[Concepts] Associated constraints infrastructure.

[saar.raz]

Summary

Added code to correctly calculate the associated constraints of a template (no enforcement yet). Depends on D41217.

[saar.raz]

• Added requires clause matching for all kinds of template redeclarations instead of just classes.

[saar.raz]

• Fixed wrong if that would cause valid instantiated requires clauses to not be accepted.

[saar.raz]

Moved function into SemaConcept.cpp.

[saar.raz]

Fixed reference to TemplateParams member in assertion.

[saar.raz]

• Consider requires clause when determining if TPL has an unexpanded pack

[rsmith]

lib/AST/DeclTemplate.cpp

203 ↗

(On Diff #159236)

Rather than producing an Expr* (which will presumably eventually need to contain synthetic && expressions), this should produce a vector of constraints (notionally and'd together), which will (eventually) be either constraint-expressions or some representation of "the constraints implied by this template parameter". If you do that, I don't think there's any reason to cache this information on the TemplateDecl; it should be cheap enough to recompute when we need it.

lib/Sema/SemaConcept.cpp

47–58 ↗

(On Diff #159236)

I'm sure we already have this "profile two expressions and check they are canonically equivalent" functionality factored out somewhere. Is there nothing like that that's appropriate to reuse here?

lib/Sema/SemaTemplate.cpp

2145–2154 ↗

(On Diff #159236)

Unrelated cleanup: please commit this separately.

lib/Sema/SemaTemplateInstantiateDecl.cpp

3141–3148 ↗

(On Diff #159236)

This is wrong (we should never substitute into a constraint-expression except as part of satisfaction or subsumption checks). Please at least add a FIXME.

lib/Serialization/ASTReaderDecl.cpp

2003–2006 ↗

(On Diff #159236)

There doesn't seem to be a good reason to serialize the cached associated constraints, even if we want to keep them (which I don't think we do).

lib/Serialization/ASTWriter.cpp

5856 ↗

(On Diff #159236)

What would be left "to do" here? For constrained parameters, there may be something interesting to track on the *Template*ParmDecl, but not here, I wouldn't think,

[saar.raz]

• Address comments by rsmith, mainly removing associated constraints caching and instead returning a smallvector of constraint expressions from getAssociatedConstraints.

[saar.raz]

• Fix bad reference to getRequiresClause on TemplateDecl in assertion

[• Quuxplusone]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

For my own edification, could you explain whether, given

#define BOOL bool
using typedef_for_bool = bool;

you'd expect to diagnose a redeclaration of B::A with associated constraint

requires bool( U() )  // extra whitespace

or

requires BOOL(U())  // different spelling of `bool`

or

requires typedef_for_bool(U())  // different spelling of `bool`

? My naive reading of N4762 temp.constr.atomic/2 says that none of these constraints (on line 10) would be "identical" to the constraint on line 6... but then I don't understand what's the salient difference between line 10 (which apparently gives no error) and line 22 (which apparently gives an error).

[saar.raz]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

Line 22 has a not (!) operator in front of the bool(), I guess you missed that?

[• Quuxplusone]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

I saw the !... but I don't understand how the compiler "knows" that !bool(U()) is "different" from bool(T()) in a way that doesn't equally apply to bool(U()).

Or suppose the constraint on line 10 was requires bool(U())==true... would that give a diagnostic?

[Rakete1111]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

bool(T()) and bool(U()) are identical because they have the same parameter mappings.

The "identical" requirement applies to the actual grammar composition of the expression, so bool(T()) would be different to bool(T()) == true.

At least that's how I understand it.

[saar.raz]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

OK, I can see where the confusion is coming from.

The way it works (in clang, at least) - is that the compiler pays no attention to the name of a template parameter for any purpose other than actually finding it in the first place - once it is found, it is 'stored' simply as bool(<template-parameter-0-0>()) where the first 0 is the depth of the template parameter list of the parameter in question (in case of a template within a template) and the second 0 is the index of the template parameter within that list.

I believe this treatment stems from [temp.over.link]p6 "When determining whether types or qualified-concept-names are equivalent, the rules above are used to compare expressions involving template parameters"

[saar.raz]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

Correction - p5 describes this better (see also the example in p4)

[• Quuxplusone]

test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

10 ↗

(On Diff #161345)

Okay, I can see how this matches the apparent intent of http://eel.is/c++draft/temp.over.link#5 . I guess a strict reading of that passage would mean that my BOOL and typedef_for_bool versions should give diagnostics, and so should

#define V(x) U
template <typename U> requires bool(V(x) ())

but no diagnostic is expected for

#define V U
template <typename U> requires bool(V ())

Anyway, sounds like this rabbit hole is out-of-scope for this review, anyway, so I'll be quiet now. :)

[saar.raz]

Rebase onto trunk

[rsmith]

include/clang/AST/DeclTemplate.h

177 ↗

(On Diff #194984)

Returning a SmallVector by value is not idiomatic. If you need to make a copy here, you should take a SmallVectorImpl by reference, so the caller can choose the inline size. If not, you should return an ArrayRef.

include/clang/Sema/Sema.h

5772–5776 ↗

(On Diff #194984)

This is not how redeclaration checking is specified any more. A redeclaration is valid only if its template-head is equivalent to that of the prior declaration, not if it has the same associated constraints. You should instead take two TemplateParameterLists, profile them, and compare the results.

lib/Serialization/ASTReaderDecl.cpp

2375 ↗

(On Diff #194984)

There's no such thing any more.

2393 ↗

(On Diff #194984)

There's no such thing any more.

[saar.raz]

• Address CR comments by rsmith
  • Remove obsolete TODOs
  • Fix redeclaration checking
  • Change getAssociatedConstraints interface
• Add requires clause to ASTNodeTraverser

[saar.raz]

Rebase onto trunk.

[saar.raz]

Remove unrelated cleanup

[rsmith]

clang/include/clang/Sema/Sema.h

5950

Missing period.

clang/test/CXX/concepts-ts/temp/concept/p4.cpp

1

Please move this test and the others to the relevant place under test/CXX/temp rather than test/CXX/concepts-ts and remove -fconcepts-ts from the RUN: lines.

clang/include/clang/AST/ASTNodeTraverser.h

public:
   }
 
   void dumpTemplateParameters(const TemplateParameterList *TPL) {
     if (!TPL)
       return;
 
     for (const auto &TP : *TPL)
       Visit(TP);
+
+    if (const Expr *RC = TPL->getRequiresClause())
+      Visit(RC);
   }
 
   void
   dumpASTTemplateArgumentListInfo(const ASTTemplateArgumentListInfo *TALI) {
     if (!TALI)
       return;
 
     for (const auto &TA : TALI->arguments())

clang/include/clang/AST/DeclTemplate.h

Expr *getRequiresClause() {
     return HasRequiresClause ? *getTrailingObjects<Expr *>() : nullptr;
   }
 
   /// The constraint-expression of the associated requires-clause.
   const Expr *getRequiresClause() const {
     return HasRequiresClause ? *getTrailingObjects<Expr *>() : nullptr;
   }
 
+  /// \brief All associated constraints derived from this template parameter
+  /// list, including the requires clause and any constraints derived from
+  /// constrained-parameters.
+  ///
+  /// The constraints in the resulting list are to be treated as if in a
+  /// conjunction ("and").
+  void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const;
+
+  bool hasAssociatedConstraints() const;
+
   SourceLocation getTemplateLoc() const { return TemplateLoc; }
   SourceLocation getLAngleLoc() const { return LAngleLoc; }
   SourceLocation getRAngleLoc() const { return RAngleLoc; }
 
   SourceRange getSourceRange() const LLVM_READONLY {
     return SourceRange(TemplateLoc, RAngleLoc);
   }
 
void clear() {
     ValueOrInherited = ArgType();
   }
 };
 
 //===----------------------------------------------------------------------===//
 // Kinds of Templates
 //===----------------------------------------------------------------------===//
 
-/// Stores the template parameter list and associated constraints for
-/// \c TemplateDecl objects that track associated constraints.
-class ConstrainedTemplateDeclInfo {
-  friend TemplateDecl;
-
-public:
-  ConstrainedTemplateDeclInfo() = default;
-
-  TemplateParameterList *getTemplateParameters() const {
-    return TemplateParams;
-  }
-
-  Expr *getAssociatedConstraints() const { return AssociatedConstraints; }
-
-protected:
-  void setTemplateParameters(TemplateParameterList *TParams) {
-    TemplateParams = TParams;
-  }
-
-  void setAssociatedConstraints(Expr *AC) { AssociatedConstraints = AC; }
-
-  TemplateParameterList *TemplateParams = nullptr;
-  Expr *AssociatedConstraints = nullptr;
-};
-
-
-/// The base class of all kinds of template declarations (e.g.,
+/// \brief The base class of all kinds of template declarations (e.g.,
 /// class, function, etc.).
 ///
 /// The TemplateDecl class stores the list of template parameters and a
 /// reference to the templated scoped declaration: the underlying AST node.
 class TemplateDecl : public NamedDecl {
   void anchor() override;
 
 protected:
+  // Construct a template decl with name, parameters, and templated element.
+  TemplateDecl(Kind DK, DeclContext *DC, SourceLocation L, DeclarationName Name,
+               TemplateParameterList *Params, NamedDecl *Decl);
+
   // Construct a template decl with the given name and parameters.
   // Used when there is no templated element (e.g., for tt-params).
-  TemplateDecl(ConstrainedTemplateDeclInfo *CTDI, Kind DK, DeclContext *DC,
-               SourceLocation L, DeclarationName Name,
-               TemplateParameterList *Params)
-      : NamedDecl(DK, DC, L, Name), TemplatedDecl(nullptr),
-        TemplateParams(CTDI) {
-    this->setTemplateParameters(Params);
-  }
-
   TemplateDecl(Kind DK, DeclContext *DC, SourceLocation L, DeclarationName Name,
                TemplateParameterList *Params)
-      : TemplateDecl(nullptr, DK, DC, L, Name, Params) {}
-
-  // Construct a template decl with name, parameters, and templated element.
-  TemplateDecl(ConstrainedTemplateDeclInfo *CTDI, Kind DK, DeclContext *DC,
-               SourceLocation L, DeclarationName Name,
-               TemplateParameterList *Params, NamedDecl *Decl)
-      : NamedDecl(DK, DC, L, Name), TemplatedDecl(Decl),
-        TemplateParams(CTDI) {
-    this->setTemplateParameters(Params);
-  }
-
-  TemplateDecl(Kind DK, DeclContext *DC, SourceLocation L, DeclarationName Name,
-               TemplateParameterList *Params, NamedDecl *Decl)
-      : TemplateDecl(nullptr, DK, DC, L, Name, Params, Decl) {}
+      : TemplateDecl(DK, DC, L, Name, Params, nullptr) {}
 
 public:
+  friend class ASTDeclReader;
+  friend class ASTDeclWriter;
+
   /// Get the list of template parameters
   TemplateParameterList *getTemplateParameters() const {
-    const auto *const CTDI =
-        TemplateParams.dyn_cast<ConstrainedTemplateDeclInfo *>();
-    return CTDI ? CTDI->getTemplateParameters()
-                : TemplateParams.get<TemplateParameterList *>();
+    return TemplateParams;
   }
 
-  /// Get the constraint-expression from the associated requires-clause (if any)
-  const Expr *getRequiresClause() const {
-    const TemplateParameterList *const TP = getTemplateParameters();
-    return TP ? TP->getRequiresClause() : nullptr;
-  }
+  /// \brief Get the total constraint-expression associated with this template,
+  /// including constraint-expressions derived from the requires-clause,
+  /// trailing requires-clause (for functions and methods) and constrained
+  /// template parameters.
+  void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const;
 
-  Expr *getAssociatedConstraints() const {
-    const auto *const C = cast<TemplateDecl>(getCanonicalDecl());
-    const auto *const CTDI =
-        C->TemplateParams.dyn_cast<ConstrainedTemplateDeclInfo *>();
-    return CTDI ? CTDI->getAssociatedConstraints() : nullptr;
-  }
+  bool hasAssociatedConstraints() const;
 
   /// Get the underlying, templated declaration.
   NamedDecl *getTemplatedDecl() const { return TemplatedDecl; }
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
 
   static bool classofKind(Kind K) {
     return K >= firstTemplate && K <= lastTemplate;
   }
 
   SourceRange getSourceRange() const override LLVM_READONLY {
     return SourceRange(getTemplateParameters()->getTemplateLoc(),
                        TemplatedDecl->getSourceRange().getEnd());
   }
 
 protected:
   NamedDecl *TemplatedDecl;
-
-  /// The template parameter list and optional requires-clause
-  /// associated with this declaration; alternatively, a
-  /// \c ConstrainedTemplateDeclInfo if the associated constraints of the
-  /// template are being tracked by this particular declaration.
-  llvm::PointerUnion<TemplateParameterList *,
-                     ConstrainedTemplateDeclInfo *>
-      TemplateParams;
+  TemplateParameterList *TemplateParams;
 
   void setTemplateParameters(TemplateParameterList *TParams) {
-    if (auto *const CTDI =
-            TemplateParams.dyn_cast<ConstrainedTemplateDeclInfo *>()) {
-      CTDI->setTemplateParameters(TParams);
-    } else {
-      TemplateParams = TParams;
-    }
-  }
-
-  void setAssociatedConstraints(Expr *AC) {
-    assert(isCanonicalDecl() &&
-           "Attaching associated constraints to non-canonical Decl");
-    TemplateParams.get<ConstrainedTemplateDeclInfo *>()
-        ->setAssociatedConstraints(AC);
-  }
 
 public:
   /// Initialize the underlying templated declaration and
   /// template parameters.
   void init(NamedDecl *templatedDecl, TemplateParameterList* templateParams) {
     assert(!TemplatedDecl && "TemplatedDecl already set!");
     assert(!TemplateParams && "TemplateParams already set!");
     TemplatedDecl = templatedDecl;
protected:
   /// Retrieves the "common" pointer shared by all (re-)declarations of
   /// the same template. Calling this routine may implicitly allocate memory
   /// for the common pointer.
   CommonBase *getCommonPtr() const;
 
   virtual CommonBase *newCommon(ASTContext &C) const = 0;
 
   // Construct a template decl with name, parameters, and templated element.
-  RedeclarableTemplateDecl(ConstrainedTemplateDeclInfo *CTDI, Kind DK,
-                           ASTContext &C, DeclContext *DC, SourceLocation L,
-                           DeclarationName Name, TemplateParameterList *Params,
-                           NamedDecl *Decl)
-      : TemplateDecl(CTDI, DK, DC, L, Name, Params, Decl), redeclarable_base(C)
-        {}
-
   RedeclarableTemplateDecl(Kind DK, ASTContext &C, DeclContext *DC,
                            SourceLocation L, DeclarationName Name,
                            TemplateParameterList *Params, NamedDecl *Decl)
-      : RedeclarableTemplateDecl(nullptr, DK, C, DC, L, Name, Params, Decl) {}
+      : TemplateDecl(DK, DC, L, Name, Params, Decl), redeclarable_base(C) {}
 
 public:
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
   friend class ASTReader;
   template <class decl_type> friend class RedeclarableTemplate;
 
   /// Retrieves the canonical declaration of this template.
return cast<ClassTemplatePartialSpecializationDecl>(
                this)->getMostRecentDecl());
   }
 
   /// Get the list of template parameters
   TemplateParameterList *getTemplateParameters() const {
     return TemplateParams;
   }
 
+  /// \brief All associated constraints of this partial specialization,
+  /// including the requires clause and any constraints derived from
+  /// constrained-parameters.
+  ///
+  /// The constraints in the resulting list are to be treated as if in a
+  /// conjunction ("and").
+  void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
+    TemplateParams->getAssociatedConstraints(AC);
+  }
+
+  bool hasAssociatedConstraints() const {
+    return TemplateParams->hasAssociatedConstraints();
+  }
+
   /// Get the template arguments as written.
   const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
     return ArgsAsWritten;
   }
 
   /// Retrieve the member class template partial specialization from
   /// which this particular class template partial specialization was
   /// instantiated.
protected:
   llvm::FoldingSetVector<ClassTemplateSpecializationDecl> &
   getSpecializations() const;
 
   /// Retrieve the set of partial specializations of this class
   /// template.
   llvm::FoldingSetVector<ClassTemplatePartialSpecializationDecl> &
   getPartialSpecializations();
 
-  ClassTemplateDecl(ConstrainedTemplateDeclInfo *CTDI, ASTContext &C,
-                    DeclContext *DC, SourceLocation L, DeclarationName Name,
-                    TemplateParameterList *Params, NamedDecl *Decl)
-      : RedeclarableTemplateDecl(CTDI, ClassTemplate, C, DC, L, Name, Params,
-                                 Decl) {}
-
   ClassTemplateDecl(ASTContext &C, DeclContext *DC, SourceLocation L,
                     DeclarationName Name, TemplateParameterList *Params,
                     NamedDecl *Decl)
-      : ClassTemplateDecl(nullptr, C, DC, L, Name, Params, Decl) {}
+      : RedeclarableTemplateDecl(ClassTemplate, C, DC, L, Name, Params, Decl) {}
 
   CommonBase *newCommon(ASTContext &C) const override;
 
   Common *getCommonPtr() const {
     return static_cast<Common *>(RedeclarableTemplateDecl::getCommonPtr());
   }
 
 public:
public:
   }
 
   /// Returns whether this template declaration defines the primary
   /// class pattern.
   bool isThisDeclarationADefinition() const {
     return getTemplatedDecl()->isThisDeclarationADefinition();
   }
 
-  // FIXME: remove default argument for AssociatedConstraints
-  /// Create a class template node.
+  /// \brief Create a class template node.
   static ClassTemplateDecl *Create(ASTContext &C, DeclContext *DC,
                                    SourceLocation L,
                                    DeclarationName Name,
                                    TemplateParameterList *Params,
-                                   NamedDecl *Decl,
-                                   Expr *AssociatedConstraints = nullptr);
+                                   NamedDecl *Decl);
 
   /// Create an empty class template node.
   static ClassTemplateDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
   /// Return the specialization with the provided arguments if it exists,
   /// otherwise return the insertion point.
   ClassTemplateSpecializationDecl *
   findSpecialization(ArrayRef<TemplateArgument> Args, void *&InsertPos);
TemplateParameterList *getTemplateParameters() const {
     return TemplateParams;
   }
 
   /// Get the template arguments as written.
   const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
     return ArgsAsWritten;
   }
 
-  /// Retrieve the member variable template partial specialization from
+  /// \brief All associated constraints of this partial specialization,
+  /// including the requires clause and any constraints derived from
+  /// constrained-parameters.
+  ///
+  /// The constraints in the resulting list are to be treated as if in a
+  /// conjunction ("and").
+  void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
+    TemplateParams->getAssociatedConstraints(AC);
+  }
+
+  bool hasAssociatedConstraints() const {
+    return TemplateParams->hasAssociatedConstraints();
+  }
+
+  /// \brief Retrieve the member variable template partial specialization from
   /// which this particular variable template partial specialization was
   /// instantiated.
   ///
   /// \code
   /// template<typename T>
   /// struct Outer {
   ///   template<typename U> U Inner;
   ///   template<typename U> U* Inner<U*> = (U*)(0); // #1
public:
   static bool classofKind(Kind K) { return K == VarTemplate; }
 };
 
 // \brief Declaration of a C++2a concept.
 class ConceptDecl : public TemplateDecl, public Mergeable<ConceptDecl> {
 protected:
   Expr *ConstraintExpr;
 
-  ConceptDecl(DeclContext *DC,
-              SourceLocation L, DeclarationName Name,
-              TemplateParameterList *Params,
-              Expr *ConstraintExpr)
-      : TemplateDecl(nullptr, Concept, DC, L, Name, Params),
+  ConceptDecl(DeclContext *DC, SourceLocation L, DeclarationName Name,
+              TemplateParameterList *Params, Expr *ConstraintExpr)
+      : TemplateDecl(Concept, DC, L, Name, Params),
         ConstraintExpr(ConstraintExpr) {};
 public:
   static ConceptDecl *Create(ASTContext &C, DeclContext *DC,
                              SourceLocation L, DeclarationName Name,
                              TemplateParameterList *Params,
                              Expr *ConstraintExpr);
   static ConceptDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 

clang/include/clang/AST/RecursiveASTVisitor.h

for (auto *C : D->clauselists())
     TRY_TO(TraverseOMPClause(C));
 })
 
 // A helper method for TemplateDecl's children.
 template <typename Derived>
 bool RecursiveASTVisitor<Derived>::TraverseTemplateParameterListHelper(
     TemplateParameterList *TPL) {
   if (TPL) {
-    for (TemplateParameterList::iterator I = TPL->begin(), E = TPL->end();
-         I != E; ++I) {
-      TRY_TO(TraverseDecl(*I));
+    for (NamedDecl *D : *TPL) {
+      TRY_TO(TraverseDecl(D));
+    }
+    if (Expr *RequiresClause = TPL->getRequiresClause()) {
+      TRY_TO(TraverseStmt(RequiresClause));
     }
   }
   return true;
 }
 
 template <typename Derived>
 template <typename T>
 bool RecursiveASTVisitor<Derived>::TraverseDeclTemplateParameterLists(T *D) {

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_concept_not_implemented : Error<
   "sorry, unimplemented concepts feature %0 used">;
 def err_non_constant_constraint_expression : Error<
   "substitution into constraint expression resulted in a non-constant "
   "expression">;
 def err_non_bool_atomic_constraint : Error<
   "atomic constraint must be of type 'bool' (found %0)">;
 
-def err_template_different_associated_constraints : Error<
-  "associated constraints differ in template redeclaration">;
+def err_template_different_requires_clause : Error<
+  "requires clause differs in template redeclaration">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
   "'%0' type specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 def warn_cxx17_compat_unicode_type : Warning<
   "'char8_t' type specifier is incompatible with C++ standards before C++20">,
   InGroup<CXXPre2aCompat>, DefaultIgnore;

clang/include/clang/Sema/Sema.h

   /// A diagnostic is emitted if it is not, and false is returned.
   bool CheckConstraintExpression(Expr *CE);
 
   bool CalculateConstraintSatisfaction(ConceptDecl *NamedConcept,
                                          MultiLevelTemplateArgumentList &MLTAL,
                                          Expr *ConstraintExpr,
                                          bool &IsSatisfied);
 
-    // ParseObjCStringLiteral - Parse Objective-C string literals.
+  /// 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);
+
   ExprResult ParseObjCStringLiteral(SourceLocation *AtLocs,
                                     ArrayRef<Expr *> Strings);
 
   ExprResult BuildObjCStringLiteral(SourceLocation AtLoc, StringLiteral *S);
 
   /// BuildObjCNumericLiteral - builds an ObjCBoxedExpr AST node for the
   /// numeric literal expression. Type of the expression will be "NSNumber *"
   /// or "id" if NSNumber is unavailable.

clang/lib/AST/ASTContext.cpp

else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(*P)) {
       }
       CanonParams.push_back(Param);
 
     } else
       CanonParams.push_back(getCanonicalTemplateTemplateParmDecl(
                                            cast<TemplateTemplateParmDecl>(*P)));
   }
 
-  assert(!TTP->getRequiresClause() &&
+  assert(!TTP->getTemplateParameters()->getRequiresClause() &&
          "Unexpected requires-clause on template template-parameter");
   Expr *const CanonRequiresClause = nullptr;
 
   TemplateTemplateParmDecl *CanonTTP
     = TemplateTemplateParmDecl::Create(*this, getTranslationUnitDecl(),
                                        SourceLocation(), TTP->getDepth(),
                                        TTP->getPosition(),
                                        TTP->isParameterPack(),

clang/lib/AST/DeclTemplate.cpp

if (!P->isTemplateParameterPack()) {
           ContainsUnexpandedParameterPack = true;
 
       // FIXME: If a default argument contains an unexpanded parameter pack, the
       // template parameter list does too.
     }
   }
   if (RequiresClause) {
     *getTrailingObjects<Expr *>() = RequiresClause;
+    if (RequiresClause->containsUnexpandedParameterPack())
+      ContainsUnexpandedParameterPack = true;
   }
 }
 
 TemplateParameterList *
 TemplateParameterList::Create(const ASTContext &C, SourceLocation TemplateLoc,
                               SourceLocation LAngleLoc,
                               ArrayRef<NamedDecl *> Params,
                               SourceLocation RAngleLoc, Expr *RequiresClause) {
static void AdoptTemplateParameterList(TemplateParameterList *Params,
   for (NamedDecl *P : *Params) {
     P->setDeclContext(Owner);
 
     if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(P))
       AdoptTemplateParameterList(TTP->getTemplateParameters(), Owner);
   }
 }
 
+void TemplateParameterList::
+getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
+  // TODO: Concepts: Collect immediately-introduced constraints.
+  if (HasRequiresClause)
+    AC.push_back(getRequiresClause());
+}
+
+bool TemplateParameterList::hasAssociatedConstraints() const {
+  // TODO: Concepts: Regard immediately-introduced constraints.
+  return HasRequiresClause;
+}
+
 namespace clang {
 
 void *allocateDefaultArgStorageChain(const ASTContext &C) {
   return new (C) char[sizeof(void*) * 2];
 }
 
 } // namespace clang
 
 //===----------------------------------------------------------------------===//
+// TemplateDecl Implementation
+//===----------------------------------------------------------------------===//
+
+TemplateDecl::TemplateDecl(Kind DK, DeclContext *DC, SourceLocation L,
+                           DeclarationName Name, TemplateParameterList *Params,
+                           NamedDecl *Decl)
+    : NamedDecl(DK, DC, L, Name), TemplatedDecl(Decl), TemplateParams(Params) {}
+
+void TemplateDecl::anchor() {}
+
+void TemplateDecl::
+getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
+  // TODO: Concepts: Append function trailing requires clause.
+  TemplateParams->getAssociatedConstraints(AC);
+}
+
+bool TemplateDecl::hasAssociatedConstraints() const {
+  // TODO: Concepts: Regard function trailing requires clause.
+  return TemplateParams->hasAssociatedConstraints();
+}
+
+//===----------------------------------------------------------------------===//
 // RedeclarableTemplateDecl Implementation
 //===----------------------------------------------------------------------===//
 
 void RedeclarableTemplateDecl::anchor() {}
 
 RedeclarableTemplateDecl::CommonBase *RedeclarableTemplateDecl::getCommonPtr() const {
   if (Common)
     return Common;

 // ClassTemplateDecl Implementation
 //===----------------------------------------------------------------------===//
 
 ClassTemplateDecl *ClassTemplateDecl::Create(ASTContext &C,
                                              DeclContext *DC,
                                              SourceLocation L,
                                              DeclarationName Name,
                                              TemplateParameterList *Params,
-                                             NamedDecl *Decl,
-                                             Expr *AssociatedConstraints) {
+                                             NamedDecl *Decl) {
   AdoptTemplateParameterList(Params, cast<DeclContext>(Decl));
 
-  if (!AssociatedConstraints) {
-    return new (C, DC) ClassTemplateDecl(C, DC, L, Name, Params, Decl);
-  }
 
-  auto *const CTDI = new (C) ConstrainedTemplateDeclInfo;
-  auto *const New =
-      new (C, DC) ClassTemplateDecl(CTDI, C, DC, L, Name, Params, Decl);
-  New->setAssociatedConstraints(AssociatedConstraints);
-  return New;
-}
-
 ClassTemplateDecl *ClassTemplateDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
   return new (C, ID) ClassTemplateDecl(C, nullptr, SourceLocation(),
                                        DeclarationName(), nullptr, nullptr);
 }
 
 void ClassTemplateDecl::LoadLazySpecializations() const {
   loadLazySpecializationsImpl();
FunctionTemplateSpecializationInfo *FunctionTemplateSpecializationInfo::Create(
 
   void *Mem =
       C.Allocate(totalSizeToAlloc<MemberSpecializationInfo *>(MSInfo ? 1 : 0));
   return new (Mem) FunctionTemplateSpecializationInfo(
       FD, Template, TSK, TemplateArgs, ArgsAsWritten, POI, MSInfo);
 }
 
 //===----------------------------------------------------------------------===//
-// TemplateDecl Implementation
-//===----------------------------------------------------------------------===//
-
-void TemplateDecl::anchor() {}
-
-//===----------------------------------------------------------------------===//
 // ClassTemplateSpecializationDecl Implementation
 //===----------------------------------------------------------------------===//
 
 ClassTemplateSpecializationDecl::
 ClassTemplateSpecializationDecl(ASTContext &Context, Kind DK, TagKind TK,
                                 DeclContext *DC, SourceLocation StartLoc,
                                 SourceLocation IdLoc,
                                 ClassTemplateDecl *SpecializedTemplate,

clang/lib/Sema/SemaConcept.cpp

if (!E.get()->EvaluateAsRValue(EvalResult, Context)) {
     for (const PartialDiagnosticAt &PDiag : EvaluationDiags)
       Diag(PDiag.first, PDiag.second);
     return true;
   }
 
   IsSatisfied = EvalResult.Val.getInt().getBoolValue();
 
   return false;
-}
+}
 No newline at end of file

clang/lib/Sema/SemaTemplate.cpp

 // Exported for use by Parser.
 SourceRange
 clang::getTemplateParamsRange(TemplateParameterList const * const *Ps,
                               unsigned N) {
   if (!N) return SourceRange();
   return SourceRange(Ps[0]->getTemplateLoc(), Ps[N-1]->getRAngleLoc());
 }
 
-namespace clang {
-/// [temp.constr.decl]p2: A template's associated constraints are
-/// defined as a single constraint-expression derived from the introduced
-/// constraint-expressions [ ... ].
-///
-/// \param Params The template parameter list and optional requires-clause.
-///
-/// \param FD The underlying templated function declaration for a function
-/// template.
-static Expr *formAssociatedConstraints(TemplateParameterList *Params,
-                                       FunctionDecl *FD);
-}
-
-static Expr *clang::formAssociatedConstraints(TemplateParameterList *Params,
-                                              FunctionDecl *FD) {
-  // FIXME: Concepts: collect additional introduced constraint-expressions
-  assert(!FD && "Cannot collect constraints from function declaration yet.");
-  return Params->getRequiresClause();
-}
-
-/// Determine whether the declaration found is acceptable as the name
+/// \brief Determine whether the declaration found is acceptable as the name
 /// of a template and, if so, return that template declaration. Otherwise,
 /// returns null.
 ///
 /// Note that this may return an UnresolvedUsingValueDecl if AllowDependent
 /// is true. In all other cases it will return a TemplateDecl (or null).
 NamedDecl *Sema::getAsTemplateNameDecl(NamedDecl *D,
                                        bool AllowFunctionTemplates,
                                        bool AllowDependent) {
if (SS.isEmpty() &&
       Diag(Shadow->getTargetDecl()->getLocation(),
            diag::note_using_decl_target);
       Diag(Shadow->getUsingDecl()->getLocation(), diag::note_using_decl) << 0;
       // Recover by ignoring the old declaration.
       PrevDecl = PrevClassTemplate = nullptr;
     }
   }
 
-  // TODO Memory management; associated constraints are not always stored.
-  Expr *const CurAC = formAssociatedConstraints(TemplateParams, nullptr);
-
   if (PrevClassTemplate) {
     // Ensure that the template parameter lists are compatible. Skip this check
     // for a friend in a dependent context: the template parameter list itself
     // could be dependent.
     if (!(TUK == TUK_Friend && CurContext->isDependentContext()) &&
         !TemplateParameterListsAreEqual(TemplateParams,
                                    PrevClassTemplate->getTemplateParameters(),
                                         /*Complain=*/true,
                                         TPL_TemplateMatch))
       return true;
 
-    // Check for matching associated constraints on redeclarations.
-    const Expr *const PrevAC = PrevClassTemplate->getAssociatedConstraints();
-    const bool RedeclACMismatch = [&] {
-      if (!(CurAC || PrevAC))
-        return false; // Nothing to check; no mismatch.
-      if (CurAC && PrevAC) {
-        llvm::FoldingSetNodeID CurACInfo, PrevACInfo;
-        CurAC->Profile(CurACInfo, Context, /*Canonical=*/true);
-        PrevAC->Profile(PrevACInfo, Context, /*Canonical=*/true);
-        if (CurACInfo == PrevACInfo)
-          return false; // All good; no mismatch.
-      }
-      return true;
-    }();
-
-    if (RedeclACMismatch) {
-      Diag(CurAC ? CurAC->getBeginLoc() : NameLoc,
-           diag::err_template_different_associated_constraints);
-      Diag(PrevAC ? PrevAC->getBeginLoc() : PrevClassTemplate->getLocation(),
-           diag::note_template_prev_declaration)
-          << /*declaration*/ 0;
-      return true;
-    }
-
     // C++ [temp.class]p4:
     //   In a redeclaration, partial specialization, explicit
     //   specialization or explicit instantiation of a class template,
     //   the class-key shall agree in kind with the original class
     //   template declaration (7.1.5.3).
     RecordDecl *PrevRecordDecl = PrevClassTemplate->getTemplatedDecl();
     if (!isAcceptableTagRedeclaration(PrevRecordDecl, Kind,
                                       TUK == TUK_Definition,  KWLoc, Name)) {
DeclResult Sema::CheckClassTemplate(
 
   // Add alignment attributes if necessary; these attributes are checked when
   // the ASTContext lays out the structure.
   if (TUK == TUK_Definition && (!SkipBody || !SkipBody->ShouldSkip)) {
     AddAlignmentAttributesForRecord(NewClass);
     AddMsStructLayoutForRecord(NewClass);
   }
 
-  // Attach the associated constraints when the declaration will not be part of
-  // a decl chain.
-  Expr *const ACtoAttach =
-      PrevClassTemplate && ShouldAddRedecl ? nullptr : CurAC;
-
   ClassTemplateDecl *NewTemplate
     = ClassTemplateDecl::Create(Context, SemanticContext, NameLoc,
                                 DeclarationName(Name), TemplateParams,
-                                NewClass, ACtoAttach);
+                                NewClass);
 
   if (ShouldAddRedecl)
     NewTemplate->setPreviousDecl(PrevClassTemplate);
 
   NewClass->setDescribedClassTemplate(NewTemplate);
 
   if (ModulePrivateLoc.isValid())
     NewTemplate->setModulePrivate();
return S.TemplateParameterListsAreEqual(NewTTP->getTemplateParameters(),
                                             OldTTP->getTemplateParameters(),
                                             Complain,
                                         (Kind == Sema::TPL_TemplateMatch
                                            ? Sema::TPL_TemplateTemplateParmMatch
                                            : Kind),
                                             TemplateArgLoc);
   }
 
+  // TODO: Concepts: Match immediately-introduced-constraint for type
+  // constraints
+
   return true;
 }
 
 /// Diagnose a known arity mismatch when comparing template argument
 /// lists.
 static
 void DiagnoseTemplateParameterListArityMismatch(Sema &S,
                                                 TemplateParameterList *New,
S.Diag(New->getTemplateLoc(), NextDiag)
     << (New->size() > Old->size())
     << (Kind != Sema::TPL_TemplateMatch)
     << SourceRange(New->getTemplateLoc(), New->getRAngleLoc());
   S.Diag(Old->getTemplateLoc(), diag::note_template_prev_declaration)
     << (Kind != Sema::TPL_TemplateMatch)
     << 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.
 ///
 /// \param New  The new template parameter list, typically written in the
 /// source code as part of a new template declaration.
 ///
 /// \param Old  The old template parameter list, typically found via
 /// name lookup of the template declared with this template parameter
Sema::TemplateParameterListsAreEqual(TemplateParameterList *New,
   if (NewParm != NewParmEnd) {
     if (Complain)
       DiagnoseTemplateParameterListArityMismatch(*this, New, Old, Kind,
                                                  TemplateArgLoc);
 
     return false;
   }
 
+  if (Kind != TPL_TemplateTemplateArgumentMatch) {
+    const Expr *NewRC = New->getRequiresClause();
+    const Expr *OldRC = Old->getRequiresClause();
+    if (!NewRC != !OldRC) {
+      if (Complain)
+        DiagnoseTemplateParameterListRequiresClauseMismatch(*this, New, Old);
+      return false;
+    }
+
+    if (NewRC) {
+      llvm::FoldingSetNodeID OldRCID, NewRCID;
+      OldRC->Profile(OldRCID, Context, /*Canonical=*/true);
+      NewRC->Profile(NewRCID, Context, /*Canonical=*/true);
+      if (OldRCID != NewRCID) {
+        if (Complain)
+          DiagnoseTemplateParameterListRequiresClauseMismatch(*this, New, Old);
+        return false;
+      }
+    }
+  }
+
   return true;
 }
 
 /// Check whether a template can be declared within this scope.
 ///
 /// If the template declaration is valid in this scope, returns
 /// false. Otherwise, issues a diagnostic and returns true.
 bool
if (TemplateParameterLists.front()->size() == 0) {
     Diag(NameLoc, diag::err_concept_no_parameters);
     return nullptr;
   }
 
   ConceptDecl *NewDecl = ConceptDecl::Create(Context, DC, NameLoc, Name,
                                              TemplateParameterLists.front(),
                                              ConstraintExpr);
                                              
-  if (NewDecl->getAssociatedConstraints()) {
+  if (NewDecl->hasAssociatedConstraints()) {
     // C++2a [temp.concept]p4:
     // A concept shall not have associated constraints.
-    // TODO: Make a test once we have actual associated constraints.
     Diag(NameLoc, diag::err_concept_no_associated_constraints);
     NewDecl->setInvalidDecl();
   }
 
   // Check for conflicting previous declaration.
   DeclarationNameInfo NameInfo(NewDecl->getDeclName(), NameLoc);
   LookupResult Previous(*this, NameInfo, LookupOrdinaryName,
                         ForVisibleRedeclaration);

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

for (auto &P : *L) {
     Params.push_back(D);
     Invalid = Invalid || !D || D->isInvalidDecl();
   }
 
   // Clean up if we had an error.
   if (Invalid)
     return nullptr;
 
-  // Note: we substitute into associated constraints later
-  Expr *const UninstantiatedRequiresClause = L->getRequiresClause();
+  // FIXME: Concepts: Substitution into requires clause should only happen when
+  // checking satisfaction.
+  Expr *InstRequiresClause = nullptr;
+  if (Expr *E = L->getRequiresClause()) {
+    ExprResult Res = SemaRef.SubstExpr(E, TemplateArgs);
+    if (Res.isInvalid() || !Res.isUsable()) {
+      return nullptr;
+    }
+    InstRequiresClause = Res.get();
+  }
 
   TemplateParameterList *InstL
     = TemplateParameterList::Create(SemaRef.Context, L->getTemplateLoc(),
                                     L->getLAngleLoc(), Params,
-                                    L->getRAngleLoc(),
-                                    UninstantiatedRequiresClause);
+                                    L->getRAngleLoc(), InstRequiresClause);
   return InstL;
 }
 
 TemplateParameterList *
 Sema::SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
                           const MultiLevelTemplateArgumentList &TemplateArgs) {
   TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs);
   return Instantiator.SubstTemplateParams(Params);

clang/lib/Serialization/ASTReader.cpp

ASTReader::ReadTemplateParameterList(ModuleFile &F,
   SourceLocation RAngleLoc = ReadSourceLocation(F, Record, Idx);
 
   unsigned NumParams = Record[Idx++];
   SmallVector<NamedDecl *, 16> Params;
   Params.reserve(NumParams);
   while (NumParams--)
     Params.push_back(ReadDeclAs<NamedDecl>(F, Record, Idx));
 
-  // TODO: Concepts
+  bool HasRequiresClause = Record[Idx++];
+  Expr *RequiresClause = HasRequiresClause ? ReadExpr(F) : nullptr;
+
   TemplateParameterList *TemplateParams = TemplateParameterList::Create(
-      getContext(), TemplateLoc, LAngleLoc, Params, RAngleLoc, nullptr);
+      getContext(), TemplateLoc, LAngleLoc, Params, RAngleLoc, RequiresClause);
   return TemplateParams;
 }
 
 void
 ASTReader::
 ReadTemplateArgumentList(SmallVectorImpl<TemplateArgument> &TemplArgs,
                          ModuleFile &F, const RecordData &Record,
                          unsigned &Idx, bool Canonicalize) {

clang/lib/Serialization/ASTReaderDecl.cpp

 }
 
 DeclID ASTDeclReader::VisitTemplateDecl(TemplateDecl *D) {
   VisitNamedDecl(D);
 
   DeclID PatternID = ReadDeclID();
   auto *TemplatedDecl = cast_or_null<NamedDecl>(Reader.GetDecl(PatternID));
   TemplateParameterList *TemplateParams = Record.readTemplateParameterList();
-  // FIXME handle associated constraints
   D->init(TemplatedDecl, TemplateParams);
 
   return PatternID;
 }
 
 void ASTDeclReader::VisitConceptDecl(ConceptDecl *D) {
   VisitTemplateDecl(D);
   D->ConstraintExpr = Record.readExpr();
ASTDeclReader::VisitClassTemplateSpecializationDeclImpl(
 
   return Redecl;
 }
 
 void ASTDeclReader::VisitClassTemplatePartialSpecializationDecl(
                                     ClassTemplatePartialSpecializationDecl *D) {
   RedeclarableResult Redecl = VisitClassTemplateSpecializationDeclImpl(D);
 
-  D->TemplateParams = Record.readTemplateParameterList();
+  TemplateParameterList *Params = Record.readTemplateParameterList();
+  D->TemplateParams = Params;
   D->ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
 
   // These are read/set from/to the first declaration.
   if (ThisDeclID == Redecl.getFirstID()) {
     D->InstantiatedFromMember.setPointer(
       ReadDeclAs<ClassTemplatePartialSpecializationDecl>());
     D->InstantiatedFromMember.setInt(Record.readInt());
   }

 ///       May require unifying ClassTemplate(Partial)SpecializationDecl and
 ///        VarTemplate(Partial)SpecializationDecl with a new data
 ///        structure Template(Partial)SpecializationDecl, and
 ///        using Template(Partial)SpecializationDecl as input type.
 void ASTDeclReader::VisitVarTemplatePartialSpecializationDecl(
     VarTemplatePartialSpecializationDecl *D) {
   RedeclarableResult Redecl = VisitVarTemplateSpecializationDeclImpl(D);
 
-  D->TemplateParams = Record.readTemplateParameterList();
+  TemplateParameterList *Params = Record.readTemplateParameterList();
+  D->TemplateParams = Params;
   D->ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
 
   // These are read/set from/to the first declaration.
   if (ThisDeclID == Redecl.getFirstID()) {
     D->InstantiatedFromMember.setPointer(
         ReadDeclAs<VarTemplatePartialSpecializationDecl>());
     D->InstantiatedFromMember.setInt(Record.readInt());
   }
 }
 
 void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   VisitTypeDecl(D);
 
   D->setDeclaredWithTypename(Record.readInt());
 
+  // TODO: Concepts: Immediately introduced constraint
   if (Record.readInt())
     D->setDefaultArgument(GetTypeSourceInfo());
 }
 
 void ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   VisitDeclaratorDecl(D);
   // TemplateParmPosition.
   D->setDepth(Record.readInt());

clang/lib/Serialization/ASTWriter.cpp

 }
 
 void ASTRecordWriter::AddTemplateParameterList(
     const TemplateParameterList *TemplateParams) {
   assert(TemplateParams && "No TemplateParams!");
   AddSourceLocation(TemplateParams->getTemplateLoc());
   AddSourceLocation(TemplateParams->getLAngleLoc());
   AddSourceLocation(TemplateParams->getRAngleLoc());
-  // TODO: Concepts
+
   Record->push_back(TemplateParams->size());
   for (const auto &P : *TemplateParams)
     AddDeclRef(P);
+  if (const Expr *RequiresClause = TemplateParams->getRequiresClause()) {
+    Record->push_back(true);
+    AddStmt(const_cast<Expr*>(RequiresClause));
+  } else {
+    Record->push_back(false);
+  }
 }
 
 /// Emit a template argument list.
 void ASTRecordWriter::AddTemplateArgumentList(
     const TemplateArgumentList *TemplateArgs) {
   assert(TemplateArgs && "No TemplateArgs!");
   Record->push_back(TemplateArgs->size());
   for (int i = 0, e = TemplateArgs->size(); i != e; ++i)

clang/lib/Serialization/ASTWriterDecl.cpp

if (D->isFirstDecl())
     AddTemplateSpecializations(D);
   Code = serialization::DECL_FUNCTION_TEMPLATE;
 }
 
 void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   VisitTypeDecl(D);
 
   Record.push_back(D->wasDeclaredWithTypename());
-
+  // TODO: Concepts - constrained parameters.
   bool OwnsDefaultArg = D->hasDefaultArgument() &&
                         !D->defaultArgumentWasInherited();
   Record.push_back(OwnsDefaultArg);
   if (OwnsDefaultArg)
     Record.AddTypeSourceInfo(D->getDefaultArgumentInfo());
 
   Code = serialization::DECL_TEMPLATE_TYPE_PARM;
 }
void ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   if (D->isExpandedParameterPack()) {
     for (unsigned I = 0, N = D->getNumExpansionTypes(); I != N; ++I) {
       Record.AddTypeRef(D->getExpansionType(I));
       Record.AddTypeSourceInfo(D->getExpansionTypeSourceInfo(I));
     }
 
     Code = serialization::DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK;
   } else {
+    // TODO: Concepts - constrained parameters.
     // Rest of NonTypeTemplateParmDecl.
     Record.push_back(D->isParameterPack());
     bool OwnsDefaultArg = D->hasDefaultArgument() &&
                           !D->defaultArgumentWasInherited();
     Record.push_back(OwnsDefaultArg);
     if (OwnsDefaultArg)
       Record.AddStmt(D->getDefaultArgument());
     Code = serialization::DECL_NON_TYPE_TEMPLATE_PARM;
void ASTDeclWriter::VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D) {
   Record.push_back(D->getPosition());
 
   if (D->isExpandedParameterPack()) {
     for (unsigned I = 0, N = D->getNumExpansionTemplateParameters();
          I != N; ++I)
       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() &&
                           !D->defaultArgumentWasInherited();
     Record.push_back(OwnsDefaultArg);
     if (OwnsDefaultArg)
       Record.AddTemplateArgumentLoc(D->getDefaultArgument());
     Code = serialization::DECL_TEMPLATE_TEMPLATE_PARM;

clang/test/CXX/concepts-ts/temp/concept/p4.cpp

+// RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
+
+template<typename T> requires true
+concept C = true; // expected-error{{concept cannot have associated constraints}}
+
+// TODO: Add test for other kinds of associated constraints once we have them.

clang/test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/class-template-decl.cpp

-// RUN: %clang_cc1 -std=c++14 -fconcepts-ts -x c++ -verify %s
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
 
 namespace nodiag {
 
 template <typename T> requires bool(T())
 struct A;
 template <typename U> requires bool(U())
 struct A;
 
 } // end namespace nodiag
 
 namespace diag {
 
 template <typename T> requires true // expected-note{{previous template declaration is here}}
 struct A;
-template <typename T> struct A; // expected-error{{associated constraints differ in template redeclaration}}
+template <typename T> struct A; // expected-error{{requires clause differs in template redeclaration}}
 
 template <typename T> struct B; // expected-note{{previous template declaration is here}}
-template <typename T> requires true // expected-error{{associated constraints differ in template redeclaration}}
+template <typename T> requires true // expected-error{{requires clause differs in template redeclaration}}
 struct B;
 
 template <typename T> requires true // expected-note{{previous template declaration is here}}
 struct C;
-template <typename T> requires !0 // expected-error{{associated constraints differ in template redeclaration}}
+template <typename T> requires !0 // expected-error{{requires clause differs in template redeclaration}}
 struct C;
 
 } // end namespace diag
 
 namespace nodiag {
 
 struct AA {
   template <typename T> requires someFunc(T())
   struct A;
 };
 
-template <typename T> requires someFunc(T())
+template <typename U> requires someFunc(U())
 struct AA::A { };
 
 struct AAF {
   template <typename T> requires someFunc(T())
   friend struct AA::A;
 };
 
 } // end namespace nodiag
 
 namespace diag {
 
 template <unsigned N>
 struct TA {
-  template <template <unsigned> class TT> requires TT<N>::happy // expected-note 2{{previous template declaration is here}}
+  template <template <unsigned> class TT> requires TT<N>::happy // expected-note {{previous template declaration is here}}
   struct A;
 
+  template <template <unsigned> class TT> requires TT<N>::happy // expected-note {{previous template declaration is here}}
+  struct B;
+
   struct AF;
 };
 
 template <unsigned N>
-template <template <unsigned> class TT> struct TA<N>::A { }; // expected-error{{associated constraints differ in template redeclaration}}
+template <template <unsigned> class TT> struct TA<N>::A { }; // expected-error{{requires clause differs in template redeclaration}}
+
+
+template <unsigned N>
+template <template <unsigned> class TT> requires TT<N + 1>::happy struct TA<N>::B { }; // expected-error{{requires clause differs in template redeclaration}}
 
 template <unsigned N>
 struct TA<N>::AF {
-  template <template <unsigned> class TT> requires TT<N + 0>::happy // expected-error{{associated constraints differ in template redeclaration}}
+  // we do not expect a diagnostic here because the template parameter list is dependent.
+  template <template <unsigned> class TT> requires TT<N + 0>::happy
   friend struct TA::A;
 };
 
 } // end namespace diag

clang/test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/func-template-decl.cpp

+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+namespace nodiag {
+
+template <typename T> requires bool(T())
+int A();
+template <typename U> requires bool(U())
+int A();
+
+} // end namespace nodiag
+
+namespace diag {
+
+namespace orig {
+  template <typename T> requires true
+  int A();
+  template <typename T>
+  int B();
+  template <typename T> requires true
+  int C();
+}
+
+template <typename T>
+int orig::A();
+// expected-error@-1{{out-of-line declaration of 'A' does not match any declaration in namespace 'diag::orig'}}
+template <typename T> requires true
+int orig::B();
+// expected-error@-1{{out-of-line declaration of 'B' does not match any declaration in namespace 'diag::orig'}}
+template <typename T> requires !0
+int orig::C();
+// expected-error@-1{{out-of-line declaration of 'C' does not match any declaration in namespace 'diag::orig'}}
+
+} // end namespace diag
+
+namespace nodiag {
+
+struct AA {
+  template <typename T> requires someFunc(T())
+  int A();
+};
+
+template <typename T> requires someFunc(T())
+int AA::A() { return sizeof(T); }
+
+} // end namespace nodiag
+
+namespace diag {
+
+template <unsigned N>
+struct TA {
+  template <template <unsigned> class TT> requires TT<N>::happy
+  int A();
+};
+
+template <unsigned N>
+template <template <unsigned> class TT> int TA<N>::A() { return sizeof(TT<N>); }
+// expected-error@-1{{out-of-line definition of 'A' does not match any declaration in 'TA<N>'}}
+
+} // end namespace diag

clang/test/CXX/concepts-ts/temp/temp.constr/temp.constr.decl/var-template-decl.cpp

+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ -verify %s
+
+namespace nodiag {
+
+struct B {
+    template <typename T> requires bool(T())
+    static int A;
+};
+
+template <typename U> requires bool(U())
+int B::A = int(U());
+
+} // end namespace nodiag
+
+namespace diag {
+
+struct B {
+    template <typename T> requires bool(T()) // expected-note{{previous template declaration is here}}
+    static int A;
+};
+
+template <typename U> requires !bool(U())  // expected-error{{requires clause differs in template redeclaration}}
+int B::A = int(U());
+
+} // end namespace diag
 No newline at end of file