo &ConceptNameInfo,
+                         NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
                          const TemplateArgumentListInfo *TemplateArgs);
 
   void diagnoseMissingTemplateArguments(TemplateName Name, SourceLocation Loc);
@@ -7160,7 +7166,8 @@ public:
                                    QualType InstantiatedParamType, Expr *Arg,
                                    TemplateArgument &Converted,
                                CheckTemplateArgumentKind CTAK = CTAK_Specified);
-  bool CheckTemplateTemplateArgument(TemplateParameterList *Params,
+  bool CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
+                                     TemplateParameterList *Params,
                                      TemplateArgumentLoc &Arg);
 
   ExprResult
@@ -7353,7 +7360,10 @@ public:
     UPPC_Lambda,
 
     /// Block expression,
-    UPPC_Block
+    UPPC_Block,
+
+    /// A type constraint,
+    UPPC_TypeConstraint
   };
 
   /// Diagnose unexpanded parameter packs.
@@ -7846,7 +7856,7 @@ public:
                                     sema::TemplateDeductionInfo &Info);
 
   bool isTemplateTemplateParameterAtLeastAsSpecializedAs(
-      TemplateParameterList *P, TemplateDecl *AArg, SourceLocation Loc);
+      TemplateParameterList *PParam, TemplateDecl *AArg, SourceLocation Loc);
 
   void MarkUsedTemplateParameters(const Expr *E, bool OnlyDeduced,
                                   unsigned Depth, llvm::SmallBitVector &Used);
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 26e76d45042..a51429264db 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -661,8 +661,9 @@ comments::FullComment *ASTContext::getCommentForDecl(
   return FC;
 }
 
-void
+void 
 ASTContext::CanonicalTemplateTemplateParm::Profile(llvm::FoldingSetNodeID &ID,
+                                                   const ASTContext &C,
                                                TemplateTemplateParmDecl *Parm) {
   ID.AddInteger(Parm->getDepth());
   ID.AddInteger(Parm->getPosition());
@@ -676,6 +677,16 @@ ASTContext::CanonicalTemplateTemplateParm::Profile(llvm::FoldingSetNodeID &ID,
     if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(*P)) {
       ID.AddInteger(0);
       ID.AddBoolean(TTP->isParameterPack());
+      const TypeConstraint *TC = TTP->getTypeConstraint();
+      ID.AddBoolean(TC != nullptr);
+      if (TC)
+        TC->getImmediatelyDeclaredConstraint()->Profile(ID, C,
+                                                        /*Canonical=*/true);
+      if (TTP->isExpandedParameterPack()) {
+        ID.AddBoolean(true);
+        ID.AddInteger(TTP->getNumExpansionParameters());
+      } else
+        ID.AddBoolean(false);
       continue;
     }
 
@@ -697,8 +708,12 @@ ASTContext::CanonicalTemplateTemplateParm::Profile(llvm::FoldingSetNodeID &ID,
 
     auto *TTP = cast<TemplateTemplateParmDecl>(*P);
     ID.AddInteger(2);
-    Profile(ID, TTP);
+    Profile(ID, C, TTP);
   }
+  Expr *RequiresClause = Parm->getTemplateParameters()->getRequiresClause();
+  ID.AddBoolean(RequiresClause != nullptr);
+  if (RequiresClause)
+    RequiresClause->Profile(ID, C, /*Canonical=*/true);
 }
 
 TemplateTemplateParmDecl *
@@ -706,7 +721,7 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
                                           TemplateTemplateParmDecl *TTP) const {
   // Check if we already have a canonical template template parameter.
   llvm::FoldingSetNodeID ID;
-  CanonicalTemplateTemplateParm::Profile(ID, TTP);
+  CanonicalTemplateTemplateParm::Profile(ID, *this, TTP);
   void *InsertPos = nullptr;
   CanonicalTemplateTemplateParm *Canonical
     = CanonTemplateTemplateParms.FindNodeOrInsertPos(ID, InsertPos);
@@ -720,15 +735,79 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
   for (TemplateParameterList::const_iterator P = Params->begin(),
                                           PEnd = Params->end();
        P != PEnd; ++P) {
-    if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(*P))
-      CanonParams.push_back(
-                  TemplateTypeParmDecl::Create(*this, getTranslationUnitDecl(),
-                                               SourceLocation(),
-                                               SourceLocation(),
-                                               TTP->getDepth(),
-                                               TTP->getIndex(), nullptr, false,
-                                               TTP->isParameterPack()));
-    else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(*P)) {
+    if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(*P)) {
+      TemplateTypeParmDecl *NewTTP = TemplateTypeParmDecl::Create(*this,
+          getTranslationUnitDecl(), SourceLocation(), SourceLocation(),
+          TTP->getDepth(), TTP->getIndex(), nullptr, false,
+          TTP->isParameterPack(), TTP->hasTypeConstraint(),
+          TTP->isExpandedParameterPack() ?
+          llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
+      if (const auto *TC = TTP->getTypeConstraint()) {
+        // This is a bit ugly - we need to form a new immediately-declared
+        // constraint that references the new parameter; this would ideally
+        // require semantic analysis (e.g. template<C T> struct S {}; - the
+        // converted arguments of C<T> could be an argument pack if C is
+        // declared as template<typename... T> concept C = ...).
+        // We don't have semantic analysis here so we dig deep into the
+        // ready-made constraint expr and change the thing manually.
+        Expr *IDC = TC->getImmediatelyDeclaredConstraint();
+        ConceptSpecializationExpr *CSE;
+        if (const auto *Fold = dyn_cast<CXXFoldExpr>(IDC))
+          CSE = cast<ConceptSpecializationExpr>(Fold->getLHS());
+        else
+          CSE = cast<ConceptSpecializationExpr>(IDC);
+        ArrayRef<TemplateArgument> OldConverted = CSE->getTemplateArguments();
+        SmallVector<TemplateArgument, 3> NewConverted;
+        NewConverted.reserve(OldConverted.size());
+
+        QualType ParamAsArgument(NewTTP->getTypeForDecl(), 0);
+        if (OldConverted.front().getKind() == TemplateArgument::Pack) {
+          // The case:
+          // template<typename... T> concept C = true;
+          // template<C<int> T> struct S; -> constraint is C<{T, int}>
+          NewConverted.push_back(ParamAsArgument);
+          for (auto &Arg : OldConverted.front().pack_elements().drop_front(1))
+            NewConverted.push_back(Arg);
+          TemplateArgument NewPack(NewConverted);
+
+          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);
+      }
+      CanonParams.push_back(NewTTP);
+    } else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(*P)) {
       QualType T = getCanonicalType(NTTP->getType());
       TypeSourceInfo *TInfo = getTrivialTypeSourceInfo(T);
       NonTypeTemplateParmDecl *Param;
@@ -767,9 +846,9 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
                                            cast<TemplateTemplateParmDecl>(*P)));
   }
 
-  assert(!TTP->getTemplateParameters()->getRequiresClause() &&
-         "Unexpected requires-clause on template template-parameter");
-  Expr *const CanonRequiresClause = nullptr;
+  Expr *CanonRequiresClause = nullptr;
+  if (Expr *RequiresClause = TTP->getTemplateParameters()->getRequiresClause())
+    CanonRequiresClause = RequiresClause;
 
   TemplateTemplateParmDecl *CanonTTP
     = TemplateTemplateParmDecl::Create(*this, getTranslationUnitDecl(),
@@ -865,7 +944,8 @@ ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
     : ConstantArrayTypes(this_()), FunctionProtoTypes(this_()),
       TemplateSpecializationTypes(this_()),
       DependentTemplateSpecializationTypes(this_()),
-      SubstTemplateTemplateParmPacks(this_()), SourceMgr(SM), LangOpts(LOpts),
+      SubstTemplateTemplateParmPacks(this_()),
+      CanonTemplateTemplateParms(this_()), SourceMgr(SM), LangOpts(LOpts),
       SanitizerBL(new SanitizerBlacklist(LangOpts.SanitizerBlacklistFiles, SM)),
       XRayFilter(new XRayFunctionFilter(LangOpts.XRayAlwaysInstrumentFiles,
                                         LangOpts.XRayNeverInstrumentFiles,
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 8eb58b32ae1..22fb67478c9 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -5097,7 +5097,7 @@ ASTNodeImporter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   // context. This context will be fixed when the actual template declaration
   // is created.
 
-  // FIXME: Import default argument.
+  // FIXME: Import default argument  and constraint expression.
 
   ExpectedSLoc BeginLocOrErr = import(D->getBeginLoc());
   if (!BeginLocOrErr)
@@ -5108,12 +5108,47 @@ ASTNodeImporter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
     return LocationOrErr.takeError();
 
   TemplateTypeParmDecl *ToD = nullptr;
-  (void)GetImportedOrCreateDecl(
+  if (GetImportedOrCreateDecl(
       ToD, D, Importer.getToContext(),
       Importer.getToContext().getTranslationUnitDecl(),
       *BeginLocOrErr, *LocationOrErr,
       D->getDepth(), D->getIndex(), Importer.Import(D->getIdentifier()),
-      D->wasDeclaredWithTypename(), D->isParameterPack());
+      D->wasDeclaredWithTypename(), D->isParameterPack(),
+      D->hasTypeConstraint()))
+    return ToD;
+
+  // Import the type-constraint
+  if (const TypeConstraint *TC = D->getTypeConstraint()) {
+    NestedNameSpecifierLoc ToNNS;
+    DeclarationName ToName;
+    SourceLocation ToNameLoc;
+    NamedDecl *ToFoundDecl;
+    ConceptDecl *ToNamedConcept;
+    Expr *ToIDC;
+    if (auto Imp = importSeq(TC->getNestedNameSpecifierLoc(),
+        TC->getConceptNameInfo().getName(), TC->getConceptNameInfo().getLoc(),
+        TC->getFoundDecl(), TC->getNamedConcept(),
+        TC->getImmediatelyDeclaredConstraint()))
+      std::tie(ToNNS, ToName, ToNameLoc, ToFoundDecl, ToNamedConcept,
+               ToIDC) = *Imp;
+    else
+      return Imp.takeError();
+
+    TemplateArgumentListInfo ToTAInfo;
+    const auto *ASTTemplateArgs = TC->getTemplateArgsAsWritten();
+    if (ASTTemplateArgs)
+      if (Error Err = ImportTemplateArgumentListInfo(*ASTTemplateArgs,
+                                                     ToTAInfo))
+        return std::move(Err);
+
+    ToD->setTypeConstraint(ToNNS, DeclarationNameInfo(ToName, ToNameLoc),
+        ToFoundDecl, ToNamedConcept,
+        ASTTemplateArgs ?
+            ASTTemplateArgumentListInfo::Create(Importer.getToContext(),
+                                                ToTAInfo) : nullptr,
+        ToIDC);
+  }
+
   return ToD;
 }
 
diff --git a/clang/lib/AST/DeclPrinter.cpp b/clang/lib/AST/DeclPrinter.cpp
index 68cc8b7a0eb..4cedcbed064 100644
--- a/clang/lib/AST/DeclPrinter.cpp
+++ b/clang/lib/AST/DeclPrinter.cpp
@@ -1053,7 +1053,9 @@ void DeclPrinter::printTemplateParameters(const TemplateParameterList *Params,
 
     if (auto TTP = dyn_cast<TemplateTypeParmDecl>(Param)) {
 
-      if (TTP->wasDeclaredWithTypename())
+      if (const TypeConstraint *TC = TTP->getTypeConstraint())
+        TC->print(Out, Policy);
+      else if (TTP->wasDeclaredWithTypename())
         Out << "typename";
       else
         Out << "class";
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 59fa7faad92..95a2e26e0df 100755
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -43,35 +43,46 @@ using namespace clang;
 // TemplateParameterList Implementation
 //===----------------------------------------------------------------------===//
 
-TemplateParameterList::TemplateParameterList(SourceLocation TemplateLoc,
+
+TemplateParameterList::TemplateParameterList(const ASTContext& C,
+                                             SourceLocation TemplateLoc,
                                              SourceLocation LAngleLoc,
                                              ArrayRef<NamedDecl *> Params,
                                              SourceLocation RAngleLoc,
                                              Expr *RequiresClause)
     : TemplateLoc(TemplateLoc), LAngleLoc(LAngleLoc), RAngleLoc(RAngleLoc),
       NumParams(Params.size()), ContainsUnexpandedParameterPack(false),
-      HasRequiresClause(static_cast<bool>(RequiresClause)) {
+      HasRequiresClause(RequiresClause != nullptr),
+      HasConstrainedParameters(false) {
   for (unsigned Idx = 0; Idx < NumParams; ++Idx) {
     NamedDecl *P = Params[Idx];
     begin()[Idx] = P;
 
-    if (!P->isTemplateParameterPack()) {
-      if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P))
-        if (NTTP->getType()->containsUnexpandedParameterPack())
-          ContainsUnexpandedParameterPack = true;
-
-      if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(P))
-        if (TTP->getTemplateParameters()->containsUnexpandedParameterPack())
-          ContainsUnexpandedParameterPack = true;
-
-      // FIXME: If a default argument contains an unexpanded parameter pack, the
-      // template parameter list does too.
+    bool IsPack = P->isTemplateParameterPack();
+    if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
+      if (!IsPack && NTTP->getType()->containsUnexpandedParameterPack())
+        ContainsUnexpandedParameterPack = true;
+      if (NTTP->hasPlaceholderTypeConstraint())
+        HasConstrainedParameters = true;
+    } else if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(P)) {
+      if (!IsPack &&
+          TTP->getTemplateParameters()->containsUnexpandedParameterPack())
+        ContainsUnexpandedParameterPack = true;
+    } else if (const TypeConstraint *TC =
+        cast<TemplateTypeParmDecl>(P)->getTypeConstraint()) {
+      if (TC->getImmediatelyDeclaredConstraint()
+          ->containsUnexpandedParameterPack())
+        ContainsUnexpandedParameterPack = true;
+      HasConstrainedParameters = true;
     }
+    // FIXME: If a default argument contains an unexpanded parameter pack, the
+    // template parameter list does too.
   }
-  if (RequiresClause) {
-    *getTrailingObjects<Expr *>() = RequiresClause;
+
+  if (HasRequiresClause) {
     if (RequiresClause->containsUnexpandedParameterPack())
       ContainsUnexpandedParameterPack = true;
+    *getTrailingObjects<Expr *>() = RequiresClause;
   }
 }
 
@@ -83,7 +94,7 @@ TemplateParameterList::Create(const ASTContext &C, SourceLocation TemplateLoc,
   void *Mem = C.Allocate(totalSizeToAlloc<NamedDecl *, Expr *>(
                              Params.size(), RequiresClause ? 1u : 0u),
                          alignof(TemplateParameterList));
-  return new (Mem) TemplateParameterList(TemplateLoc, LAngleLoc, Params,
+  return new (Mem) TemplateParameterList(C, TemplateLoc, LAngleLoc, Params,
                                          RAngleLoc, RequiresClause);
 }
 
@@ -91,11 +102,23 @@ unsigned TemplateParameterList::getMinRequiredArguments() const {
   unsigned NumRequiredArgs = 0;
   for (const NamedDecl *P : asArray()) {
     if (P->isTemplateParameterPack()) {
-      if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P))
+      if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
         if (NTTP->isExpandedParameterPack()) {
           NumRequiredArgs += NTTP->getNumExpansionTypes();
           continue;
         }
+      } else if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
+        if (TTP->isExpandedParameterPack()) {
+          NumRequiredArgs += TTP->getNumExpansionParameters();
+          continue;
+        }
+      } else {
+        const auto *TP = cast<TemplateTemplateParmDecl>(P);
+        if (TP->isExpandedParameterPack()) {
+          NumRequiredArgs += TP->getNumExpansionTemplateParameters();
+          continue;
+        }
+      }
 
       break;
     }
@@ -140,14 +163,17 @@ static void AdoptTemplateParameterList(TemplateParameterList *Params,
 
 void TemplateParameterList::
 getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
-  // TODO: Concepts: Collect immediately-introduced constraints.
+  if (HasConstrainedParameters)
+    for (const NamedDecl *Param : *this)
+      if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
+        if (const auto *TC = TTP->getTypeConstraint())
+          AC.push_back(TC->getImmediatelyDeclaredConstraint());
   if (HasRequiresClause)
     AC.push_back(getRequiresClause());
 }
 
 bool TemplateParameterList::hasAssociatedConstraints() const {
-  // TODO: Concepts: Regard immediately-introduced constraints.
-  return HasRequiresClause;
+  return HasRequiresClause || HasConstrainedParameters;
 }
 
 namespace clang {
@@ -558,9 +584,14 @@ TemplateTypeParmDecl *
 TemplateTypeParmDecl::Create(const ASTContext &C, DeclContext *DC,
                              SourceLocation KeyLoc, SourceLocation NameLoc,
                              unsigned D, unsigned P, IdentifierInfo *Id,
-                             bool Typename, bool ParameterPack) {
+                             bool Typename, bool ParameterPack,
+                             bool HasTypeConstraint,
+                             Optional<unsigned> NumExpanded) {
   auto *TTPDecl =
-      new (C, DC) TemplateTypeParmDecl(DC, KeyLoc, NameLoc, Id, Typename);
+      new (C, DC,
+           additionalSizeToAlloc<TypeConstraint>(HasTypeConstraint ? 1 : 0))
+      TemplateTypeParmDecl(DC, KeyLoc, NameLoc, Id, Typename,
+                           HasTypeConstraint, NumExpanded);
   QualType TTPType = C.getTemplateTypeParmType(D, P, ParameterPack, TTPDecl);
   TTPDecl->setTypeForDecl(TTPType.getTypePtr());
   return TTPDecl;
@@ -569,7 +600,17 @@ TemplateTypeParmDecl::Create(const ASTContext &C, DeclContext *DC,
 TemplateTypeParmDecl *
 TemplateTypeParmDecl::CreateDeserialized(const ASTContext &C, unsigned ID) {
   return new (C, ID) TemplateTypeParmDecl(nullptr, SourceLocation(),
-                                          SourceLocation(), nullptr, false);
+                                          SourceLocation(), nullptr, false,
+                                          false, None);
+}
+
+TemplateTypeParmDecl *
+TemplateTypeParmDecl::CreateDeserialized(const ASTContext &C, unsigned ID,
+                                         bool HasTypeConstraint) {
+  return new (C, ID,
+              additionalSizeToAlloc<TypeConstraint>(HasTypeConstraint ? 1 : 0))
+         TemplateTypeParmDecl(nullptr, SourceLocation(), SourceLocation(),
+                              nullptr, false, HasTypeConstraint, None);
 }
 
 SourceLocation TemplateTypeParmDecl::getDefaultArgumentLoc() const {
@@ -602,6 +643,20 @@ bool TemplateTypeParmDecl::isParameterPack() const {
   return getTypeForDecl()->castAs<TemplateTypeParmType>()->isParameterPack();
 }
 
+void TemplateTypeParmDecl::setTypeConstraint(NestedNameSpecifierLoc NNS,
+    DeclarationNameInfo NameInfo, NamedDecl *FoundDecl, ConceptDecl *CD,
+    const ASTTemplateArgumentListInfo *ArgsAsWritten,
+    Expr *ImmediatelyDeclaredConstraint) {
+  assert(HasTypeConstraint &&
+         "HasTypeConstraint=true must be passed at construction in order to "
+         "call setTypeConstraint");
+  assert(!TypeConstraintInitialized &&
+         "TypeConstraint was already initialized!");
+  new (getTrailingObjects<TypeConstraint>()) TypeConstraint(NNS, NameInfo,
+      FoundDecl, CD, ArgsAsWritten, ImmediatelyDeclaredConstraint);
+  TypeConstraintInitialized = true;
+}
+
 //===----------------------------------------------------------------------===//
 // NonTypeTemplateParmDecl Method Implementations
 //===----------------------------------------------------------------------===//
@@ -1256,7 +1311,8 @@ createMakeIntegerSeqParameterList(const ASTContext &C, DeclContext *DC) {
   // typename T
   auto *T = TemplateTypeParmDecl::Create(
       C, DC, SourceLocation(), SourceLocation(), /*Depth=*/1, /*Position=*/0,
-      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false);
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false,
+      /*HasTypeConstraint=*/false);
   T->setImplicit(true);
 
   // T ...Ints
@@ -1281,7 +1337,8 @@ createMakeIntegerSeqParameterList(const ASTContext &C, DeclContext *DC) {
   // typename T
   auto *TemplateTypeParm = TemplateTypeParmDecl::Create(
       C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/1,
-      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false);
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false,
+      /*HasTypeConstraint=*/false);
   TemplateTypeParm->setImplicit(true);
 
   // T N
@@ -1309,7 +1366,8 @@ createTypePackElementParameterList(const ASTContext &C, DeclContext *DC) {
   // typename ...T
   auto *Ts = TemplateTypeParmDecl::Create(
       C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/1,
-      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/true);
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/true,
+      /*HasTypeConstraint=*/false);
   Ts->setImplicit(true);
 
   // template <std::size_t Index, typename ...T>
@@ -1339,3 +1397,15 @@ BuiltinTemplateDecl::BuiltinTemplateDecl(const ASTContext &C, DeclContext *DC,
     : TemplateDecl(BuiltinTemplate, DC, SourceLocation(), Name,
                    createBuiltinTemplateParameterList(C, DC, BTK)),
       BTK(BTK) {}
+
+void TypeConstraint::print(llvm::raw_ostream &OS, PrintingPolicy Policy) const {
+  if (NestedNameSpec)
+    NestedNameSpec.getNestedNameSpecifier()->print(OS, Policy);
+  ConceptName.printName(OS, Policy);
+  if (hasExplicitTemplateArgs()) {
+    OS << "<";
+    for (auto &ArgLoc : ArgsAsWritten->arguments())
+      ArgLoc.getArgument().print(Policy, OS);
+    OS << ">";
+  }
+}
\ No newline at end of file
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index deb76820afe..422227d787b 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -1765,9 +1765,9 @@ CUDAKernelCallExpr *CUDAKernelCallExpr::CreateEmpty(const ASTContext &Ctx,
   return new (Mem) CUDAKernelCallExpr(NumArgs, Empty);
 }
 
-ConceptSpecializationExpr::ConceptSpecializationExpr(ASTContext &C,
+ConceptSpecializationExpr::ConceptSpecializationExpr(const ASTContext &C,
     NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
-    SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
+    DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
     ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
     ArrayRef<TemplateArgument> ConvertedArgs,
     const ConstraintSatisfaction *Satisfaction)
@@ -1776,34 +1776,31 @@ ConceptSpecializationExpr::ConceptSpecializationExpr(ASTContext &C,
            // All the flags below are set in setTemplateArguments.
            /*ValueDependent=*/!Satisfaction, /*InstantiationDependent=*/false,
            /*ContainsUnexpandedParameterPacks=*/false),
-      NestedNameSpec(NNS), TemplateKWLoc(TemplateKWLoc),
-      ConceptNameLoc(ConceptNameLoc), FoundDecl(FoundDecl),
-      NamedConcept(NamedConcept), NumTemplateArgs(ConvertedArgs.size()),
+      ConceptReference(NNS, TemplateKWLoc, ConceptNameInfo, FoundDecl,
+                       NamedConcept, ArgsAsWritten),
+      NumTemplateArgs(ConvertedArgs.size()),
       Satisfaction(Satisfaction ?
                    ASTConstraintSatisfaction::Create(C, *Satisfaction) :
                    nullptr) {
-  setTemplateArguments(ArgsAsWritten, ConvertedArgs);
+  setTemplateArguments(ConvertedArgs);
 }
 
 ConceptSpecializationExpr::ConceptSpecializationExpr(EmptyShell Empty,
     unsigned NumTemplateArgs)
-    : Expr(ConceptSpecializationExprClass, Empty),
+    : Expr(ConceptSpecializationExprClass, Empty), ConceptReference(),
       NumTemplateArgs(NumTemplateArgs) { }
 
 void ConceptSpecializationExpr::setTemplateArguments(
-    const ASTTemplateArgumentListInfo *ArgsAsWritten,
     ArrayRef<TemplateArgument> Converted) {
   assert(Converted.size() == NumTemplateArgs);
-  assert(!this->ArgsAsWritten && "setTemplateArguments can only be used once");
-  this->ArgsAsWritten = ArgsAsWritten;
   std::uninitialized_copy(Converted.begin(), Converted.end(),
                           getTrailingObjects<TemplateArgument>());
   bool IsInstantiationDependent = false;
   bool ContainsUnexpandedParameterPack = false;
-  for (const TemplateArgumentLoc& LocInfo : ArgsAsWritten->arguments()) {
-    if (LocInfo.getArgument().isInstantiationDependent())
+  for (const TemplateArgument& Arg : Converted) {
+    if (Arg.isInstantiationDependent())
       IsInstantiationDependent = true;
-    if (LocInfo.getArgument().containsUnexpandedParameterPack())
+    if (Arg.containsUnexpandedParameterPack())
       ContainsUnexpandedParameterPack = true;
     if (ContainsUnexpandedParameterPack && IsInstantiationDependent)
       break;
@@ -1821,9 +1818,10 @@ void ConceptSpecializationExpr::setTemplateArguments(
 }
 
 ConceptSpecializationExpr *
-ConceptSpecializationExpr::Create(ASTContext &C, NestedNameSpecifierLoc NNS,
+ConceptSpecializationExpr::Create(const ASTContext &C,
+                                  NestedNameSpecifierLoc NNS,
                                   SourceLocation TemplateKWLoc,
-                                  SourceLocation ConceptNameLoc,
+                                  DeclarationNameInfo ConceptNameInfo,
                                   NamedDecl *FoundDecl,
                                   ConceptDecl *NamedConcept,
                                const ASTTemplateArgumentListInfo *ArgsAsWritten,
@@ -1832,7 +1830,7 @@ ConceptSpecializationExpr::Create(ASTContext &C, NestedNameSpecifierLoc NNS,
   void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
                                 ConvertedArgs.size()));
   return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
-                                                ConceptNameLoc, FoundDecl,
+                                                ConceptNameInfo, FoundDecl,
                                                 NamedConcept, ArgsAsWritten,
                                                 ConvertedArgs, Satisfaction);
 }
diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
index 3b89c630b45..27fdca1c4b9 100644
--- a/clang/lib/AST/ODRHash.cpp
+++ b/clang/lib/AST/ODRHash.cpp
@@ -380,6 +380,11 @@ public:
     }
     Hash.AddBoolean(D->isParameterPack());
 
+    const TypeConstraint *TC = D->getTypeConstraint();
+    Hash.AddBoolean(TC != nullptr);
+    if (TC)
+      AddStmt(TC->getImmediatelyDeclaredConstraint());
+
     Inherited::VisitTemplateTypeParmDecl(D);
   }
 
diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
index fa7f4130b76..965ad17fcfa 100644
--- a/clang/lib/AST/TextNodeDumper.cpp
+++ b/clang/lib/AST/TextNodeDumper.cpp
@@ -1694,7 +1694,16 @@ void TextNodeDumper::VisitBuiltinTemplateDecl(const BuiltinTemplateDecl *D) {
 }
 
 void TextNodeDumper::VisitTemplateTypeParmDecl(const TemplateTypeParmDecl *D) {
-  if (D->wasDeclaredWithTypename())
+  if (const auto *TC = D->getTypeConstraint()) {
+    OS << " ";
+    dumpBareDeclRef(TC->getNamedConcept());
+    if (TC->getNamedConcept() != TC->getFoundDecl()) {
+      OS << " (";
+      dumpBareDeclRef(TC->getFoundDecl());
+      OS << ")";
+    }
+    Visit(TC->getImmediatelyDeclaredConstraint());
+  } else if (D->wasDeclaredWithTypename())
     OS << " typename";
   else
     OS << " class";
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index c94ccee6028..69a3ed9cbad 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -5121,6 +5121,13 @@ bool Parser::isDeclarationSpecifier(bool DisambiguatingWithExpression) {
     return !DisambiguatingWithExpression ||
            !isStartOfObjCClassMessageMissingOpenBracket();
 
+    // placeholder-type-specifier
+  case tok::annot_template_id: {
+    TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
+    return TemplateId->Kind == TNK_Concept_template &&
+        (NextToken().is(tok::kw_auto) || NextToken().is(tok::kw_decltype));
+  }
+
   case tok::kw___declspec:
   case tok::kw___cdecl:
   case tok::kw___stdcall:
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index f4ffa08b2a1..e685d5ea8a9 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -142,11 +142,16 @@ void Parser::CheckForTemplateAndDigraph(Token &Next, ParsedType ObjectType,
 ///
 /// \param OnlyNamespace If true, only considers namespaces in lookup.
 ///
+///
 /// \returns true if there was an error parsing a scope specifier
-bool Parser::ParseOptionalCXXScopeSpecifier(
-    CXXScopeSpec &SS, ParsedType ObjectType, bool EnteringContext,
-    bool *MayBePseudoDestructor, bool IsTypename, IdentifierInfo **LastII,
-    bool OnlyNamespace, bool InUsingDeclaration) {
+bool Parser::ParseOptionalCXXScopeSpecifier(CXXScopeSpec &SS,
+                                            ParsedType ObjectType,
+                                            bool EnteringContext,
+                                            bool *MayBePseudoDestructor,
+                                            bool IsTypename,
+                                            IdentifierInfo **LastII,
+                                            bool OnlyNamespace,
+                                            bool InUsingDeclaration) {
   assert(getLangOpts().CPlusPlus &&
          "Call sites of this function should be guarded by checking for C++");
 
@@ -476,6 +481,7 @@ bool Parser::ParseOptionalCXXScopeSpecifier(
     // nested-name-specifier:
     //   type-name '<'
     if (Next.is(tok::less)) {
+
       TemplateTy Template;
       UnqualifiedId TemplateName;
       TemplateName.setIdentifier(&II, Tok.getLocation());
@@ -500,7 +506,8 @@ bool Parser::ParseOptionalCXXScopeSpecifier(
         // template-id to be translated into a type annotation,
         // because some clients (e.g., the parsing of class template
         // specializations) still want to see the original template-id
-        // token.
+        // token, and it might not be a type at all (e.g. a concept name in a
+        // type-constraint).
         ConsumeToken();
         if (AnnotateTemplateIdToken(Template, TNK, SS, SourceLocation(),
                                     TemplateName, false))
diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index 35cee596bb0..1b9301b6591 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/clang/lib/Parse/ParseTemplate.cpp
@@ -12,6 +12,7 @@
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclTemplate.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/Parse/ParseDiagnostic.h"
 #include "clang/Parse/Parser.h"
 #include "clang/Parse/RAIIObjectsForParser.h"
@@ -498,7 +499,10 @@ Parser::ParseTemplateParameterList(const unsigned Depth,
 
 /// Determine whether the parser is at the start of a template
 /// type parameter.
-bool Parser::isStartOfTemplateTypeParameter() {
+/// \param ScopeError will receive true if there was an error parsing a
+/// scope specifier at the current location.
+bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
+  ScopeError = false;
   if (Tok.is(tok::kw_class)) {
     // "class" may be the start of an elaborated-type-specifier or a
     // type-parameter. Per C++ [temp.param]p3, we prefer the type-parameter.
@@ -531,6 +535,40 @@ bool Parser::isStartOfTemplateTypeParameter() {
     }
   }
 
+  bool WasScopeAnnotation = Tok.is(tok::annot_cxxscope);
+  CXXScopeSpec SS;
+  ScopeError =
+      ParseOptionalCXXScopeSpecifier(SS, ParsedType(),
+                                     /*EnteringContext=*/false,
+                                     /*MayBePseudoDestructor=*/nullptr,
+                                     // If this is not a type-constraint, then
+                                     // this scope-spec is part of the typename
+                                     // of a non-type template parameter
+                                     /*IsTypename=*/true, /*LastII=*/nullptr,
+                                     // We won't find concepts in
+                                     // non-namespaces anyway, so might as well
+                                     // parse this correctly for possible type
+                                     // names.
+                                     /*OnlyNamespace=*/false);
+  if (ScopeError)
+    return false;
+  if (TryAnnotateTypeConstraint(SS))
+    return false;
+  bool IsTypeConstraint = isTypeConstraintAnnotation();
+  if (!IsTypeConstraint && SS.isNotEmpty()) {
+    // This isn't a type-constraint but we've already parsed this scope
+    // specifier - annotate it.
+    AnnotateScopeToken(SS, /*isNewAnnotation=*/!WasScopeAnnotation);
+    return false;
+  }
+
+  if (IsTypeConstraint &&
+      // Next token might be 'auto' or 'decltype', indicating that this
+      // type-constraint is in fact part of a placeholder-type-specifier of a
+      // non-type template parameter.
+      !NextToken().isOneOf(tok::kw_auto, tok::kw_decltype))
+    return true;
+
   // 'typedef' is a reasonably-common typo/thinko for 'typename', and is
   // ill-formed otherwise.
   if (Tok.isNot(tok::kw_typename) && Tok.isNot(tok::kw_typedef))
@@ -574,24 +612,37 @@ bool Parser::isStartOfTemplateTypeParameter() {
 ///         type-parameter
 ///         parameter-declaration
 ///
-///       type-parameter: (see below)
-///         'class' ...[opt] identifier[opt]
-///         'class' identifier[opt] '=' type-id
-///         'typename' ...[opt] identifier[opt]
-///         'typename' identifier[opt] '=' type-id
-///         'template' '<' template-parameter-list '>'
-///               'class' ...[opt] identifier[opt]
-///         'template' '<' template-parameter-list '>' 'class' identifier[opt]
-///               = id-expression
+///       type-parameter: (See below)
+///         type-parameter-key ...[opt] identifier[opt]
+///         type-parameter-key identifier[opt] = type-id
+/// (C++2a) type-constraint ...[opt] identifier[opt]
+/// (C++2a) type-constraint identifier[opt] = type-id
+///         'template' '<' template-parameter-list '>' type-parameter-key
+///               ...[opt] identifier[opt]
+///         'template' '<' template-parameter-list '>' type-parameter-key
+///               identifier[opt] '=' id-expression
+///
+///       type-parameter-key:
+///         class
+///         typename
+///
 NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
-  if (isStartOfTemplateTypeParameter()) {
-    // Is there just a typo in the input code? ('typedef' instead of 'typename')
+  // We could be facing a type-constraint, which (could) start a type parameter.
+  // Annotate it now (we might end up not using it if we determine this
+  // type-constraint is in fact part of a placeholder-type-specifier of a
+  // non-type template parameter.
+
+  bool ScopeError;
+  if (isStartOfTemplateTypeParameter(ScopeError)) {
+    // Is there just a typo in the input code? ('typedef' instead of
+    // 'typename')
     if (Tok.is(tok::kw_typedef)) {
       Diag(Tok.getLocation(), diag::err_expected_template_parameter);
 
       Diag(Tok.getLocation(), diag::note_meant_to_use_typename)
           << FixItHint::CreateReplacement(CharSourceRange::getCharRange(
-                                              Tok.getLocation(), Tok.getEndLoc()),
+                                              Tok.getLocation(),
+                                              Tok.getEndLoc()),
                                           "typename");
 
       Tok.setKind(tok::kw_typename);
@@ -599,7 +650,26 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
 
     return ParseTypeParameter(Depth, Position);
   }
-
+  if (ScopeError) {
+    // We return an invalid parameter as opposed to null to avoid having bogus
+    // diagnostics about an empty template parameter list.
+    // FIXME: Fix ParseTemplateParameterList to better handle nullptr results
+    //  from here.
+    // Return a NTTP as if there was an error in a scope specifier, the user
+    // probably meant to write the type of a NTTP.
+    DeclSpec DS(getAttrFactory());
+    DS.SetTypeSpecError();
+    Declarator D(DS, DeclaratorContext::TemplateParamContext);
+    D.SetIdentifier(nullptr, Tok.getLocation());
+    D.setInvalidType(true);
+    NamedDecl *ErrorParam = Actions.ActOnNonTypeTemplateParameter(
+        getCurScope(), D, Depth, Position, /*EqualLoc=*/SourceLocation(),
+        /*DefaultArg=*/nullptr);
+    ErrorParam->setInvalidDecl(true);
+    SkipUntil(tok::comma, tok::greater, tok::greatergreater,
+              StopAtSemi | StopBeforeMatch);
+    return ErrorParam;
+  }
   if (Tok.is(tok::kw_template))
     return ParseTemplateTemplateParameter(Depth, Position);
 
@@ -609,6 +679,56 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
   return ParseNonTypeTemplateParameter(Depth, Position);
 }
 
+/// Check whether the current token is a template-id annotation denoting a
+/// type-constraint.
+bool Parser::isTypeConstraintAnnotation() {
+  if (Tok.isNot(tok::annot_template_id))
+    return false;
+  const auto *ExistingAnnot =
+      static_cast<TemplateIdAnnotation *>(Tok.getAnnotationValue());
+  return ExistingAnnot->Kind == TNK_Concept_template;
+}
+
+/// Try parsing a type-constraint construct at the current location, after the
+/// optional scope specifier.
+///
+///     type-constraint:
+///       nested-name-specifier[opt] concept-name
+///       nested-name-specifier[opt] concept-name
+///           '<' template-argument-list[opt] '>'[opt]
+///
+/// \returns true if an error occurred, and false otherwise.
+bool Parser::TryAnnotateTypeConstraint(CXXScopeSpec &SS) {
+  if (!getLangOpts().ConceptsTS || Tok.isNot(tok::identifier))
+    return false;
+
+  UnqualifiedId PossibleConceptName;
+  PossibleConceptName.setIdentifier(Tok.getIdentifierInfo(),
+                                    Tok.getLocation());
+
+  TemplateTy PossibleConcept;
+  bool MemberOfUnknownSpecialization = false;
+  auto TNK = Actions.isTemplateName(getCurScope(), SS,
+                                    /*hasTemplateKeyword=*/false,
+                                    PossibleConceptName,
+                                    /*ObjectType=*/ParsedType(),
+                                    /*EnteringContext=*/false,
+                                    PossibleConcept,
+                                    MemberOfUnknownSpecialization);
+  assert(!MemberOfUnknownSpecialization
+         && "Member when we only allowed namespace scope qualifiers??");
+  if (!PossibleConcept || TNK != TNK_Concept_template)
+    return false;
+
+  // At this point we're sure we're dealing with a constrained parameter. It
+  // may or may not have a template parameter list following the concept name.
+  return AnnotateTemplateIdToken(PossibleConcept, TNK, SS,
+                                 /*TemplateKWLoc=*/SourceLocation(),
+                                 PossibleConceptName,
+                                 /*AllowTypeAnnotation=*/false,
+                                 /*TypeConstraint=*/true);
+}
+
 /// ParseTypeParameter - Parse a template type parameter (C++ [temp.param]).
 /// Other kinds of template parameters are parsed in
 /// ParseTemplateTemplateParameter and ParseNonTypeTemplateParameter.
@@ -619,12 +739,25 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
 ///         'typename' ...[opt][C++0x] identifier[opt]
 ///         'typename' identifier[opt] '=' type-id
 NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
-  assert(Tok.isOneOf(tok::kw_class, tok::kw_typename) &&
-         "A type-parameter starts with 'class' or 'typename'");
-
-  // Consume the 'class' or 'typename' keyword.
-  bool TypenameKeyword = Tok.is(tok::kw_typename);
-  SourceLocation KeyLoc = ConsumeToken();
+  assert(Tok.isOneOf(tok::kw_class, tok::kw_typename, tok::annot_template_id) &&
+         "A type-parameter starts with 'class', 'typename' or a "
+         "type-constraint");
+
+  TemplateIdAnnotation *TypeConstraint = nullptr;
+  bool TypenameKeyword = false;
+  SourceLocation KeyLoc;
+  if (Tok.is(tok::annot_template_id)) {
+    // Consume the 'type-constraint'.
+    TypeConstraint =
+        static_cast<TemplateIdAnnotation *>(Tok.getAnnotationValue());
+    assert(TypeConstraint->Kind == TNK_Concept_template &&
+           "stray non-concept template-id annotation");
+    KeyLoc = ConsumeAnnotationToken();
+  } else {
+    // Consume the 'class' or 'typename' keyword.
+    TypenameKeyword = Tok.is(tok::kw_typename);
+    KeyLoc = ConsumeToken();
+  }
 
   // Grab the ellipsis (if given).
   SourceLocation EllipsisLoc;
@@ -664,9 +797,19 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
     DefaultArg = ParseTypeName(/*Range=*/nullptr,
                                DeclaratorContext::TemplateTypeArgContext).get();
 
-  return Actions.ActOnTypeParameter(getCurScope(), TypenameKeyword, EllipsisLoc,
-                                    KeyLoc, ParamName, NameLoc, Depth, Position,
-                                    EqualLoc, DefaultArg);
+  NamedDecl *NewDecl = Actions.ActOnTypeParameter(getCurScope(),
+                                                  TypenameKeyword, EllipsisLoc,
+                                                  KeyLoc, ParamName, NameLoc,
+                                                  Depth, Position, EqualLoc,
+                                                  DefaultArg,
+                                                  TypeConstraint != nullptr);
+
+  if (TypeConstraint)
+    Actions.ActOnTypeConstraint(TypeConstraint,
+                                cast<TemplateTypeParmDecl>(NewDecl),
+                                EllipsisLoc);
+
+  return NewDecl;
 }
 
 /// ParseTemplateTemplateParameter - Handle the parsing of template
@@ -829,7 +972,7 @@ Parser::ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position) {
 
   // Create the parameter.
   return Actions.ActOnNonTypeTemplateParameter(getCurScope(), ParamDecl,
-                                               Depth, Position, EqualLoc,
+                                               Depth, Position, EqualLoc, 
                                                DefaultArg.get());
 }
 
@@ -1105,6 +1248,10 @@ Parser::ParseTemplateIdAfterTemplateName(bool ConsumeLastToken,
 /// simple-template-id is always replaced with a template-id
 /// annotation token.
 ///
+/// \param TypeConstraint if true, then this is actually a type-constraint,
+/// meaning that the template argument list can be omitted (and the template in
+/// question must be a concept).
+///
 /// If an unrecoverable parse error occurs and no annotation token can be
 /// formed, this function returns true.
 ///
@@ -1112,10 +1259,15 @@ bool Parser::AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
                                      CXXScopeSpec &SS,
                                      SourceLocation TemplateKWLoc,
                                      UnqualifiedId &TemplateName,
-                                     bool AllowTypeAnnotation) {
+                                     bool AllowTypeAnnotation,
+                                     bool TypeConstraint) {
   assert(getLangOpts().CPlusPlus && "Can only annotate template-ids in C++");
-  assert(Template && Tok.is(tok::less) &&
+  assert(Template && (Tok.is(tok::less) || TypeConstraint) &&
          "Parser isn't at the beginning of a template-id");
+  assert(!(TypeConstraint && AllowTypeAnnotation) && "type-constraint can't be "
+                                                     "a type annotation");
+  assert((!TypeConstraint || TNK == TNK_Concept_template) && "type-constraint "
+         "must accompany a concept name");
 
   // Consume the template-name.
   SourceLocation TemplateNameLoc = TemplateName.getSourceRange().getBegin();
@@ -1123,17 +1275,19 @@ bool Parser::AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
   // Parse the enclosed template argument list.
   SourceLocation LAngleLoc, RAngleLoc;
   TemplateArgList TemplateArgs;
-  bool Invalid = ParseTemplateIdAfterTemplateName(false, LAngleLoc,
-                                                  TemplateArgs,
-                                                  RAngleLoc);
-
-  if (Invalid) {
-    // If we failed to parse the template ID but skipped ahead to a >, we're not
-    // going to be able to form a token annotation.  Eat the '>' if present.
-    TryConsumeToken(tok::greater);
-    // FIXME: Annotate the token stream so we don't produce the same errors
-    // again if we're doing this annotation as part of a tentative parse.
-    return true;
+  if (!TypeConstraint || Tok.is(tok::less)) {
+    bool Invalid = ParseTemplateIdAfterTemplateName(false, LAngleLoc,
+                                                    TemplateArgs,
+                                                    RAngleLoc);
+
+    if (Invalid) {
+      // If we failed to parse the template ID but skipped ahead to a >, we're not
+      // going to be able to form a token annotation.  Eat the '>' if present.
+      TryConsumeToken(tok::greater);
+      // FIXME: Annotate the token stream so we don't produce the same errors
+      // again if we're doing this annotation as part of a tentative parse.
+      return true;
+    }
   }
 
   ASTTemplateArgsPtr TemplateArgsPtr(TemplateArgs);
@@ -1371,8 +1525,9 @@ ParsedTemplateArgument Parser::ParseTemplateArgument() {
   // Parse a non-type template argument.
   SourceLocation Loc = Tok.getLocation();
   ExprResult ExprArg = ParseConstantExpressionInExprEvalContext(MaybeTypeCast);
-  if (ExprArg.isInvalid() || !ExprArg.get())
+  if (ExprArg.isInvalid() || !ExprArg.get()) {
     return ParsedTemplateArgument();
+  }
 
   return ParsedTemplateArgument(ParsedTemplateArgument::NonType,
                                 ExprArg.get(), Loc);
diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index e14c63d557a..7260977c634 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -2986,7 +2986,11 @@ static void AddTemplateParameterChunks(
     if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(*P)) {
       if (TTP->wasDeclaredWithTypename())
         PlaceholderStr = "typename";
-      else
+      else if (const auto *TC = TTP->getTypeConstraint()) {
+        llvm::raw_string_ostream OS(PlaceholderStr);
+        TC->print(OS, Policy);
+        OS.flush();
+      } else
         PlaceholderStr = "class";
 
       if (TTP->getIdentifier()) {
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 6f22326e8fc..1184446796e 100755
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -976,20 +976,23 @@ ParsedTemplateArgument Sema::ActOnTemplateTypeArgument(TypeResult ParsedType) {
 /// If the type parameter has a default argument, it will be added
 /// later via ActOnTypeParameterDefault.
 NamedDecl *Sema::ActOnTypeParameter(Scope *S, bool Typename,
-                               SourceLocation EllipsisLoc,
-                               SourceLocation KeyLoc,
-                               IdentifierInfo *ParamName,
-                               SourceLocation ParamNameLoc,
-                               unsigned Depth, unsigned Position,
-                               SourceLocation EqualLoc,
-                               ParsedType DefaultArg) {
+                                    SourceLocation EllipsisLoc,
+                                    SourceLocation KeyLoc,
+                                    IdentifierInfo *ParamName,
+                                    SourceLocation ParamNameLoc,
+                                    unsigned Depth, unsigned Position,
+                                    SourceLocation EqualLoc,
+                                    ParsedType DefaultArg,
+                                    bool HasTypeConstraint) {
   assert(S->isTemplateParamScope() &&
          "Template type parameter not in template parameter scope!");
 
   bool IsParameterPack = EllipsisLoc.isValid();
-  TemplateTypeParmDecl *Param = TemplateTypeParmDecl::Create(
-      Context, Context.getTranslationUnitDecl(), KeyLoc, ParamNameLoc, Depth,
-      Position, ParamName, Typename, IsParameterPack);
+  TemplateTypeParmDecl *Param
+    = TemplateTypeParmDecl::Create(Context, Context.getTranslationUnitDecl(),
+                                   KeyLoc, ParamNameLoc, Depth, Position,
+                                   ParamName, Typename, IsParameterPack,
+                                   HasTypeConstraint);
   Param->setAccess(AS_public);
 
   if (Param->isParameterPack())
@@ -1036,6 +1039,125 @@ NamedDecl *Sema::ActOnTypeParameter(Scope *S, bool Typename,
   return Param;
 }
 
+/// Convert the parser's template argument list representation into our form.
+static TemplateArgumentListInfo
+makeTemplateArgumentListInfo(Sema &S, TemplateIdAnnotation &TemplateId) {
+  TemplateArgumentListInfo TemplateArgs(TemplateId.LAngleLoc,
+                                        TemplateId.RAngleLoc);
+  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId.getTemplateArgs(),
+                                     TemplateId.NumArgs);
+  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgs);
+  return TemplateArgs;
+}
+
+bool Sema::ActOnTypeConstraint(TemplateIdAnnotation *TypeConstr,
+                               TemplateTypeParmDecl *ConstrainedParameter,
+                               SourceLocation EllipsisLoc) {
+  ConceptDecl *CD =
+      cast<ConceptDecl>(TypeConstr->Template.get().getAsTemplateDecl());
+
+  // C++2a [temp.param]p4:
+  //     [...] The concept designated by a type-constraint shall be a type
+  //     concept ([temp.concept]).
+  if (!CD->isTypeConcept()) {
+    Diag(TypeConstr->TemplateNameLoc,
+         diag::err_type_constraint_non_type_concept);
+    return true;
+  }
+
+  bool WereArgsSpecified = TypeConstr->LAngleLoc.isValid();
+
+  if (!WereArgsSpecified &&
+      CD->getTemplateParameters()->getMinRequiredArguments() > 1) {
+    Diag(TypeConstr->TemplateNameLoc,
+         diag::err_type_constraint_missing_arguments) << CD;
+    return true;
+  }
+
+  TemplateArgumentListInfo TemplateArgs;
+  if (TypeConstr->LAngleLoc.isValid()) {
+    TemplateArgs =
+        makeTemplateArgumentListInfo(*this, *TypeConstr);
+  }
+  return AttachTypeConstraint(
+      TypeConstr->SS.isSet() ? TypeConstr->SS.getWithLocInContext(Context) :
+      NestedNameSpecifierLoc(),
+      DeclarationNameInfo(DeclarationName(TypeConstr->Name),
+                          TypeConstr->TemplateNameLoc), CD,
+      TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
+      ConstrainedParameter, EllipsisLoc);
+}
+
+/// Attach a type-constraint to a template parameter.
+/// \returns true if an error occured. This can happen if the
+/// immediately-declared constraint could not be formed (e.g. incorrect number
+/// of arguments for the named concept).
+bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
+                                DeclarationNameInfo NameInfo,
+                                ConceptDecl *NamedConcept,
+                                const TemplateArgumentListInfo *TemplateArgs,
+                                TemplateTypeParmDecl *ConstrainedParameter,
+                                SourceLocation EllipsisLoc) {
+  // C++2a [temp.param]p4:
+  //     [...] If Q is of the form C<A1, ..., An>, then let E' be
+  //     C<T, A1, ..., An>. Otherwise, let E' be C<T>. [...]
+  const ASTTemplateArgumentListInfo *ArgsAsWritten =
+    TemplateArgs ? ASTTemplateArgumentListInfo::Create(Context,
+                                                       *TemplateArgs) : nullptr;
+
+  QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
+  TemplateArgumentListInfo ConstraintArgs;
+  ConstraintArgs.addArgument(
+    TemplateArgumentLoc(
+        TemplateArgument(ParamAsArgument),
+        TemplateArgumentLocInfo(
+            Context.getTrivialTypeSourceInfo(ParamAsArgument,
+                ConstrainedParameter->getLocation()))));
+  if (TemplateArgs) {
+    ConstraintArgs.setRAngleLoc(TemplateArgs->getRAngleLoc());
+    ConstraintArgs.setLAngleLoc(TemplateArgs->getLAngleLoc());
+    for (const TemplateArgumentLoc &ArgLoc : TemplateArgs->arguments())
+      ConstraintArgs.addArgument(ArgLoc);
+  }
+
+  // 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,
+                                          ImmediatelyDeclaredConstraint.get());
+  return false;
+}
+
 /// Check that the type of a non-type template parameter is
 /// well-formed.
 ///
@@ -1868,7 +1990,23 @@ private:
           SemaRef.Context, DC, TTP->getBeginLoc(), TTP->getLocation(),
           /*Depth*/ 0, Depth1IndexAdjustment + TTP->getIndex(),
           TTP->getIdentifier(), TTP->wasDeclaredWithTypename(),
-          TTP->isParameterPack());
+          TTP->isParameterPack(), TTP->hasTypeConstraint(),
+          TTP->isExpandedParameterPack() ?
+          llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
+      if (const auto *TC = TTP->getTypeConstraint()) {
+        TemplateArgumentListInfo TransformedArgs;
+        const auto *ArgsAsWritten = TC->getTemplateArgsAsWritten();
+        if (SemaRef.Subst(ArgsAsWritten->getTemplateArgs(),
+                          ArgsAsWritten->NumTemplateArgs, TransformedArgs,
+                          Args))
+          SemaRef.AttachTypeConstraint(
+              TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
+              TC->getNamedConcept(), &TransformedArgs, NewTTP,
+              NewTTP->isParameterPack()
+                 ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
+                     ->getEllipsisLoc()
+                 : SourceLocation());
+      }
       if (TTP->hasDefaultArgument()) {
         TypeSourceInfo *InstantiatedDefaultArg =
             SemaRef.SubstType(TTP->getDefaultArgumentInfo(), Args,
@@ -2177,6 +2315,17 @@ static bool DiagnoseUnexpandedParameterPacks(Sema &S,
   TemplateParameterList *Params = TTP->getTemplateParameters();
   for (unsigned I = 0, N = Params->size(); I != N; ++I) {
     NamedDecl *P = Params->getParam(I);
+    if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
+      if (!TTP->isParameterPack())
+        if (const TypeConstraint *TC = TTP->getTypeConstraint())
+          if (TC->hasExplicitTemplateArgs())
+            for (auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
+              if (S.DiagnoseUnexpandedParameterPack(ArgLoc,
+                                                    Sema::UPPC_TypeConstraint))
+                return true;
+      continue;
+    }
+
     if (NonTypeTemplateParmDecl *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
       if (!NTTP->isParameterPack() &&
           S.DiagnoseUnexpandedParameterPack(NTTP->getLocation(),
@@ -3711,17 +3860,6 @@ static bool isSameAsPrimaryTemplate(TemplateParameterList *Params,
   return true;
 }
 
-/// Convert the parser's template argument list representation into our form.
-static TemplateArgumentListInfo
-makeTemplateArgumentListInfo(Sema &S, TemplateIdAnnotation &TemplateId) {
-  TemplateArgumentListInfo TemplateArgs(TemplateId.LAngleLoc,
-                                        TemplateId.RAngleLoc);
-  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId.getTemplateArgs(),
-                                     TemplateId.NumArgs);
-  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgs);
-  return TemplateArgs;
-}
-
 template<typename PartialSpecDecl>
 static void checkMoreSpecializedThanPrimary(Sema &S, PartialSpecDecl *Partial) {
   if (Partial->getDeclContext()->isDependentContext())
@@ -4233,14 +4371,14 @@ void Sema::diagnoseMissingTemplateArguments(TemplateName Name,
 ExprResult
 Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
                              SourceLocation TemplateKWLoc,
-                             SourceLocation ConceptNameLoc,
+                             const DeclarationNameInfo &ConceptNameInfo,
                              NamedDecl *FoundDecl,
                              ConceptDecl *NamedConcept,
                              const TemplateArgumentListInfo *TemplateArgs) {
   assert(NamedConcept && "A concept template id without a template?");
 
   llvm::SmallVector<TemplateArgument, 4> Converted;
-  if (CheckTemplateArgumentList(NamedConcept, ConceptNameLoc,
+  if (CheckTemplateArgumentList(NamedConcept, ConceptNameInfo.getLoc(),
                            const_cast<TemplateArgumentListInfo&>(*TemplateArgs),
                                 /*PartialTemplateArgs=*/false, Converted,
                                 /*UpdateArgsWithConversion=*/false))
@@ -4259,14 +4397,14 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
                                   {NamedConcept->getConstraintExpr()},
                                   Converted,
                                   SourceRange(SS.isSet() ? SS.getBeginLoc() :
-                                                           ConceptNameLoc,
-                                              TemplateArgs->getRAngleLoc()),
-                                  Satisfaction))
+                                                       ConceptNameInfo.getLoc(),
+                                                TemplateArgs->getRAngleLoc()),
+                                    Satisfaction))
       return ExprError();
 
   return ConceptSpecializationExpr::Create(Context,
       SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc{},
-      TemplateKWLoc, ConceptNameLoc, FoundDecl, NamedConcept,
+      TemplateKWLoc, ConceptNameInfo, FoundDecl, NamedConcept,
       ASTTemplateArgumentListInfo::Create(Context, *TemplateArgs), Converted,
       AreArgsDependent ? nullptr : &Satisfaction);
 }
@@ -4312,8 +4450,7 @@ ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
   }
 
   if (R.getAsSingle<ConceptDecl>()) {
-    return CheckConceptTemplateId(SS, TemplateKWLoc,
-                                  R.getLookupNameInfo().getBeginLoc(),
+    return CheckConceptTemplateId(SS, TemplateKWLoc, R.getLookupNameInfo(),
                                   R.getFoundDecl(),
                                   R.getAsSingle<ConceptDecl>(), TemplateArgs);
   }
@@ -5116,7 +5253,7 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
 
   case TemplateArgument::Template:
   case TemplateArgument::TemplateExpansion:
-    if (CheckTemplateTemplateArgument(Params, Arg))
+    if (CheckTemplateTemplateArgument(TempParm, Params, Arg))
       return true;
 
     Converted.push_back(Arg.getArgument());
@@ -5156,6 +5293,12 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
 /// In \c A<int,int>::B, \c NTs and \c TTs have expanded pack size 2, and \c Us
 /// is not a pack expansion, so returns an empty Optional.
 static Optional<unsigned> getExpandedPackSize(NamedDecl *Param) {
+  if (TemplateTypeParmDecl *TTP
+        = dyn_cast<TemplateTypeParmDecl>(Param)) {
+    if (TTP->isExpandedParameterPack())
+      return TTP->getNumExpansionParameters();
+  }
+
   if (NonTypeTemplateParmDecl *NTTP
         = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
     if (NTTP->isExpandedParameterPack())
@@ -6894,7 +7037,8 @@ static void DiagnoseTemplateParameterListArityMismatch(
 ///
 /// This routine implements the semantics of C++ [temp.arg.template].
 /// It returns true if an error occurred, and false otherwise.
-bool Sema::CheckTemplateTemplateArgument(TemplateParameterList *Params,
+bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
+                                         TemplateParameterList *Params,
                                          TemplateArgumentLoc &Arg) {
   TemplateName Name = Arg.getArgument().getAsTemplateOrTemplatePattern();
   TemplateDecl *Template = Name.getAsTemplateDecl();
@@ -6933,6 +7077,9 @@ bool Sema::CheckTemplateTemplateArgument(TemplateParameterList *Params,
   // C++1z [temp.arg.template]p3: (DR 150)
   //   A template-argument matches a template template-parameter P when P
   //   is at least as specialized as the template-argument A.
+  // FIXME: We should enable RelaxedTemplateTemplateArgs by default as it is a
+  //  defect report resolution from C++17 and shouldn't be introduced by
+  //  concepts.
   if (getLangOpts().RelaxedTemplateTemplateArgs) {
     // Quick check for the common case:
     //   If P contains a parameter pack, then A [...] matches P if each of A's
@@ -6940,12 +7087,39 @@ bool Sema::CheckTemplateTemplateArgument(TemplateParameterList *Params,
     //   the template-parameter-list of P.
     if (TemplateParameterListsAreEqual(
             Template->getTemplateParameters(), Params, false,
-            TPL_TemplateTemplateArgumentMatch, Arg.getLocation()))
+            TPL_TemplateTemplateArgumentMatch, Arg.getLocation()) &&
+        // If the argument has no associated constraints, then the parameter is
+        // definitely at least as specialized as the argument.
+        // Otherwise - we need a more thorough check.
+        !Template->hasAssociatedConstraints())
       return false;
 
     if (isTemplateTemplateParameterAtLeastAsSpecializedAs(Params, Template,
-                                                          Arg.getLocation()))
+                                                          Arg.getLocation())) {
+      // C++2a[temp.func.order]p2
+      //   [...] If both deductions succeed, the partial ordering selects the
+      //   more constrained template as described by the rules in
+      //   [temp.constr.order].
+      SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
+      Params->getAssociatedConstraints(ParamsAC);
+      Template->getAssociatedConstraints(TemplateAC);
+      bool IsParamAtLeastAsConstrained;
+      if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
+                                 IsParamAtLeastAsConstrained))
+        return true;
+      if (!IsParamAtLeastAsConstrained) {
+        Diag(Arg.getLocation(),
+             diag::err_template_template_parameter_not_at_least_as_constrained)
+            << Template << Param << Arg.getSourceRange();
+        Diag(Param->getLocation(), diag::note_entity_declared_at) << Param;
+        Diag(Template->getLocation(), diag::note_entity_declared_at)
+            << Template;
+        MaybeEmitAmbiguousAtomicConstraintsDiagnostic(Param, ParamsAC, Template,
+                                                      TemplateAC);
+        return true;
+      }
       return false;
+    }
     // FIXME: Produce better diagnostics for deduction failures.
   }
 
@@ -7123,7 +7297,6 @@ static bool MatchTemplateParameterKind(Sema &S, NamedDecl *New, NamedDecl *Old,
                                        bool Complain,
                                      Sema::TemplateParameterListEqualKind Kind,
                                        SourceLocation TemplateArgLoc) {
-  // TODO: Concepts: Check constrained-parameter constraints here.
   // Check the actual kind (type, non-type, template).
   if (Old->getKind() != New->getKind()) {
     if (Complain) {
@@ -7177,50 +7350,73 @@ static bool MatchTemplateParameterKind(Sema &S, NamedDecl *New, NamedDecl *Old,
     // template parameter and one of the non-type template parameter types
     // is dependent, then we must wait until template instantiation time
     // to actually compare the arguments.
-    if (Kind == Sema::TPL_TemplateTemplateArgumentMatch &&
-        (OldNTTP->getType()->isDependentType() ||
-         NewNTTP->getType()->isDependentType()))
-      return true;
-
-    if (!S.Context.hasSameType(OldNTTP->getType(), NewNTTP->getType())) {
-      if (Complain) {
-        unsigned NextDiag = diag::err_template_nontype_parm_different_type;
-        if (TemplateArgLoc.isValid()) {
-          S.Diag(TemplateArgLoc,
-                 diag::err_template_arg_template_params_mismatch);
-          NextDiag = diag::note_template_nontype_parm_different_type;
+    if (Kind != Sema::TPL_TemplateTemplateArgumentMatch ||
+        (!OldNTTP->getType()->isDependentType() &&
+         !NewNTTP->getType()->isDependentType()))
+      if (!S.Context.hasSameType(OldNTTP->getType(), NewNTTP->getType())) {
+        if (Complain) {
+          unsigned NextDiag = diag::err_template_nontype_parm_different_type;
+          if (TemplateArgLoc.isValid()) {
+            S.Diag(TemplateArgLoc,
+                   diag::err_template_arg_template_params_mismatch);
+            NextDiag = diag::note_template_nontype_parm_different_type;
+          }
+          S.Diag(NewNTTP->getLocation(), NextDiag)
+            << NewNTTP->getType()
+            << (Kind != Sema::TPL_TemplateMatch);
+          S.Diag(OldNTTP->getLocation(),
+                 diag::note_template_nontype_parm_prev_declaration)
+            << OldNTTP->getType();
         }
-        S.Diag(NewNTTP->getLocation(), NextDiag)
-          << NewNTTP->getType()
-          << (Kind != Sema::TPL_TemplateMatch);
-        S.Diag(OldNTTP->getLocation(),
-               diag::note_template_nontype_parm_prev_declaration)
-          << OldNTTP->getType();
-      }
 
-      return false;
-    }
-
-    return true;
+        return false;
+      }
   }
-
   // For template template parameters, check the template parameter types.
   // The template parameter lists of template template
   // parameters must agree.
-  if (TemplateTemplateParmDecl *OldTTP
+  else if (TemplateTemplateParmDecl *OldTTP
                                     = dyn_cast<TemplateTemplateParmDecl>(Old)) {
     TemplateTemplateParmDecl *NewTTP = cast<TemplateTemplateParmDecl>(New);
-    return S.TemplateParameterListsAreEqual(NewTTP->getTemplateParameters(),
-                                            OldTTP->getTemplateParameters(),
-                                            Complain,
+    if (!S.TemplateParameterListsAreEqual(NewTTP->getTemplateParameters(),
+                                          OldTTP->getTemplateParameters(),
+                                          Complain,
                                         (Kind == Sema::TPL_TemplateMatch
                                            ? Sema::TPL_TemplateTemplateParmMatch
                                            : Kind),
-                                            TemplateArgLoc);
-  }
+                                          TemplateArgLoc))
+      return false;
+  } else if (Kind != Sema::TPL_TemplateTemplateArgumentMatch) {
+    const Expr *NewC = nullptr, *OldC = nullptr;
+    if (const auto *TC = cast<TemplateTypeParmDecl>(New)->getTypeConstraint())
+      NewC = TC->getImmediatelyDeclaredConstraint();
+    if (const auto *TC = cast<TemplateTypeParmDecl>(Old)->getTypeConstraint())
+      OldC = TC->getImmediatelyDeclaredConstraint();
+
+    auto Diagnose = [&] {
+      S.Diag(NewC ? NewC->getBeginLoc() : New->getBeginLoc(),
+           diag::err_template_different_type_constraint);
+      S.Diag(OldC ? OldC->getBeginLoc() : Old->getBeginLoc(),
+           diag::note_template_prev_declaration) << /*declaration*/0;
+    };
 
-  // TODO: Concepts: Match immediately-introduced-constraint for type
-  // constraints
+    if (!NewC != !OldC) {
+      if (Complain)
+        Diagnose();
+      return false;
+    }
+
+    if (NewC) {
+      llvm::FoldingSetNodeID OldCID, NewCID;
+      OldC->Profile(OldCID, S.Context, /*Canonical=*/true);
+      NewC->Profile(NewCID, S.Context, /*Canonical=*/true);
+      if (OldCID != NewCID) {
+        if (Complain)
+          Diagnose();
+        return false;
+      }
+    }
+  }
 
   return true;
 }
@@ -7247,15 +7443,6 @@ void DiagnoseTemplateParameterListArityMismatch(Sema &S,
     << SourceRange(Old->getTemplateLoc(), Old->getRAngleLoc());
 }
 
-static void
-DiagnoseTemplateParameterListRequiresClauseMismatch(Sema &S,
-                                                    TemplateParameterList *New,
-                                                    TemplateParameterList *Old){
-  S.Diag(New->getTemplateLoc(), diag::err_template_different_requires_clause);
-  S.Diag(Old->getTemplateLoc(),  diag::note_template_prev_declaration)
-      << /*declaration*/0;
-}
-
 /// Determine whether the given template parameter lists are
 /// equivalent.
 ///
@@ -7348,9 +7535,17 @@ Sema::TemplateParameterListsAreEqual(TemplateParameterList *New,
   if (Kind != TPL_TemplateTemplateArgumentMatch) {
     const Expr *NewRC = New->getRequiresClause();
     const Expr *OldRC = Old->getRequiresClause();
+
+    auto Diagnose = [&] {
+      Diag(NewRC ? NewRC->getBeginLoc() : New->getTemplateLoc(),
+           diag::err_template_different_requires_clause);
+      Diag(OldRC ? OldRC->getBeginLoc() : Old->getTemplateLoc(),
+           diag::note_template_prev_declaration) << /*declaration*/0;
+    };
+
     if (!NewRC != !OldRC) {
       if (Complain)
-        DiagnoseTemplateParameterListRequiresClauseMismatch(*this, New, Old);
+        Diagnose();
       return false;
     }
 
@@ -7360,7 +7555,7 @@ Sema::TemplateParameterListsAreEqual(TemplateParameterList *New,
       NewRC->Profile(NewRCID, Context, /*Canonical=*/true);
       if (OldRCID != NewRCID) {
         if (Complain)
-          DiagnoseTemplateParameterListRequiresClauseMismatch(*this, New, Old);
+          Diagnose();
         return false;
       }
     }
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index e626948cb5d..1b9f1b2144d 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -643,6 +643,10 @@ static TemplateParameter makeTemplateParameter(Decl *D) {
 
 /// If \p Param is an expanded parameter pack, get the number of expansions.
 static Optional<unsigned> getExpandedPackSize(NamedDecl *Param) {
+  if (auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
+    if (TTP->isExpandedParameterPack())
+      return TTP->getNumExpansionParameters();
+
   if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param))
     if (NTTP->isExpandedParameterPack())
       return NTTP->getNumExpansionTypes();
@@ -4542,11 +4546,12 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
 
   // Build template<class TemplParam> void Func(FuncParam);
   TemplateTypeParmDecl *TemplParam = TemplateTypeParmDecl::Create(
-      Context, nullptr, SourceLocation(), Loc, Depth, 0, nullptr, false, false);
+      Context, nullptr, SourceLocation(), Loc, Depth, 0, nullptr, false, false,
+      false);
   QualType TemplArg = QualType(TemplParam->getTypeForDecl(), 0);
   NamedDecl *TemplParamPtr = TemplParam;
   FixedSizeTemplateParameterListStorage<1, false> TemplateParamsSt(
-      Loc, Loc, TemplParamPtr, Loc, nullptr);
+      Context, Loc, Loc, TemplParamPtr, Loc, nullptr);
 
   QualType FuncParam =
       SubstituteDeducedTypeTransform(*this, TemplArg, /*UseTypeSugar*/false)
@@ -5276,9 +5281,8 @@ Sema::getMoreSpecializedPartialSpecialization(
       return nullptr;
     if (IsAtLeastAsConstrained(PS2, AC2, PS1, AC1, AtLeastAsConstrained2))
       return nullptr;
-    if (AtLeastAsConstrained1 == AtLeastAsConstrained2) {
+    if (AtLeastAsConstrained1 == AtLeastAsConstrained2)
       return nullptr;
-    }
     return AtLeastAsConstrained1 ? PS1 : PS2;
   }
 
@@ -5352,7 +5356,8 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
     SFINAETrap Trap(*this);
 
     Context.getInjectedTemplateArgs(P, PArgs);
-    TemplateArgumentListInfo PArgList(P->getLAngleLoc(), P->getRAngleLoc());
+    TemplateArgumentListInfo PArgList(P->getLAngleLoc(),
+                                      P->getRAngleLoc());
     for (unsigned I = 0, N = P->size(); I != N; ++I) {
       // Unwrap packs that getInjectedTemplateArgs wrapped around pack
       // expansions, to form an "as written" argument list.
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index ce94c036eb1..64500d0a26d 100755
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2479,13 +2479,68 @@ Decl *TemplateDeclInstantiator::VisitTemplateTypeParmDecl(
   // TODO: don't always clone when decls are refcounted.
   assert(D->getTypeForDecl()->isTemplateTypeParmType());
 
+  Optional<unsigned> NumExpanded;
+
+  if (const TypeConstraint *TC = D->getTypeConstraint()) {
+    if (D->isPackExpansion() && !D->isExpandedParameterPack()) {
+      assert(TC->getTemplateArgsAsWritten() &&
+             "type parameter can only be an expansion when explicit arguments "
+             "are specified");
+      // The template type parameter pack's type is a pack expansion of types.
+      // Determine whether we need to expand this parameter pack into separate
+      // types.
+      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+      for (auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
+        SemaRef.collectUnexpandedParameterPacks(ArgLoc, Unexpanded);
+
+      // Determine whether the set of unexpanded parameter packs can and should
+      // be expanded.
+      bool Expand = true;
+      bool RetainExpansion = false;
+      if (SemaRef.CheckParameterPacksForExpansion(
+              cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
+                  ->getEllipsisLoc(),
+              SourceRange(TC->getConceptNameLoc(),
+                          TC->hasExplicitTemplateArgs() ?
+                          TC->getTemplateArgsAsWritten()->getRAngleLoc() :
+                          TC->getConceptNameInfo().getEndLoc()),
+              Unexpanded, TemplateArgs, Expand, RetainExpansion, NumExpanded))
+        return nullptr;
+    }
+  }
+
   TemplateTypeParmDecl *Inst = TemplateTypeParmDecl::Create(
       SemaRef.Context, Owner, D->getBeginLoc(), D->getLocation(),
       D->getDepth() - TemplateArgs.getNumSubstitutedLevels(), D->getIndex(),
-      D->getIdentifier(), D->wasDeclaredWithTypename(), D->isParameterPack());
+      D->getIdentifier(), D->wasDeclaredWithTypename(), D->isParameterPack(),
+      D->hasTypeConstraint(), NumExpanded);
+
   Inst->setAccess(AS_public);
   Inst->setImplicit(D->isImplicit());
-
+  if (auto *TC = D->getTypeConstraint()) {
+    // TODO: Concepts: do not instantiate the constraint (delayed constraint
+    // substitution)
+    const ASTTemplateArgumentListInfo *TemplArgInfo
+      = TC->getTemplateArgsAsWritten();
+    TemplateArgumentListInfo InstArgs;
+
+    if (TemplArgInfo) {
+      InstArgs.setLAngleLoc(TemplArgInfo->LAngleLoc);
+      InstArgs.setRAngleLoc(TemplArgInfo->RAngleLoc);
+      if (SemaRef.Subst(TemplArgInfo->getTemplateArgs(),
+                        TemplArgInfo->NumTemplateArgs,
+                        InstArgs, TemplateArgs))
+        return nullptr;
+    }
+    if (SemaRef.AttachTypeConstraint(
+            TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
+            TC->getNamedConcept(), &InstArgs, Inst,
+            D->isParameterPack()
+                ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
+                    ->getEllipsisLoc()
+                : SourceLocation()))
+      return nullptr;
+  }
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
     TypeSourceInfo *InstantiatedDefaultArg =
         SemaRef.SubstType(D->getDefaultArgumentInfo(), TemplateArgs,
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index d77542be53f..3884fdae8fe 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -3016,7 +3016,8 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
                 SemaRef.Context, SemaRef.Context.getTranslationUnitDecl(),
                 /*KeyLoc*/ SourceLocation(), /*NameLoc*/ D.getBeginLoc(),
                 TemplateParameterDepth, AutoParameterPosition,
-                /*Identifier*/ nullptr, false, IsParameterPack);
+                /*Identifier*/ nullptr, false, IsParameterPack,
+                /*HasTypeConstraint=*/false);
         CorrespondingTemplateParam->setImplicit();
         LSI->TemplateParams.push_back(CorrespondingTemplateParam);
         // Replace the 'auto' in the function parameter with this invented
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 1d775dab67b..3b827fbc950 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -3064,13 +3064,13 @@ public:
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
   ExprResult RebuildConceptSpecializationExpr(NestedNameSpecifierLoc NNS,
-      SourceLocation TemplateKWLoc, SourceLocation ConceptNameLoc,
+      SourceLocation TemplateKWLoc, DeclarationNameInfo ConceptNameInfo,
       NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
       TemplateArgumentListInfo *TALI) {
     CXXScopeSpec SS;
     SS.Adopt(NNS);
     ExprResult Result = getSema().CheckConceptTemplateId(SS, TemplateKWLoc,
-                                                         ConceptNameLoc,
+                                                         ConceptNameInfo,
                                                          FoundDecl,
                                                          NamedConcept, TALI);
     if (Result.isInvalid())
@@ -11175,7 +11175,7 @@ TreeTransform<Derived>::TransformConceptSpecializationExpr(
 
   return getDerived().RebuildConceptSpecializationExpr(
       E->getNestedNameSpecifierLoc(), E->getTemplateKWLoc(),
-      E->getConceptNameLoc(), E->getFoundDecl(), E->getNamedConcept(),
+      E->getConceptNameInfo(), E->getFoundDecl(), E->getNamedConcept(),
       &TransArgs);
 }
 
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 3bcc9ad5805..96a7d5ae0a3 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2313,7 +2313,20 @@ void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
 
   D->setDeclaredWithTypename(Record.readInt());
 
-  // TODO: Concepts: Immediately introduced constraint
+  if (Record.readInt()) {
+    NestedNameSpecifierLoc NNS = Record.readNestedNameSpecifierLoc();
+    DeclarationNameInfo DN = Record.readDeclarationNameInfo();
+    ConceptDecl *NamedConcept = cast<ConceptDecl>(Record.readDecl());
+    const ASTTemplateArgumentListInfo *ArgsAsWritten = nullptr;
+    if (Record.readInt())
+        ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
+    Expr *ImmediatelyDeclaredConstraint = Record.readExpr();
+    D->setTypeConstraint(NNS, DN, /*FoundDecl=*/nullptr, NamedConcept,
+                         ArgsAsWritten, ImmediatelyDeclaredConstraint);
+    if ((D->ExpandedParameterPack = Record.readInt()))
+      D->NumExpanded = Record.readInt();
+  }
+
   if (Record.readInt())
     D->setDefaultArgument(readTypeSourceInfo());
 }
@@ -3800,9 +3813,12 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
   case DECL_FUNCTION_TEMPLATE:
     D = FunctionTemplateDecl::CreateDeserialized(Context, ID);
     break;
-  case DECL_TEMPLATE_TYPE_PARM:
-    D = TemplateTypeParmDecl::CreateDeserialized(Context, ID);
+  case DECL_TEMPLATE_TYPE_PARM: {
+    bool HasTypeConstraint = Record.readInt();
+    D = TemplateTypeParmDecl::CreateDeserialized(Context, ID,
+                                                 HasTypeConstraint);
     break;
+  }
   case DECL_NON_TYPE_TEMPLATE_PARM:
     D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID);
     break;
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index 533502116ed..f558c26b5f1 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -730,15 +730,13 @@ void ASTStmtReader::VisitConceptSpecializationExpr(
   unsigned NumTemplateArgs = Record.readInt();
   E->NestedNameSpec = Record.readNestedNameSpecifierLoc();
   E->TemplateKWLoc = Record.readSourceLocation();
-  E->ConceptNameLoc = Record.readSourceLocation();
-  E->FoundDecl = readDeclAs<NamedDecl>();
+  E->ConceptName = Record.readDeclarationNameInfo();
   E->NamedConcept = readDeclAs<ConceptDecl>();
-  const ASTTemplateArgumentListInfo *ArgsAsWritten =
-      Record.readASTTemplateArgumentListInfo();
+  E->ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
   llvm::SmallVector<TemplateArgument, 4> Args;
   for (unsigned I = 0; I < NumTemplateArgs; ++I)
     Args.push_back(Record.readTemplateArgument());
-  E->setTemplateArguments(ArgsAsWritten, Args);
+  E->setTemplateArguments(Args);
   ConstraintSatisfaction Satisfaction;
   Satisfaction.IsSatisfied = Record.readInt();
   if (!Satisfaction.IsSatisfied) {
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index 6b8a9d9719d..b2a8c118d40 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1637,10 +1637,26 @@ void ASTDeclWriter::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
 }
 
 void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
+  Record.push_back(D->hasTypeConstraint());
   VisitTypeDecl(D);
 
   Record.push_back(D->wasDeclaredWithTypename());
-  // TODO: Concepts - constrained parameters.
+
+  const TypeConstraint *TC = D->getTypeConstraint();
+  Record.push_back(TC != nullptr);
+  if (TC) {
+    Record.AddNestedNameSpecifierLoc(TC->getNestedNameSpecifierLoc());
+    Record.AddDeclarationNameInfo(TC->getConceptNameInfo());
+    Record.AddDeclRef(TC->getNamedConcept());
+    Record.push_back(TC->getTemplateArgsAsWritten() != nullptr);
+    if (TC->getTemplateArgsAsWritten())
+      Record.AddASTTemplateArgumentListInfo(TC->getTemplateArgsAsWritten());
+    Record.AddStmt(TC->getImmediatelyDeclaredConstraint());
+    Record.push_back(D->isExpandedParameterPack());
+    if (D->isExpandedParameterPack())
+      Record.push_back(D->getNumExpansionParameters());
+  }
+
   bool OwnsDefaultArg = D->hasDefaultArgument() &&
                         !D->defaultArgumentWasInherited();
   Record.push_back(OwnsDefaultArg);
@@ -1670,7 +1686,6 @@ void ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
 
     Code = serialization::DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK;
   } else {
-    // TODO: Concepts - constrained parameters.
     // Rest of NonTypeTemplateParmDecl.
     Record.push_back(D->isParameterPack());
     bool OwnsDefaultArg = D->hasDefaultArgument() &&
@@ -1700,7 +1715,6 @@ void ASTDeclWriter::VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D) {
       Record.AddTemplateParameterList(D->getExpansionTemplateParameters(I));
     Code = serialization::DECL_EXPANDED_TEMPLATE_TEMPLATE_PARM_PACK;
   } else {
-    // TODO: Concepts - constrained parameters.
     // Rest of TemplateTemplateParmDecl.
     Record.push_back(D->isParameterPack());
     bool OwnsDefaultArg = D->hasDefaultArgument() &&
diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index e671c78fe97..9231f3b2b9b 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -395,8 +395,7 @@ void ASTStmtWriter::VisitConceptSpecializationExpr(
   Record.push_back(TemplateArgs.size());
   Record.AddNestedNameSpecifierLoc(E->getNestedNameSpecifierLoc());
   Record.AddSourceLocation(E->getTemplateKWLoc());
-  Record.AddSourceLocation(E->getConceptNameLoc());
-  Record.AddDeclRef(E->getFoundDecl());
+  Record.AddDeclarationNameInfo(E->getConceptNameInfo());
   Record.AddDeclRef(E->getNamedConcept());
   Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
   for (const TemplateArgument &Arg : TemplateArgs)
diff --git a/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp b/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp
new file mode 100644
index 00000000000..593336163fa
--- /dev/null
+++ b/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp
@@ -0,0 +1,34 @@
+// RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -frelaxed-template-template-args -verify %s
+
+template<typename T> concept C = T::f();
+// expected-note@-1{{similar constraint}}
+template<typename T> concept D = C<T> && T::g();
+template<typename T> concept F = T::f();
+// expected-note@-1{{similar constraint expressions not considered equivalent}}
+template<template<C> class P> struct S1 { }; // expected-note 2{{'P' declared here}}
+
+template<C> struct X { }; // expected-note{{'X' declared here}}
+
+template<D> struct Y { }; // expected-note 2{{'Y' declared here}}
+template<typename T> struct Z { };
+template<F> struct W { }; // expected-note{{'W' declared here}}
+
+S1<X> s11;
+S1<Y> s12; // expected-error{{template template argument 'Y' is more constrained than template template parameter 'P'}}
+S1<Z> s13;
+S1<W> s14; // expected-error{{template template argument 'W' is more constrained than template template parameter 'P'}}
+
+template<template<typename> class P> struct S2 { }; // expected-note 2{{'P' declared here}}
+
+S2<X> s21; // expected-error{{template template argument 'X' is more constrained than template template parameter 'P'}}
+S2<Y> s22; // expected-error{{template template argument 'Y' is more constrained than template template parameter 'P'}}
+S2<Z> s23;
+
+template <template <typename...> class C>
+struct S3;
+
+template <C T>
+using N = typename T::type;
+
+using s31 = S3<N>;
+using s32 = S3<Z>;
diff --git a/clang/test/CXX/temp/temp.constr/temp.constr.decl/class-template-decl.cpp b/clang/test/CXX/temp/temp.constr/temp.constr.decl/class-template-decl.cpp
index 6f7b80e26a6..2a1a4e1b7ff 100644
--- a/clang/test/CXX/temp/temp.constr/temp.constr.decl/class-template-decl.cpp
+++ b/clang/test/CXX/temp/temp.constr/temp.constr.decl/class-template-decl.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++2a -x c++ -verify %s
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
 
 namespace nodiag {
 
@@ -7,6 +7,14 @@ struct A;
 template <typename U> requires (bool(U()))
 struct A;
 
+template<typename T>
+concept C1 = true;
+
+template <C1 T> requires (bool(T()))
+struct B;
+template <C1 U> requires (bool(U()))
+struct B;
+
 } // end namespace nodiag
 
 namespace diag {
@@ -24,6 +32,14 @@ struct C;
 template <typename T> requires (!0) // expected-error{{requires clause differs in template redeclaration}}
 struct C;
 
+template<typename T>
+concept C1 = true;
+
+template <C1 T> // expected-note{{previous template declaration is here}}
+struct D;
+template <typename T> requires C1<T> // expected-error{{type constraint differs in template redeclaration}}
+struct D;
+
 } // end namespace diag
 
 namespace nodiag {
diff --git a/clang/test/CXX/temp/temp.constr/temp.constr.decl/p3.cpp b/clang/test/CXX/temp/temp.constr/temp.constr.decl/p3.cpp
new file mode 100644
index 00000000000..c6fe82d8de4
--- /dev/null
+++ b/clang/test/CXX/temp/temp.constr/temp.constr.decl/p3.cpp
@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+template<typename T>
+struct X {
+    using Y = typename T::invalid;
+};
+
+template<typename T>
+concept Invalid = X<T>{};
+
+template<typename T>
+concept False = false; // expected-note{{because 'false' evaluated to false}}
+
+template<typename T>
+concept True = true;
+
+// TODO: Concepts: Uncomment trailing requires clauses here when we have correct substitution.
+//template<True T>
+//  requires False<T>
+//void g1() requires Invalid<T>;
+//
+//using g1i = decltype(g1<int>());
+
+template<False T> // expected-note{{because 'int' does not satisfy 'False'}}
+  requires Invalid<T>
+void g2(); // requires Invalid<T>;
+// expected-note@-1{{candidate template ignored: constraints not satisfied [with T = int]}}
+
+using g2i = decltype(g2<int>());
+// expected-error@-1{{no matching function for call to 'g2'}}
\ No newline at end of file
diff --git a/clang/test/CXX/temp/temp.param/p10-2a.cpp b/clang/test/CXX/temp/temp.param/p10-2a.cpp
new file mode 100644
index 00000000000..de40f8cca5d
--- /dev/null
+++ b/clang/test/CXX/temp/temp.param/p10-2a.cpp
@@ -0,0 +1,117 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+template<typename T>
+concept C1 = sizeof(T) == 1;
+// expected-note@-1 2{{because 'sizeof(short) == 1' (2 == 1) evaluated to false}}
+// expected-note@-2 {{because 'sizeof(int) == 1' (4 == 1) evaluated to false}}
+
+template<C1 T> // expected-note {{because 'int' does not satisfy 'C1'}}
+using A = T;
+
+using a1 = A<int>; // expected-error {{constraints not satisfied for alias template 'A' [with T = int]}}
+using a2 = A<char>;
+
+template<typename T>
+concept C2 = sizeof(T) == 2;
+// expected-note@-1 {{because 'sizeof(char) == 2' (1 == 2) evaluated to false}}
+
+template<C1 T1, C2 T2>
+// expected-note@-1 2{{because 'short' does not satisfy 'C1'}}
+// expected-note@-2 {{because 'char' does not satisfy 'C2'}}
+using B = T1;
+
+using b1 = B<char, short>;
+using b2 = B<char, char>; // expected-error {{constraints not satisfied for alias template 'B' [with T1 = char, T2 = char]}}
+using b3 = B<short, short>; // expected-error {{constraints not satisfied for alias template 'B' [with T1 = short, T2 = short]}}
+using b4 = B<short, char>; // expected-error {{constraints not satisfied for alias template 'B' [with T1 = short, T2 = char]}}
+
+template<typename... T>
+concept C3 = (sizeof(T) + ...) == 12;
+// expected-note@-1 {{because 'sizeof(char [11]) == 12' (11 == 12) evaluated to false}}
+// expected-note@-2 {{because 'sizeof(char [10]) == 12' (10 == 12) evaluated to false}}
+// expected-note@-3 3{{because 'sizeof(int) == 12' (4 == 12) evaluated to false}}
+// expected-note@-4 6{{because 'sizeof(short) == 12' (2 == 12) evaluated to false}}
+
+template<C3 T1, C3 T2, C3 T3>
+// expected-note@-1 {{because 'char [11]' does not satisfy 'C3'}}
+// expected-note@-2 {{because 'char [10]' does not satisfy 'C3'}}
+using C = T2;
+
+using c1 = C<char[12], int[3], short[6]>;
+using c2 = C<char[12], char[11], char[10]>;
+// expected-error@-1 {{constraints not satisfied for alias template 'C' [with T1 = char [12], T2 = char [11], T3 = char [10]]}}
+using c3 = C<char[12], char[12], char[10]>;
+// expected-error@-1 {{constraints not satisfied for alias template 'C' [with T1 = char [12], T2 = char [12], T3 = char [10]]}}
+
+template<C3... Ts>
+// expected-note@-1 {{because 'int' does not satisfy 'C3'}}
+// expected-note@-2 2{{and 'int' does not satisfy 'C3'}}
+// expected-note@-3 {{because 'short' does not satisfy 'C3'}}
+// expected-note@-4 5{{and 'short' does not satisfy 'C3'}}
+using D = int;
+
+using d1 = D<char[12], int[3], short[6]>;
+using d2 = D<int, int, int>;
+// expected-error@-1 {{constraints not satisfied for alias template 'D' [with Ts = <int, int, int>}}
+using d3 = D<short, short, short, short, short, short>;
+// expected-error@-1 {{constraints not satisfied for alias template 'D' [with Ts = <short, short, short, short, short, short>}}
+
+template<typename T>
+concept C4 = sizeof(T) == 4;
+// expected-note@-1 3{{because 'sizeof(char) == 4' (1 == 4) evaluated to false}}
+
+template<C4... Ts>
+// expected-note@-1 2{{because 'char' does not satisfy 'C4'}}
+// expected-note@-2 {{and 'char' does not satisfy 'C4'}}
+using E = int;
+
+using e1 = E<int>;
+using e2 = E<char, int>; // expected-error {{constraints not satisfied for alias template 'E' [with Ts = <char, int>]}}
+using e3 = E<char, char>; // expected-error {{constraints not satisfied for alias template 'E' [with Ts = <char, char>]}}
+using e4 = E<>;
+
+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, typename U>
+concept Same = is_same_v<T, U>; // expected-note {{because 'is_same_v<long, int>' evaluated to false}}
+
+template<Same<int> T> // expected-note {{because 'Same<long, int>' evaluated to false}}
+using F = T;
+
+using f1 = F<int>;
+using f2 = F<long>; // expected-error {{constraints not satisfied for alias template 'F' [with T = long]}}
+
+template<typename T, typename... Ts>
+concept OneOf = (is_same_v<T, Ts> || ...);
+// expected-note@-1 2{{because 'is_same_v<char, char [1]>' evaluated to false}}
+// expected-note@-2 2{{and 'is_same_v<char, char [2]>' evaluated to false}}
+// expected-note@-3 {{because 'is_same_v<short, int>' evaluated to false}}
+// expected-note@-4 {{and 'is_same_v<short, long>' evaluated to false}}
+// expected-note@-5 {{and 'is_same_v<short, char>' evaluated to false}}
+// expected-note@-6 3{{because 'is_same_v<int, char [1]>' evaluated to false}}
+// expected-note@-7 3{{and 'is_same_v<int, char [2]>' evaluated to false}}
+
+template<OneOf<char[1], char[2]> T, OneOf<int, long, char> U>
+// expected-note@-1 2{{because 'OneOf<char, char [1], char [2]>' evaluated to false}}
+// expected-note@-2 {{because 'OneOf<short, int, long, char>' evaluated to false}}
+using G = T;
+
+using g1 = G<char[1], int>;
+using g2 = G<char, int>; // expected-error{{constraints not satisfied for alias template 'G' [with T = char, U = int]}}
+using g3 = G<char[1], short>; // expected-error{{constraints not satisfied for alias template 'G' [with T = char [1], U = short]}}
+using g4 = G<char, short>; // expected-error{{constraints not satisfied for alias template 'G' [with T = char, U = short]}}
+
+template<OneOf<char[1], char[2]>... Ts>
+// expected-note@-1 2{{because 'OneOf<int, char [1], char [2]>' evaluated to false}}
+// expected-note@-2 {{and 'OneOf<int, char [1], char [2]>' evaluated to false}}
+using H = int;
+
+using h1 = H<char[1], int>;
+// expected-error@-1 {{constraints not satisfied for alias template 'H' [with Ts = <char [1], int>]}}
+using h2 = H<int, int>;
+// expected-error@-1 {{constraints not satisfied for alias template 'H' [with Ts = <int, int>]}}
+using h3 = H<char[1], char[2]>;
\ No newline at end of file
diff --git a/clang/test/Parser/cxx2a-constrained-template-param-with-partial-id.cpp b/clang/test/Parser/cxx2a-constrained-template-param-with-partial-id.cpp
new file mode 100644
index 00000000000..2d80475e1eb
--- /dev/null
+++ b/clang/test/Parser/cxx2a-constrained-template-param-with-partial-id.cpp
@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+template<typename T, int a>
+concept C1 = true;
+
+template<C1 T>
+// expected-error@-1 {{'C1' requires more than 1 template argument; provide the remaining arguments explicitly to use it here}}
+using badA = T[10];
+
+template<C1<0> T>
+using A = T[10];
+
+using a = A<int>;
+
+namespace ns {
+  template<typename T, typename U, typename... X>
+  concept C2 = true;
+}
+
+template<ns::C2 T1, ::ns::C2 T2>
+// expected-error@-1 2{{'C2' requires more than 1 template argument; provide the remaining arguments explicitly to use it here}}
+requires (sizeof(T1) <= sizeof(T2))
+struct badB { };
+
+template<ns::C2<int> T1, ::ns::C2<char, T1> T2>
+  requires (sizeof(T1) <= sizeof(T2))
+struct B { };
+
+using b = B<int, int>;
+
+template<ns::C2... T1>
+// expected-error@-1 {{'C2' requires more than 1 template argument; provide the remaining arguments explicitly to use it here}}
+struct badC { };
+
+template<ns::C2<int>... T1>
+struct C { };
+
+using c1 = C<char, char, char>;
+using c2 = C<char, char, char, char>;
\ No newline at end of file
diff --git a/clang/test/Parser/cxx2a-constrained-template-param.cpp b/clang/test/Parser/cxx2a-constrained-template-param.cpp
new file mode 100644
index 00000000000..2ee544e970b
--- /dev/null
+++ b/clang/test/Parser/cxx2a-constrained-template-param.cpp
@@ -0,0 +1,52 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+namespace type
+{
+  template<typename T>
+  concept C1 = true;
+
+  template<C1 T, C1 U = int>
+  using A = T[10];
+
+  using a = A<int>;
+
+  namespace ns {
+    template<typename T, int a = 0>
+    concept C2 = true;
+  }
+
+  template<ns::C2 T1, ::type::ns::C2 T2> requires (sizeof(T1) <= sizeof(T2))
+  struct B { };
+
+  using b = B<int, int>;
+
+  template<ns::C2... T1>
+  struct C { };
+
+  using c1 = C<char, char, char>;
+  using c2 = C<char, char, char, char>;
+}
+
+namespace non_type
+{
+  template<int v>
+  concept C1 = true;
+
+  template<C1 v, C1 u = 0> // expected-error{{expected a type}} // expected-note{{declared here}}
+  // expected-error@-1 2{{concept named in type constraint is not a type concept}}
+  // expected-error@-2 {{expected ',' or '>' in template-parameter-list}}
+  int A = v; // expected-error{{'v' does not refer to a value}}
+}
+
+namespace temp
+{
+  template<typename>
+  struct test1 { }; // expected-note{{template is declared here}}
+
+  template<template<typename> typename T>
+  concept C1 = true;
+
+  template<C1 TT, C1 UU = test1> // expected-error{{use of class template 'test1' requires template arguments}}
+  // expected-error@-1 2{{concept named in type constraint is not a type concept}}
+  using A = TT<int>; // expected-error{{expected ';' after alias declaration}}
+}
\ No newline at end of file
diff --git a/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp b/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp
new file mode 100644
index 00000000000..4daf31a292c
--- /dev/null
+++ b/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp
@@ -0,0 +1,36 @@
+// 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, typename U>
+concept same_as = is_same_v<T, U>;
+// expected-note@-1{{because 'is_same_v<int, _Bool>' evaluated to false}}
+
+template<typename T, typename... Us>
+concept either = (is_same_v<T, Us> || ...);
+
+template<typename... Ts>
+struct T {
+    template<same_as<Ts>... Us>
+    // expected-note@-1{{because 'same_as<int, _Bool>' evaluated to false}}
+    static void foo(Us... u, int x) { };
+    // expected-note@-1{{candidate template ignored: deduced too few arguments}}
+    // expected-note@-2{{candidate template ignored: constraints not satisfied}}
+
+    template<typename... Us>
+    struct S {
+        template<either<Ts, Us...>... Vs>
+        static void foo(Vs... v);
+    };
+};
+
+int main() {
+  T<int, bool>::foo(1); // expected-error{{no matching function for call to 'foo'}}
+  T<int, bool>::foo(1, 2, 3); // expected-error{{no matching function for call to 'foo'}}
+  T<int, bool>::S<char>::foo(1, 'a');
+  T<int, bool>::S<char>::foo('a', true);
+}
diff --git a/clang/test/SemaTemplate/ms-delayed-default-template-args.cpp b/clang/test/SemaTemplate/ms-delayed-default-template-args.cpp
index 0c054694247..beb64f8273d 100644
--- a/clang/test/SemaTemplate/ms-delayed-default-template-args.cpp
+++ b/clang/test/SemaTemplate/ms-delayed-default-template-args.cpp
@@ -94,12 +94,11 @@ namespace test_undeclared_nontype_parm_arg {
 template <typename T> struct Bar { T x; };
 
 template <Bar<Xylophone> *P> // expected-error {{use of undeclared identifier 'Xylophone'}}
-// expected-note@-1 {{template parameter is declared here}}
 struct Foo { };
 
 typedef int Xylophone;
 Bar<Xylophone> g;
-template struct Foo<&g>; // expected-error {{cannot be converted}}
+template struct Foo<&g>; // expected-error {{value of type}}
 }
 
 #endif
diff --git a/clang/tools/libclang/CIndex.cpp b/clang/tools/libclang/CIndex.cpp
index 94084e169fa..6f3490471de 100644
--- a/clang/tools/libclang/CIndex.cpp
+++ b/clang/tools/libclang/CIndex.cpp
@@ -757,6 +757,11 @@ bool CursorVisitor::VisitClassTemplatePartialSpecializationDecl(
 }
 
 bool CursorVisitor::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
+  if (const auto *TC = D->getTypeConstraint())
+    if (Visit(MakeCXCursor(TC->getImmediatelyDeclaredConstraint(), StmtParent,
+                           TU, RegionOfInterest)))
+      return true;
+
   // Visit the default argument.
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited())
     if (TypeSourceInfo *DefArg = D->getDefaultArgumentInfo())
@@ -5050,7 +5055,12 @@ CXString clang_getCursorDisplayName(CXCursor C) {
       // There is no parameter name, which makes this tricky. Try to come up
       // with something useful that isn't too long.
       if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
-        OS << (TTP->wasDeclaredWithTypename()? "typename" : "class");
+        if (const auto *TC = TTP->getTypeConstraint()) {
+          TC->getConceptNameInfo().printName(OS, Policy);
+          if (TC->hasExplicitTemplateArgs())
+            OS << "<...>";
+        } else
+          OS << (TTP->wasDeclaredWithTypename()? "typename" : "class");
       else if (NonTypeTemplateParmDecl *NTTP
                                     = dyn_cast<NonTypeTemplateParmDecl>(Param))
         OS << NTTP->getType().getAsString(Policy);