[clang] befa8cf - Re-apply "Deferred Concept Instantiation Implementation""

Erich Keane via cfe-commits cfe-commits at lists.llvm.org
Fri Jul 1 06:51:43 PDT 2022


Author: Erich Keane
Date: 2022-07-01T06:51:38-07:00
New Revision: befa8cf087dbb8159a4d9dc8fa4d6748d6d5049a

URL: https://github.com/llvm/llvm-project/commit/befa8cf087dbb8159a4d9dc8fa4d6748d6d5049a
DIFF: https://github.com/llvm/llvm-project/commit/befa8cf087dbb8159a4d9dc8fa4d6748d6d5049a.diff

LOG: Re-apply "Deferred Concept Instantiation Implementation""

This reverts commit d4d47e574ecae562ab32f8ac7fa3f4d424bb6574.

This fixes the lldb crash that was observed by ensuring that our
friend-'template contains reference to' TreeTransform properly handles a
TemplateDecl.

Added: 
    clang/test/SemaTemplate/concepts-friends.cpp
    clang/test/SemaTemplate/deferred-concept-inst.cpp
    clang/test/SemaTemplate/trailing-return-short-circuit.cpp

Modified: 
    clang/docs/ReleaseNotes.rst
    clang/include/clang/AST/Decl.h
    clang/include/clang/AST/DeclBase.h
    clang/include/clang/Sema/Sema.h
    clang/include/clang/Sema/Template.h
    clang/lib/AST/ASTImporter.cpp
    clang/lib/AST/Decl.cpp
    clang/lib/AST/DeclBase.cpp
    clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
    clang/lib/Sema/SemaConcept.cpp
    clang/lib/Sema/SemaOverload.cpp
    clang/lib/Sema/SemaTemplate.cpp
    clang/lib/Sema/SemaTemplateDeduction.cpp
    clang/lib/Sema/SemaTemplateInstantiate.cpp
    clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
    clang/lib/Sema/TreeTransform.h
    clang/lib/Serialization/ASTReaderDecl.cpp
    clang/lib/Serialization/ASTWriterDecl.cpp
    clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
    clang/test/CXX/temp/temp.constr/temp.constr.order/class-template-partial-specializations.cpp
    clang/test/CXX/temp/temp.constr/temp.constr.order/var-template-partial-specializations.cpp
    clang/test/SemaTemplate/concepts.cpp
    clang/test/SemaTemplate/instantiate-requires-clause.cpp

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3342f6208c4f8..26d769deee6d4 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -451,10 +451,11 @@ C++20 Feature Support
 - No longer attempt to evaluate a consteval UDL function call at runtime when
   it is called through a template instantiation. This fixes
   `Issue 54578 <https://github.com/llvm/llvm-project/issues/54578>`_.
-
-- Implemented ``__builtin_source_location()``, which enables library support
-  for ``std::source_location``.
-
+- Implemented `__builtin_source_location()` which enables library support for std::source_location.
+- Clang now correctly delays the instantiation of function constraints until
+  the time of checking, which should now allow the libstdc++ ranges implementation
+  to work for at least trivial examples.  This fixes
+  `Issue 44178 <https://github.com/llvm/llvm-project/issues/44178>`_.
 - The mangling scheme for C++20 modules has incompatibly changed. The
   initial mangling was discovered not to be reversible, and the weak
   ownership design decision did not give the backwards compatibility

diff  --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 66fab94b45b8a..0a7dc63d1c25d 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -1890,7 +1890,9 @@ class FunctionDecl : public DeclaratorDecl,
     TK_FunctionTemplateSpecialization,
     // A function template specialization that hasn't yet been resolved to a
     // particular specialized function template.
-    TK_DependentFunctionTemplateSpecialization
+    TK_DependentFunctionTemplateSpecialization,
+    // A non templated function which is in a dependent scope.
+    TK_DependentNonTemplate
   };
 
   /// Stashed information about a defaulted function definition whose body has
@@ -1939,20 +1941,21 @@ class FunctionDecl : public DeclaratorDecl,
   /// The template or declaration that this declaration
   /// describes or was instantiated from, respectively.
   ///
-  /// For non-templates, this value will be NULL. For function
-  /// declarations that describe a function template, this will be a
-  /// pointer to a FunctionTemplateDecl. For member functions
-  /// of class template specializations, this will be a MemberSpecializationInfo
-  /// pointer containing information about the specialization.
-  /// For function template specializations, this will be a
-  /// FunctionTemplateSpecializationInfo, which contains information about
-  /// the template being specialized and the template arguments involved in
-  /// that specialization.
-  llvm::PointerUnion<FunctionTemplateDecl *,
-                     MemberSpecializationInfo *,
+  /// For non-templates this value will be NULL, unless this non-template
+  /// function declaration was declared directly inside of a function template,
+  /// in which case this will have a pointer to a FunctionDecl, stored in the
+  /// NamedDecl. For function declarations that describe a function template,
+  /// this will be a pointer to a FunctionTemplateDecl, stored in the NamedDecl.
+  /// For member functions of class template specializations, this will be a
+  /// MemberSpecializationInfo pointer containing information about the
+  /// specialization. For function template specializations, this will be a
+  /// FunctionTemplateSpecializationInfo, which contains information about the
+  /// template being specialized and the template arguments involved in that
+  /// specialization.
+  llvm::PointerUnion<NamedDecl *, MemberSpecializationInfo *,
                      FunctionTemplateSpecializationInfo *,
                      DependentFunctionTemplateSpecializationInfo *>
-    TemplateOrSpecialization;
+      TemplateOrSpecialization;
 
   /// Provides source/type location info for the declaration name embedded in
   /// the DeclaratorDecl base class.
@@ -2695,6 +2698,11 @@ class FunctionDecl : public DeclaratorDecl,
     setInstantiationOfMemberFunction(getASTContext(), FD, TSK);
   }
 
+  /// Specify that this function declaration was instantiated from FunctionDecl
+  /// FD. This is only used if this is a function declaration declared locally
+  /// inside of a function template.
+  void setInstantiatedFromDecl(FunctionDecl *FD);
+
   /// Retrieves the function template that is described by this
   /// function declaration.
   ///
@@ -2709,6 +2717,8 @@ class FunctionDecl : public DeclaratorDecl,
   /// FunctionTemplateDecl from a FunctionDecl.
   FunctionTemplateDecl *getDescribedFunctionTemplate() const;
 
+  FunctionDecl *getInstantiatedFromDecl() const;
+
   void setDescribedFunctionTemplate(FunctionTemplateDecl *Template);
 
   /// Determine whether this function is a function template

diff  --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 52fe8dd6b1e57..3a5cb5cb7b723 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -920,10 +920,10 @@ class alignas(8) Decl {
 
   /// If this decl is defined inside a function/method/block it returns
   /// the corresponding DeclContext, otherwise it returns null.
-  const DeclContext *getParentFunctionOrMethod() const;
-  DeclContext *getParentFunctionOrMethod() {
-    return const_cast<DeclContext*>(
-                    const_cast<const Decl*>(this)->getParentFunctionOrMethod());
+  const DeclContext *getParentFunctionOrMethod(bool Lexical = false) const;
+  DeclContext *getParentFunctionOrMethod(bool Lexical = false) {
+    return const_cast<DeclContext *>(
+        const_cast<const Decl *>(this)->getParentFunctionOrMethod(Lexical));
   }
 
   /// Retrieves the "canonical" declaration of the given declaration.

diff  --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index f979a2cae818a..8f40313c55c71 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -3622,6 +3622,11 @@ class Sema final {
                   bool ConsiderCudaAttrs = true,
                   bool ConsiderRequiresClauses = true);
 
+  // Calculates whether the expression Constraint depends on an enclosing
+  // template, for the purposes of [temp.friend] p9.
+  bool ConstraintExpressionDependsOnEnclosingTemplate(unsigned TemplateDepth,
+                                                      const Expr *Constraint);
+
   enum class AllowedExplicit {
     /// Allow no explicit functions to be used.
     None,
@@ -7091,6 +7096,21 @@ class Sema final {
       LocalInstantiationScope &Scope,
       const MultiLevelTemplateArgumentList &TemplateArgs);
 
+  /// used by SetupConstraintCheckingTemplateArgumentsAndScope to recursively(in
+  /// the case of lambdas) set up the LocalInstantiationScope of the current
+  /// function.
+  bool SetupConstraintScope(
+      FunctionDecl *FD, llvm::Optional<ArrayRef<TemplateArgument>> TemplateArgs,
+      MultiLevelTemplateArgumentList MLTAL, LocalInstantiationScope &Scope);
+
+  /// Used during constraint checking, sets up the constraint template arguemnt
+  /// lists, and calls SetupConstraintScope to set up the
+  /// LocalInstantiationScope to have the proper set of ParVarDecls configured.
+  llvm::Optional<MultiLevelTemplateArgumentList>
+  SetupConstraintCheckingTemplateArgumentsAndScope(
+      FunctionDecl *FD, llvm::Optional<ArrayRef<TemplateArgument>> TemplateArgs,
+      LocalInstantiationScope &Scope);
+
 public:
   const NormalizedConstraint *
   getNormalizedAssociatedConstraints(
@@ -7121,8 +7141,10 @@ class Sema final {
   /// check (either a concept or a constrained entity).
   /// \param ConstraintExprs a list of constraint expressions, treated as if
   /// they were 'AND'ed together.
-  /// \param TemplateArgs the list of template arguments to substitute into the
-  /// constraint expression.
+  /// \param TemplateArgList the multi-level list of template arguments to
+  /// substitute into the constraint expression. This should be relative to the
+  /// top-level (hence multi-level), since we need to instantiate fully at the
+  /// time of checking.
   /// \param TemplateIDRange The source range of the template id that
   /// caused the constraints check.
   /// \param Satisfaction if true is returned, will contain details of the
@@ -7132,7 +7154,40 @@ class Sema final {
   /// false otherwise.
   bool CheckConstraintSatisfaction(
       const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
-      ArrayRef<TemplateArgument> TemplateArgs,
+      const MultiLevelTemplateArgumentList &TemplateArgList,
+      SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction) {
+    llvm::SmallVector<Expr *, 4> Converted;
+    return CheckConstraintSatisfaction(Template, ConstraintExprs, Converted,
+                                       TemplateArgList, TemplateIDRange,
+                                       Satisfaction);
+  }
+
+  /// \brief Check whether the given list of constraint expressions are
+  /// satisfied (as if in a 'conjunction') given template arguments.
+  /// Additionally, takes an empty list of Expressions which is populated with
+  /// the instantiated versions of the ConstraintExprs.
+  /// \param Template the template-like entity that triggered the constraints
+  /// check (either a concept or a constrained entity).
+  /// \param ConstraintExprs a list of constraint expressions, treated as if
+  /// they were 'AND'ed together.
+  /// \param ConvertedConstraints a out parameter that will get populated with
+  /// the instantiated version of the ConstraintExprs if we successfully checked
+  /// satisfaction.
+  /// \param TemplateArgList the multi-level list of template arguments to
+  /// substitute into the constraint expression. This should be relative to the
+  /// top-level (hence multi-level), since we need to instantiate fully at the
+  /// time of checking.
+  /// \param TemplateIDRange The source range of the template id that
+  /// caused the constraints check.
+  /// \param Satisfaction if true is returned, will contain details of the
+  /// satisfaction, with enough information to diagnose an unsatisfied
+  /// expression.
+  /// \returns true if an error occurred and satisfaction could not be checked,
+  /// false otherwise.
+  bool CheckConstraintSatisfaction(
+      const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
+      llvm::SmallVectorImpl<Expr *> &ConvertedConstraints,
+      const MultiLevelTemplateArgumentList &TemplateArgList,
       SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction);
 
   /// \brief Check whether the given non-dependent constraint expression is
@@ -7168,9 +7223,9 @@ class Sema final {
   ///
   /// \returns true if the constrains are not satisfied or could not be checked
   /// for satisfaction, false if the constraints are satisfied.
-  bool EnsureTemplateArgumentListConstraints(TemplateDecl *Template,
-                                       ArrayRef<TemplateArgument> TemplateArgs,
-                                             SourceRange TemplateIDRange);
+  bool EnsureTemplateArgumentListConstraints(
+      TemplateDecl *Template, MultiLevelTemplateArgumentList TemplateArgs,
+      SourceRange TemplateIDRange);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.
@@ -8904,7 +8959,8 @@ class Sema final {
 
   MultiLevelTemplateArgumentList getTemplateInstantiationArgs(
       const NamedDecl *D, const TemplateArgumentList *Innermost = nullptr,
-      bool RelativeToPrimary = false, const FunctionDecl *Pattern = nullptr);
+      bool RelativeToPrimary = false, const FunctionDecl *Pattern = nullptr,
+      bool LookBeyondLambda = false, bool IncludeContainingStruct = false);
 
   /// A context in which code is being synthesized (where a source location
   /// alone is not sufficient to identify the context). This covers template
@@ -9637,6 +9693,11 @@ class Sema final {
                       ExtParameterInfoBuilder &ParamInfos);
   ExprResult SubstExpr(Expr *E,
                        const MultiLevelTemplateArgumentList &TemplateArgs);
+  // Unlike the above, this evaluates constraints, which should only happen at
+  // 'constraint checking' time.
+  ExprResult
+  SubstConstraintExpr(Expr *E,
+                      const MultiLevelTemplateArgumentList &TemplateArgs);
 
   /// Substitute the given template arguments into a list of
   /// expressions, expanding pack expansions if required.
@@ -9660,13 +9721,14 @@ class Sema final {
 
   TemplateParameterList *
   SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
-                      const MultiLevelTemplateArgumentList &TemplateArgs);
+                      const MultiLevelTemplateArgumentList &TemplateArgs,
+                      bool InstantiateConstraints = false);
 
   bool
   SubstTemplateArguments(ArrayRef<TemplateArgumentLoc> Args,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
-                         TemplateArgumentListInfo &Outputs);
-
+                         TemplateArgumentListInfo &Outputs,
+                         bool InstantiateConstraints = false);
 
   Decl *SubstDecl(Decl *D, DeclContext *Owner,
                   const MultiLevelTemplateArgumentList &TemplateArgs);
@@ -9758,7 +9820,8 @@ class Sema final {
                     const MultiLevelTemplateArgumentList &TemplateArgs);
 
   bool SubstTypeConstraint(TemplateTypeParmDecl *Inst, const TypeConstraint *TC,
-                           const MultiLevelTemplateArgumentList &TemplateArgs);
+                           const MultiLevelTemplateArgumentList &TemplateArgs,
+                           bool isEvaluatingAConstraint);
 
   bool InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                   ParmVarDecl *Param);

diff  --git a/clang/include/clang/Sema/Template.h b/clang/include/clang/Sema/Template.h
index 5dcde77b5dd37..602faf8607412 100644
--- a/clang/include/clang/Sema/Template.h
+++ b/clang/include/clang/Sema/Template.h
@@ -75,6 +75,8 @@ enum class TemplateSubstitutionKind : char {
   class MultiLevelTemplateArgumentList {
     /// The template argument list at a certain template depth
     using ArgList = ArrayRef<TemplateArgument>;
+    using ArgListsIterator = SmallVector<ArgList, 4>::iterator;
+    using ConstArgListsIterator = SmallVector<ArgList, 4>::const_iterator;
 
     /// The template argument lists, stored from the innermost template
     /// argument list (first) to the outermost template argument list (last).
@@ -121,6 +123,12 @@ enum class TemplateSubstitutionKind : char {
       return TemplateArgumentLists.size();
     }
 
+    /// Determine the number of substituted args at 'Depth'.
+    unsigned getNumSubstitutedArgs(unsigned Depth) const {
+      assert(NumRetainedOuterLevels <= Depth && Depth < getNumLevels());
+      return TemplateArgumentLists[getNumLevels() - Depth - 1].size();
+    }
+
     unsigned getNumRetainedOuterLevels() const {
       return NumRetainedOuterLevels;
     }
@@ -158,6 +166,14 @@ enum class TemplateSubstitutionKind : char {
       return !(*this)(Depth, Index).isNull();
     }
 
+    bool isAnyArgInstantiationDependent() const {
+      for (ArgList List : TemplateArgumentLists)
+        for (const TemplateArgument &TA : List)
+          if (TA.isInstantiationDependent())
+            return true;
+      return false;
+    }
+
     /// Clear out a specific template argument.
     void setArgument(unsigned Depth, unsigned Index,
                      TemplateArgument Arg) {
@@ -183,6 +199,14 @@ enum class TemplateSubstitutionKind : char {
       TemplateArgumentLists.push_back(Args);
     }
 
+    /// Replaces the current 'innermost' level with the provided argument list.
+    /// This is useful for type deduction cases where we need to get the entire
+    /// list from the AST, but then add the deduced innermost list.
+    void replaceInnermostTemplateArguments(ArgList Args) {
+      assert(TemplateArgumentLists.size() > 0 && "Replacing in an empty list?");
+      TemplateArgumentLists[0] = Args;
+    }
+
     /// Add an outermost level that we are not substituting. We have no
     /// arguments at this level, and do not remove it from the depth of inner
     /// template parameters that we instantiate.
@@ -197,6 +221,16 @@ enum class TemplateSubstitutionKind : char {
     const ArgList &getInnermost() const {
       return TemplateArgumentLists.front();
     }
+
+    /// Retrieve the outermost template argument list.
+    const ArgList &getOutermost() const { return TemplateArgumentLists.back(); }
+
+    ArgListsIterator begin() { return TemplateArgumentLists.begin(); }
+    ConstArgListsIterator begin() const {
+      return TemplateArgumentLists.begin();
+    }
+    ArgListsIterator end() { return TemplateArgumentLists.end(); }
+    ConstArgListsIterator end() const { return TemplateArgumentLists.end(); }
   };
 
   /// The context in which partial ordering of function templates occurs.
@@ -469,6 +503,7 @@ enum class TemplateSubstitutionKind : char {
     const MultiLevelTemplateArgumentList &TemplateArgs;
     Sema::LateInstantiatedAttrVec* LateAttrs = nullptr;
     LocalInstantiationScope *StartingScope = nullptr;
+    bool EvaluatingAConstraint = false;
 
     /// A list of out-of-line class template partial
     /// specializations that will need to be instantiated after the
@@ -487,10 +522,12 @@ enum class TemplateSubstitutionKind : char {
 
   public:
     TemplateDeclInstantiator(Sema &SemaRef, DeclContext *Owner,
-                             const MultiLevelTemplateArgumentList &TemplateArgs)
+                             const MultiLevelTemplateArgumentList &TemplateArgs,
+                             bool EvaluatingConstraint = false)
         : SemaRef(SemaRef),
           SubstIndex(SemaRef, SemaRef.ArgumentPackSubstitutionIndex),
-          Owner(Owner), TemplateArgs(TemplateArgs) {}
+          Owner(Owner), TemplateArgs(TemplateArgs),
+          EvaluatingAConstraint(EvaluatingConstraint) {}
 
 // Define all the decl visitors using DeclNodes.inc
 #define DECL(DERIVED, BASE) \

diff  --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 8d930ead97d3e..7a20807aa1f90 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -3109,6 +3109,11 @@ Error ASTNodeImporter::ImportTemplateInformation(
   case FunctionDecl::TK_FunctionTemplate:
     return Error::success();
 
+  case FunctionDecl::TK_DependentNonTemplate:
+    if (Expected<FunctionDecl *> InstFDOrErr =
+            import(FromFD->getInstantiatedFromDecl()))
+      ToFD->setInstantiatedFromDecl(*InstFDOrErr);
+    return Error::success();
   case FunctionDecl::TK_MemberSpecialization: {
     TemplateSpecializationKind TSK = FromFD->getTemplateSpecializationKind();
 

diff  --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 5e5101203e6cf..3bf3bd66ff326 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3732,8 +3732,12 @@ const IdentifierInfo *FunctionDecl::getLiteralIdentifier() const {
 FunctionDecl::TemplatedKind FunctionDecl::getTemplatedKind() const {
   if (TemplateOrSpecialization.isNull())
     return TK_NonTemplate;
-  if (TemplateOrSpecialization.is<FunctionTemplateDecl *>())
+  if (auto *ND = TemplateOrSpecialization.dyn_cast<NamedDecl *>()) {
+    if (isa<FunctionDecl>(ND))
+      return TK_DependentNonTemplate;
+    assert(isa<FunctionTemplateDecl>(ND) && "No other types it could be?");
     return TK_FunctionTemplate;
+  }
   if (TemplateOrSpecialization.is<MemberSpecializationInfo *>())
     return TK_MemberSpecialization;
   if (TemplateOrSpecialization.is<FunctionTemplateSpecializationInfo *>())
@@ -3774,15 +3778,28 @@ FunctionDecl::setInstantiationOfMemberFunction(ASTContext &C,
 }
 
 FunctionTemplateDecl *FunctionDecl::getDescribedFunctionTemplate() const {
-  return TemplateOrSpecialization.dyn_cast<FunctionTemplateDecl *>();
+  return dyn_cast_or_null<FunctionTemplateDecl>(
+      TemplateOrSpecialization.dyn_cast<NamedDecl *>());
 }
 
-void FunctionDecl::setDescribedFunctionTemplate(FunctionTemplateDecl *Template) {
+void FunctionDecl::setDescribedFunctionTemplate(
+    FunctionTemplateDecl *Template) {
   assert(TemplateOrSpecialization.isNull() &&
          "Member function is already a specialization");
   TemplateOrSpecialization = Template;
 }
 
+void FunctionDecl::setInstantiatedFromDecl(FunctionDecl *FD) {
+  assert(TemplateOrSpecialization.isNull() &&
+         "function is already a specialization");
+  TemplateOrSpecialization = FD;
+}
+
+FunctionDecl *FunctionDecl::getInstantiatedFromDecl() const {
+  return dyn_cast_or_null<FunctionDecl>(
+      TemplateOrSpecialization.dyn_cast<NamedDecl *>());
+}
+
 bool FunctionDecl::isImplicitlyInstantiable() const {
   // If the function is invalid, it can't be implicitly instantiated.
   if (isInvalidDecl())

diff  --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 13dd6da3f24fe..9c1deaedf6b07 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -283,8 +283,9 @@ unsigned Decl::getTemplateDepth() const {
   return cast<Decl>(DC)->getTemplateDepth();
 }
 
-const DeclContext *Decl::getParentFunctionOrMethod() const {
-  for (const DeclContext *DC = getDeclContext();
+const DeclContext *Decl::getParentFunctionOrMethod(bool Lexical) const {
+  for (const DeclContext *DC = Lexical ? getLexicalDeclContext()
+                                       : getDeclContext();
        DC && !DC->isTranslationUnit() && !DC->isNamespace();
        DC = DC->getParent())
     if (DC->isFunctionOrMethod())

diff  --git a/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp b/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
index bffa66c2d9448..1a785182e3634 100644
--- a/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
+++ b/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
@@ -303,6 +303,7 @@ class ExtractAPIVisitor : public RecursiveASTVisitor<ExtractAPIVisitor> {
     // Skip templated functions.
     switch (Decl->getTemplatedKind()) {
     case FunctionDecl::TK_NonTemplate:
+    case FunctionDecl::TK_DependentNonTemplate:
       break;
     case FunctionDecl::TK_MemberSpecialization:
     case FunctionDecl::TK_FunctionTemplateSpecialization:

diff  --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 239e5dc4394c3..9edba27bea5ea 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -30,6 +30,7 @@ using namespace sema;
 
 namespace {
 class LogicalBinOp {
+  SourceLocation Loc;
   OverloadedOperatorKind Op = OO_None;
   const Expr *LHS = nullptr;
   const Expr *RHS = nullptr;
@@ -40,12 +41,14 @@ class LogicalBinOp {
       Op = BinaryOperator::getOverloadedOperator(BO->getOpcode());
       LHS = BO->getLHS();
       RHS = BO->getRHS();
+      Loc = BO->getExprLoc();
     } else if (auto *OO = dyn_cast<CXXOperatorCallExpr>(E)) {
       // If OO is not || or && it might not have exactly 2 arguments.
       if (OO->getNumArgs() == 2) {
         Op = OO->getOperator();
         LHS = OO->getArg(0);
         RHS = OO->getArg(1);
+        Loc = OO->getOperatorLoc();
       }
     }
   }
@@ -56,6 +59,26 @@ class LogicalBinOp {
 
   const Expr *getLHS() const { return LHS; }
   const Expr *getRHS() const { return RHS; }
+
+  ExprResult recreateBinOp(Sema &SemaRef, ExprResult LHS) const {
+    return recreateBinOp(SemaRef, LHS, const_cast<Expr *>(getRHS()));
+  }
+
+  ExprResult recreateBinOp(Sema &SemaRef, ExprResult LHS,
+                           ExprResult RHS) const {
+    assert((isAnd() || isOr()) && "Not the right kind of op?");
+    assert((!LHS.isInvalid() && !RHS.isInvalid()) && "not good expressions?");
+
+    if (!LHS.isUsable() || !RHS.isUsable())
+      return ExprEmpty();
+
+    // We should just be able to 'normalize' these to the builtin Binary
+    // Operator, since that is how they are evaluated in constriant checks.
+    return BinaryOperator::Create(SemaRef.Context, LHS.get(), RHS.get(),
+                                  BinaryOperator::getOverloadedOpcode(Op),
+                                  SemaRef.Context.BoolTy, VK_PRValue,
+                                  OK_Ordinary, Loc, FPOptionsOverride{});
+  }
 };
 }
 
@@ -122,16 +145,18 @@ bool Sema::CheckConstraintExpression(const Expr *ConstraintExpression,
 }
 
 template <typename AtomicEvaluator>
-static bool
+static ExprResult
 calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
                                 ConstraintSatisfaction &Satisfaction,
                                 AtomicEvaluator &&Evaluator) {
   ConstraintExpr = ConstraintExpr->IgnoreParenImpCasts();
 
   if (LogicalBinOp BO = ConstraintExpr) {
-    if (calculateConstraintSatisfaction(S, BO.getLHS(), Satisfaction,
-                                        Evaluator))
-      return true;
+    ExprResult LHSRes = calculateConstraintSatisfaction(
+        S, BO.getLHS(), Satisfaction, Evaluator);
+
+    if (LHSRes.isInvalid())
+      return ExprError();
 
     bool IsLHSSatisfied = Satisfaction.IsSatisfied;
 
@@ -142,7 +167,7 @@ calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
       //    is checked. If that is satisfied, the disjunction is satisfied.
       //    Otherwise, the disjunction is satisfied if and only if the second
       //    operand is satisfied.
-      return false;
+      return BO.recreateBinOp(S, LHSRes);
 
     if (BO.isAnd() && !IsLHSSatisfied)
       // [temp.constr.op] p2
@@ -151,12 +176,21 @@ calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
       //    is checked. If that is not satisfied, the conjunction is not
       //    satisfied. Otherwise, the conjunction is satisfied if and only if
       //    the second operand is satisfied.
-      return false;
+      return BO.recreateBinOp(S, LHSRes);
 
-    return calculateConstraintSatisfaction(
+    ExprResult RHSRes = calculateConstraintSatisfaction(
         S, BO.getRHS(), Satisfaction, std::forward<AtomicEvaluator>(Evaluator));
-  } else if (auto *C = dyn_cast<ExprWithCleanups>(ConstraintExpr)) {
-    return calculateConstraintSatisfaction(S, C->getSubExpr(), Satisfaction,
+    if (RHSRes.isInvalid())
+      return ExprError();
+
+    return BO.recreateBinOp(S, LHSRes, RHSRes);
+  }
+
+  if (auto *C = dyn_cast<ExprWithCleanups>(ConstraintExpr)) {
+    // These aren't evaluated, so we don't care about cleanups, so we can just
+    // evaluate these as if the cleanups didn't exist.
+    return calculateConstraintSatisfaction(
+        S, C->getSubExpr(), Satisfaction,
         std::forward<AtomicEvaluator>(Evaluator));
   }
 
@@ -164,11 +198,11 @@ calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
   ExprResult SubstitutedAtomicExpr = Evaluator(ConstraintExpr);
 
   if (SubstitutedAtomicExpr.isInvalid())
-    return true;
+    return ExprError();
 
   if (!SubstitutedAtomicExpr.isUsable())
     // Evaluator has decided satisfaction without yielding an expression.
-    return false;
+    return ExprEmpty();
 
   EnterExpressionEvaluationContext ConstantEvaluated(
       S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
@@ -185,7 +219,7 @@ calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
         << SubstitutedAtomicExpr.get()->getSourceRange();
     for (const PartialDiagnosticAt &PDiag : EvaluationDiags)
       S.Diag(PDiag.first, PDiag.second);
-    return true;
+    return ExprError();
   }
 
   assert(EvalResult.Val.isInt() &&
@@ -195,13 +229,13 @@ calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
     Satisfaction.Details.emplace_back(ConstraintExpr,
                                       SubstitutedAtomicExpr.get());
 
-  return false;
+  return SubstitutedAtomicExpr;
 }
 
-static bool calculateConstraintSatisfaction(
-    Sema &S, const NamedDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
-    SourceLocation TemplateNameLoc, MultiLevelTemplateArgumentList &MLTAL,
-    const Expr *ConstraintExpr, ConstraintSatisfaction &Satisfaction) {
+static ExprResult calculateConstraintSatisfaction(
+    Sema &S, const NamedDecl *Template, SourceLocation TemplateNameLoc,
+    const MultiLevelTemplateArgumentList &MLTAL, const Expr *ConstraintExpr,
+    ConstraintSatisfaction &Satisfaction) {
   return calculateConstraintSatisfaction(
       S, ConstraintExpr, Satisfaction, [&](const Expr *AtomicExpr) {
         EnterExpressionEvaluationContext ConstantEvaluated(
@@ -219,8 +253,8 @@ static bool calculateConstraintSatisfaction(
             return ExprError();
           // We do not want error diagnostics escaping here.
           Sema::SFINAETrap Trap(S);
-          SubstitutedExpression = S.SubstExpr(const_cast<Expr *>(AtomicExpr),
-                                              MLTAL);
+          SubstitutedExpression =
+              S.SubstConstraintExpr(const_cast<Expr *>(AtomicExpr), MLTAL);
           // Substitution might have stripped off a contextual conversion to
           // bool if this is the operand of an '&&' or '||'. For example, we
           // might lose an lvalue-to-rvalue conversion here. If so, put it back
@@ -268,72 +302,92 @@ static bool calculateConstraintSatisfaction(
       });
 }
 
-static bool CheckConstraintSatisfaction(Sema &S, const NamedDecl *Template,
-                                        ArrayRef<const Expr *> ConstraintExprs,
-                                        ArrayRef<TemplateArgument> TemplateArgs,
-                                        SourceRange TemplateIDRange,
-                                        ConstraintSatisfaction &Satisfaction) {
+static bool CheckConstraintSatisfaction(
+    Sema &S, const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
+    llvm::SmallVectorImpl<Expr *> &Converted,
+    const MultiLevelTemplateArgumentList &TemplateArgsList,
+    SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction) {
   if (ConstraintExprs.empty()) {
     Satisfaction.IsSatisfied = true;
     return false;
   }
 
-  for (auto& Arg : TemplateArgs)
-    if (Arg.isInstantiationDependent()) {
-      // No need to check satisfaction for dependent constraint expressions.
-      Satisfaction.IsSatisfied = true;
-      return false;
-    }
+  if (TemplateArgsList.isAnyArgInstantiationDependent()) {
+    //  No need to check satisfaction for dependent constraint expressions.
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  ArrayRef<TemplateArgument> TemplateArgs =
+      TemplateArgsList.getNumSubstitutedLevels() > 0
+          ? TemplateArgsList.getOutermost()
+          : ArrayRef<TemplateArgument>{};
 
-  Sema::InstantiatingTemplate Inst(S, TemplateIDRange.getBegin(),
+  Sema::InstantiatingTemplate Inst(
+      S, TemplateIDRange.getBegin(),
       Sema::InstantiatingTemplate::ConstraintsCheck{},
       const_cast<NamedDecl *>(Template), TemplateArgs, TemplateIDRange);
   if (Inst.isInvalid())
     return true;
 
-  MultiLevelTemplateArgumentList MLTAL;
-  MLTAL.addOuterTemplateArguments(TemplateArgs);
-
   for (const Expr *ConstraintExpr : ConstraintExprs) {
-    if (calculateConstraintSatisfaction(S, Template, TemplateArgs,
-                                        TemplateIDRange.getBegin(), MLTAL,
-                                        ConstraintExpr, Satisfaction))
+    ExprResult Res = calculateConstraintSatisfaction(
+        S, Template, TemplateIDRange.getBegin(), TemplateArgsList,
+        ConstraintExpr, Satisfaction);
+    if (Res.isInvalid())
       return true;
-    if (!Satisfaction.IsSatisfied)
+
+    Converted.push_back(Res.get());
+    if (!Satisfaction.IsSatisfied) {
+      // Backfill the 'converted' list with nulls so we can keep the Converted
+      // and unconverted lists in sync.
+      Converted.append(ConstraintExprs.size() - Converted.size(), nullptr);
       // [temp.constr.op] p2
-      //   [...] To determine if a conjunction is satisfied, the satisfaction
-      //   of the first operand is checked. If that is not satisfied, the
-      //   conjunction is not satisfied. [...]
+      // [...] To determine if a conjunction is satisfied, the satisfaction
+      // of the first operand is checked. If that is not satisfied, the
+      // conjunction is not satisfied. [...]
       return false;
+    }
   }
   return false;
 }
 
 bool Sema::CheckConstraintSatisfaction(
     const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
-    ArrayRef<TemplateArgument> TemplateArgs, SourceRange TemplateIDRange,
-    ConstraintSatisfaction &OutSatisfaction) {
+    llvm::SmallVectorImpl<Expr *> &ConvertedConstraints,
+    const MultiLevelTemplateArgumentList &TemplateArgsList,
+    SourceRange TemplateIDRange, ConstraintSatisfaction &OutSatisfaction) {
   if (ConstraintExprs.empty()) {
     OutSatisfaction.IsSatisfied = true;
     return false;
   }
   if (!Template) {
     return ::CheckConstraintSatisfaction(*this, nullptr, ConstraintExprs,
-                                         TemplateArgs, TemplateIDRange,
-                                         OutSatisfaction);
+                                         ConvertedConstraints, TemplateArgsList,
+                                         TemplateIDRange, OutSatisfaction);
   }
+
+  // A list of the template argument list flattened in a predictible manner for
+  // the purposes of caching. The ConstraintSatisfaction type is in AST so it
+  // has no access to the MultiLevelTemplateArgumentList, so this has to happen
+  // here.
+  llvm::SmallVector<TemplateArgument, 4> FlattenedArgs;
+  for (ArrayRef<TemplateArgument> List : TemplateArgsList)
+    FlattenedArgs.insert(FlattenedArgs.end(), List.begin(), List.end());
+
   llvm::FoldingSetNodeID ID;
-  ConstraintSatisfaction::Profile(ID, Context, Template, TemplateArgs);
+  ConstraintSatisfaction::Profile(ID, Context, Template, FlattenedArgs);
+
   void *InsertPos;
   if (auto *Cached = SatisfactionCache.FindNodeOrInsertPos(ID, InsertPos)) {
     OutSatisfaction = *Cached;
     return false;
   }
   auto Satisfaction =
-      std::make_unique<ConstraintSatisfaction>(Template, TemplateArgs);
+      std::make_unique<ConstraintSatisfaction>(Template, FlattenedArgs);
   if (::CheckConstraintSatisfaction(*this, Template, ConstraintExprs,
-                                    TemplateArgs, TemplateIDRange,
-                                    *Satisfaction)) {
+                                    ConvertedConstraints, TemplateArgsList,
+                                    TemplateIDRange, *Satisfaction)) {
     return true;
   }
   OutSatisfaction = *Satisfaction;
@@ -347,21 +401,118 @@ bool Sema::CheckConstraintSatisfaction(
 bool Sema::CheckConstraintSatisfaction(const Expr *ConstraintExpr,
                                        ConstraintSatisfaction &Satisfaction) {
   return calculateConstraintSatisfaction(
-      *this, ConstraintExpr, Satisfaction,
-      [this](const Expr *AtomicExpr) -> ExprResult {
-        // We only do this to immitate lvalue-to-rvalue conversion.
-        return PerformContextuallyConvertToBool(const_cast<Expr *>(AtomicExpr));
-      });
+             *this, ConstraintExpr, Satisfaction,
+             [this](const Expr *AtomicExpr) -> ExprResult {
+               // We only do this to immitate lvalue-to-rvalue conversion.
+               return PerformContextuallyConvertToBool(
+                   const_cast<Expr *>(AtomicExpr));
+             })
+      .isInvalid();
+}
+
+bool Sema::SetupConstraintScope(
+    FunctionDecl *FD, llvm::Optional<ArrayRef<TemplateArgument>> TemplateArgs,
+    MultiLevelTemplateArgumentList MLTAL, LocalInstantiationScope &Scope) {
+  if (FD->isTemplateInstantiation() && FD->getPrimaryTemplate()) {
+    FunctionTemplateDecl *PrimaryTemplate = FD->getPrimaryTemplate();
+    InstantiatingTemplate Inst(
+        *this, FD->getPointOfInstantiation(),
+        Sema::InstantiatingTemplate::ConstraintsCheck{}, PrimaryTemplate,
+        TemplateArgs ? *TemplateArgs : ArrayRef<TemplateArgument>{},
+        SourceRange());
+    if (Inst.isInvalid())
+      return true;
+
+    // addInstantiatedParametersToScope creates a map of 'uninstantiated' to
+    // 'instantiated' parameters and adds it to the context. For the case where
+    // this function is a template being instantiated NOW, we also need to add
+    // the list of current template arguments to the list so that they also can
+    // be picked out of the map.
+    if (auto *SpecArgs = FD->getTemplateSpecializationArgs()) {
+      MultiLevelTemplateArgumentList JustTemplArgs(*SpecArgs);
+      if (addInstantiatedParametersToScope(
+              FD, PrimaryTemplate->getTemplatedDecl(), Scope, JustTemplArgs))
+        return true;
+    }
+
+    // If this is a member function, make sure we get the parameters that
+    // reference the original primary template.
+    if (const auto *FromMemTempl =
+            PrimaryTemplate->getInstantiatedFromMemberTemplate()) {
+      if (addInstantiatedParametersToScope(FD, FromMemTempl->getTemplatedDecl(),
+                                           Scope, MLTAL))
+        return true;
+    }
+
+    return false;
+  }
+
+  if (FD->getTemplatedKind() == FunctionDecl::TK_MemberSpecialization ||
+      FD->getTemplatedKind() == FunctionDecl::TK_DependentNonTemplate) {
+    FunctionDecl *InstantiatedFrom =
+        FD->getTemplatedKind() == FunctionDecl::TK_MemberSpecialization
+            ? FD->getInstantiatedFromMemberFunction()
+            : FD->getInstantiatedFromDecl();
+
+    InstantiatingTemplate Inst(
+        *this, FD->getPointOfInstantiation(),
+        Sema::InstantiatingTemplate::ConstraintsCheck{}, InstantiatedFrom,
+        TemplateArgs ? *TemplateArgs : ArrayRef<TemplateArgument>{},
+        SourceRange());
+    if (Inst.isInvalid())
+      return true;
+
+    // Case where this was not a template, but instantiated as a
+    // child-function.
+    if (addInstantiatedParametersToScope(FD, InstantiatedFrom, Scope, MLTAL))
+      return true;
+  }
+
+  return false;
+}
+
+// This function collects all of the template arguments for the purposes of
+// constraint-instantiation and checking.
+llvm::Optional<MultiLevelTemplateArgumentList>
+Sema::SetupConstraintCheckingTemplateArgumentsAndScope(
+    FunctionDecl *FD, llvm::Optional<ArrayRef<TemplateArgument>> TemplateArgs,
+    LocalInstantiationScope &Scope) {
+  MultiLevelTemplateArgumentList MLTAL;
+
+  // Collect the list of template arguments relative to the 'primary' template.
+  // We need the entire list, since the constraint is completely uninstantiated
+  // at this point.
+  MLTAL = getTemplateInstantiationArgs(FD, nullptr, /*RelativeToPrimary*/ true,
+                                       /*Pattern*/ nullptr,
+                                       /*LookBeyondLambda*/ true);
+  if (SetupConstraintScope(FD, TemplateArgs, MLTAL, Scope))
+    return {};
+
+  return MLTAL;
 }
 
 bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
                                     ConstraintSatisfaction &Satisfaction,
                                     SourceLocation UsageLoc) {
-  const Expr *RC = FD->getTrailingRequiresClause();
-  if (RC->isInstantiationDependent()) {
+  // Don't check constraints if the function is dependent. Also don't check if
+  // this is a function template specialization, as the call to
+  // CheckinstantiatedFunctionTemplateConstraints after this will check it
+  // better.
+  if (FD->isDependentContext() ||
+      FD->getTemplatedKind() ==
+          FunctionDecl::TK_FunctionTemplateSpecialization) {
     Satisfaction.IsSatisfied = true;
     return false;
   }
+
+  ContextRAII SavedContext{
+      *this, cast<DeclContext>(
+                 const_cast<FunctionDecl *>(FD)->getNonClosureContext())};
+  LocalInstantiationScope Scope(*this, true);
+  llvm::Optional<MultiLevelTemplateArgumentList> MLTAL =
+      SetupConstraintCheckingTemplateArgumentsAndScope(
+          const_cast<FunctionDecl *>(FD), {}, Scope);
+
   Qualifiers ThisQuals;
   CXXRecordDecl *Record = nullptr;
   if (auto *Method = dyn_cast<CXXMethodDecl>(FD)) {
@@ -372,14 +523,27 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
   // We substitute with empty arguments in order to rebuild the atomic
   // constraint in a constant-evaluated context.
   // FIXME: Should this be a dedicated TreeTransform?
-  return CheckConstraintSatisfaction(
-      FD, {RC}, /*TemplateArgs=*/{},
-      SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
-      Satisfaction);
+  const Expr *RC = FD->getTrailingRequiresClause();
+  llvm::SmallVector<Expr *, 1> Converted;
+
+  if (CheckConstraintSatisfaction(
+          FD, {RC}, Converted, *MLTAL,
+          SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
+          Satisfaction))
+    return true;
+
+  // FIXME: we need to do this for the function constraints for
+  // comparison of constraints to work, but do we also need to do it for
+  // CheckInstantiatedFunctionConstraints?  That one is more 
diff icult, but we
+  // seem to always just pick up the constraints from the primary template.
+  assert(Converted.size() <= 1 && "Got more expressions converted?");
+  if (!Converted.empty() && Converted[0] != nullptr)
+    const_cast<FunctionDecl *>(FD)->setTrailingRequiresClause(Converted[0]);
+  return false;
 }
 
 bool Sema::EnsureTemplateArgumentListConstraints(
-    TemplateDecl *TD, ArrayRef<TemplateArgument> TemplateArgs,
+    TemplateDecl *TD, MultiLevelTemplateArgumentList TemplateArgs,
     SourceRange TemplateIDRange) {
   ConstraintSatisfaction Satisfaction;
   llvm::SmallVector<const Expr *, 3> AssociatedConstraints;
@@ -392,7 +556,8 @@ bool Sema::EnsureTemplateArgumentListConstraints(
     SmallString<128> TemplateArgString;
     TemplateArgString = " ";
     TemplateArgString += getTemplateArgumentBindingsText(
-        TD->getTemplateParameters(), TemplateArgs.data(), TemplateArgs.size());
+        TD->getTemplateParameters(), TemplateArgs.getInnermost().data(),
+        TemplateArgs.getInnermost().size());
 
     Diag(TemplateIDRange.getBegin(),
          diag::err_template_arg_list_constraints_not_satisfied)
@@ -424,21 +589,13 @@ bool Sema::CheckInstantiatedFunctionTemplateConstraints(
   Sema::ContextRAII savedContext(*this, Decl);
   LocalInstantiationScope Scope(*this);
 
-  // If this is not an explicit specialization - we need to get the instantiated
-  // version of the template arguments and add them to scope for the
-  // substitution.
-  if (Decl->isTemplateInstantiation()) {
-    InstantiatingTemplate Inst(*this, Decl->getPointOfInstantiation(),
-        InstantiatingTemplate::ConstraintsCheck{}, Decl->getPrimaryTemplate(),
-        TemplateArgs, SourceRange());
-    if (Inst.isInvalid())
-      return true;
-    MultiLevelTemplateArgumentList MLTAL(
-        *Decl->getTemplateSpecializationArgs());
-    if (addInstantiatedParametersToScope(
-            Decl, Decl->getPrimaryTemplate()->getTemplatedDecl(), Scope, MLTAL))
-      return true;
-  }
+  Optional<MultiLevelTemplateArgumentList> MLTAL =
+      SetupConstraintCheckingTemplateArgumentsAndScope(Decl, TemplateArgs,
+                                                       Scope);
+
+  if (!MLTAL)
+    return true;
+
   Qualifiers ThisQuals;
   CXXRecordDecl *Record = nullptr;
   if (auto *Method = dyn_cast<CXXMethodDecl>(Decl)) {
@@ -446,7 +603,8 @@ bool Sema::CheckInstantiatedFunctionTemplateConstraints(
     Record = Method->getParent();
   }
   CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
-  return CheckConstraintSatisfaction(Template, TemplateAC, TemplateArgs,
+  llvm::SmallVector<Expr *, 1> Converted;
+  return CheckConstraintSatisfaction(Template, TemplateAC, Converted, *MLTAL,
                                      PointOfInstantiation, Satisfaction);
 }
 
@@ -721,22 +879,22 @@ Sema::getNormalizedAssociatedConstraints(
   return CacheEntry->second;
 }
 
-static bool substituteParameterMappings(Sema &S, NormalizedConstraint &N,
-    ConceptDecl *Concept, ArrayRef<TemplateArgument> TemplateArgs,
-    const ASTTemplateArgumentListInfo *ArgsAsWritten) {
+static bool
+substituteParameterMappings(Sema &S, NormalizedConstraint &N,
+                            ConceptDecl *Concept,
+                            const MultiLevelTemplateArgumentList &MLTAL,
+                            const ASTTemplateArgumentListInfo *ArgsAsWritten) {
   if (!N.isAtomic()) {
-    if (substituteParameterMappings(S, N.getLHS(), Concept, TemplateArgs,
+    if (substituteParameterMappings(S, N.getLHS(), Concept, MLTAL,
                                     ArgsAsWritten))
       return true;
-    return substituteParameterMappings(S, N.getRHS(), Concept, TemplateArgs,
+    return substituteParameterMappings(S, N.getRHS(), Concept, MLTAL,
                                        ArgsAsWritten);
   }
   TemplateParameterList *TemplateParams = Concept->getTemplateParameters();
 
   AtomicConstraint &Atomic = *N.getAtomicConstraint();
   TemplateArgumentListInfo SubstArgs;
-  MultiLevelTemplateArgumentList MLTAL;
-  MLTAL.addOuterTemplateArguments(TemplateArgs);
   if (!Atomic.ParameterMapping) {
     llvm::SmallBitVector OccurringIndices(TemplateParams->size());
     S.MarkUsedTemplateParameters(Atomic.ConstraintExpr, /*OnlyDeduced=*/false,
@@ -766,7 +924,8 @@ static bool substituteParameterMappings(Sema &S, NormalizedConstraint &N,
       Sema::InstantiatingTemplate::ParameterMappingSubstitution{}, Concept,
       SourceRange(ArgsAsWritten->arguments()[0].getSourceRange().getBegin(),
                   ArgsAsWritten->arguments().back().getSourceRange().getEnd()));
-  if (S.SubstTemplateArguments(*Atomic.ParameterMapping, MLTAL, SubstArgs))
+  if (S.SubstTemplateArguments(*Atomic.ParameterMapping, MLTAL, SubstArgs,
+                               /*InstantiateConstraints=*/true))
     return true;
   Atomic.ParameterMapping.emplace(
         MutableArrayRef<TemplateArgumentLoc>(
@@ -777,6 +936,20 @@ static bool substituteParameterMappings(Sema &S, NormalizedConstraint &N,
   return false;
 }
 
+static bool substituteParameterMappings(Sema &S, NormalizedConstraint &N,
+                                        const ConceptSpecializationExpr *CSE) {
+  TemplateArgumentList TAL{TemplateArgumentList::OnStack,
+                           CSE->getTemplateArguments()};
+  MultiLevelTemplateArgumentList MLTAL =
+      S.getTemplateInstantiationArgs(CSE->getNamedConcept(), &TAL,
+                                     /*RelativeToPrimary*/ true,
+                                     /*Pattern*/ nullptr,
+                                     /*LookBeyondLambda*/ true);
+
+  return substituteParameterMappings(S, N, CSE->getNamedConcept(), MLTAL,
+                                     CSE->getTemplateArgsAsWritten());
+}
+
 Optional<NormalizedConstraint>
 NormalizedConstraint::fromConstraintExprs(Sema &S, NamedDecl *D,
                                           ArrayRef<const Expr *> E) {
@@ -839,9 +1012,7 @@ NormalizedConstraint::fromConstraintExpr(Sema &S, NamedDecl *D, const Expr *E) {
     Optional<NormalizedConstraint> New;
     New.emplace(S.Context, *SubNF);
 
-    if (substituteParameterMappings(
-            S, *New, CSE->getNamedConcept(),
-            CSE->getTemplateArguments(), CSE->getTemplateArgsAsWritten()))
+    if (substituteParameterMappings(S, *New, CSE))
       return None;
 
     return New;

diff  --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index c226ed6254790..644114c9485ee 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -992,6 +992,78 @@ static bool checkArgPlaceholdersForOverload(Sema &S, MultiExprArg Args,
   return false;
 }
 
+// Figure out the to-translation-unit depth for this function declaration for
+// the purpose of seeing if they 
diff er by constraints. This isn't the same as
+// getTemplateDepth, because it includes already instantiated parents.
+static unsigned CalculateTemplateDepthForConstraints(Sema &S,
+                                                     FunctionDecl *FD) {
+  MultiLevelTemplateArgumentList MLTAL =
+      S.getTemplateInstantiationArgs(FD, nullptr, /*RelativeToPrimary*/ true,
+                                     /*Pattern*/ nullptr,
+                                     /*LookBeyondLambda*/ true);
+  return MLTAL.getNumSubstitutedLevels();
+}
+
+// Friend definitions can appear identical but be 
diff erent declarations based
+// on the last sentence of the rule below (others included for clarification):
+// C++20 [temp.friend] p9: A non-template friend declaration
+// with a requires-clause shall be a definition.  A friend function template
+// with a constraint that depends on a template parameter from an enclosing
+// template shall be a definition.  Such a constrained friend function or
+// function template declaration does not declare the same function or function
+// template as a declaration in any other scope.
+static bool FriendsDifferByConstraints(Sema &S, DeclContext *CurContext,
+                                       FunctionDecl *Old, FunctionDecl *New,
+                                       Scope *Scope) {
+  // If these aren't friends, than they aren't friends that 
diff ere by
+  // constraints.
+  if (!Old->getFriendObjectKind() || !New->getFriendObjectKind())
+    return false;
+
+  // If the the two functions share lexical declaration context, they are not in
+  // separate instantations, and thus in the same scope.
+  if (New->getLexicalDeclContext() == Old->getLexicalDeclContext())
+    return false;
+
+  if (!Old->getDescribedFunctionTemplate()) {
+    assert(!New->getDescribedFunctionTemplate() &&
+           "How would these be the same if they aren't both templates?");
+
+    // If these friends don't have constraints, they aren't constrained, and
+    // thus don't fall under temp.friend p9. Else the simple presence of a
+    // constraint makes them unique.
+    return Old->getTrailingRequiresClause();
+  }
+
+  SmallVector<const Expr *, 3> OldAC;
+  Old->getDescribedFunctionTemplate()->getAssociatedConstraints(OldAC);
+
+#ifndef NDEBUG
+  SmallVector<const Expr *, 3> NewAC;
+  New->getDescribedFunctionTemplate()->getAssociatedConstraints(NewAC);
+  assert(OldAC.size() == NewAC.size() &&
+         "Difference should have been noticed earlier if sizes of constraints "
+         "aren't the same");
+#endif
+  // If there are no constraints, these are not constrained friend function or
+  // friend function templates.
+  if (OldAC.size() == 0)
+    return false;
+
+  unsigned OldTemplateDepth = CalculateTemplateDepthForConstraints(S, Old);
+
+  // At this point, if the constrained function template declaration depends on
+  // a template parameter from an enclosing template, they are not the same
+  // function.  Since these were deemed identical before we got here, we only
+  // have to look into 1 side to see if they refer to a containing template.
+  for (const Expr *Constraint : OldAC)
+    if (S.ConstraintExpressionDependsOnEnclosingTemplate(OldTemplateDepth,
+                                                         Constraint))
+      return true;
+
+  return false;
+}
+
 /// Determine whether the given New declaration is an overload of the
 /// declarations in Old. This routine returns Ovl_Match or Ovl_NonFunction if
 /// New and Old cannot be overloaded, e.g., if New has the same signature as
@@ -1068,6 +1140,15 @@ Sema::CheckOverload(Scope *S, FunctionDecl *New, const LookupResult &Old,
             !shouldLinkPossiblyHiddenDecl(*I, New))
           continue;
 
+        // C++20 [temp.friend] p9: A non-template friend declaration with a
+        // requires-clause shall be a definition.  A friend function template
+        // with a constraint that depends on a template parameter from an
+        // enclosing template shall be a definition.  Such a constrained friend
+        // function or function template declaration does not declare the same
+        // function or function template as a declaration in any other scope.
+        if (FriendsDifferByConstraints(*this, CurContext, OldF, New, S))
+          continue;
+
         Match = *I;
         return Ovl_Match;
       }

diff  --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index dbfe6164bda2c..8e300f0b05b45 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1685,6 +1685,59 @@ NamedDecl *Sema::ActOnTemplateTemplateParameter(Scope* S,
   return Param;
 }
 
+namespace {
+class ConstraintRefersToContainingTemplateChecker
+    : public TreeTransform<ConstraintRefersToContainingTemplateChecker> {
+  bool Result = false;
+  unsigned TemplateDepth = 0;
+
+public:
+  using inherited = TreeTransform<ConstraintRefersToContainingTemplateChecker>;
+
+  ConstraintRefersToContainingTemplateChecker(Sema &SemaRef,
+                                              unsigned TemplateDepth)
+      : inherited(SemaRef), TemplateDepth(TemplateDepth) {}
+  bool getResult() const { return Result; }
+
+  // This should be the only template parm type that we have to deal with.
+  // SubstTempalteTypeParmPack, SubstNonTypeTemplateParmPack, and
+  // FunctionParmPackExpr are all partially substituted, which cannot happen
+  // with concepts at this point in translation.
+  QualType TransformTemplateTypeParmType(TypeLocBuilder &TLB,
+                                         TemplateTypeParmTypeLoc TL) {
+    assert(TL.getDecl()->getDepth() <= TemplateDepth &&
+           "Nothing should reference a value below the actual template depth, "
+           "depth is likely wrong");
+    if (TL.getDecl()->getDepth() != TemplateDepth)
+      Result = true;
+    return inherited::TransformTemplateTypeParmType(TLB, TL);
+  }
+
+  Decl *TransformDecl(SourceLocation Loc, Decl *D) {
+    // FIXME : This is possibly an incomplete list, but it is unclear what other
+    // Decl kinds could be used to refer to the template parameters.  This is a
+    // best guess so far based on examples currently available, but the
+    // unreachable should catch future instances/cases.
+    if (auto *TD = dyn_cast<TypedefNameDecl>(D))
+      TransformType(TD->getUnderlyingType());
+    else if (auto *VD = dyn_cast<ValueDecl>(D))
+      TransformType(VD->getType());
+    else if (auto *TD = dyn_cast<TemplateDecl>(D))
+      TransformTemplateParameterList(TD->getTemplateParameters());
+    else
+      llvm_unreachable("Don't know how to handle this declaration type yet");
+    return D;
+  }
+};
+} // namespace
+
+bool Sema::ConstraintExpressionDependsOnEnclosingTemplate(
+    unsigned TemplateDepth, const Expr *Constraint) {
+  ConstraintRefersToContainingTemplateChecker Checker(*this, TemplateDepth);
+  Checker.TransformExpr(const_cast<Expr *>(Constraint));
+  return Checker.getResult();
+}
+
 /// ActOnTemplateParameterList - Builds a TemplateParameterList, optionally
 /// constrained by RequiresClause, that contains the template parameters in
 /// Params.
@@ -2286,7 +2339,8 @@ struct ConvertConstructorToDeductionGuideTransform {
           TTP->isExpandedParameterPack() ?
           llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
       if (const auto *TC = TTP->getTypeConstraint())
-        SemaRef.SubstTypeConstraint(NewTTP, TC, Args);
+        SemaRef.SubstTypeConstraint(NewTTP, TC, Args,
+                                    /*IsEvaluatingAConstraint*/ false);
       if (TTP->hasDefaultArgument()) {
         TypeSourceInfo *InstantiatedDefaultArg =
             SemaRef.SubstType(TTP->getDefaultArgumentInfo(), Args,
@@ -4703,9 +4757,11 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
   bool AreArgsDependent =
       TemplateSpecializationType::anyDependentTemplateArguments(*TemplateArgs,
                                                                 Converted);
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(Converted);
   if (!AreArgsDependent &&
       CheckConstraintSatisfaction(
-          NamedConcept, {NamedConcept->getConstraintExpr()}, Converted,
+          NamedConcept, {NamedConcept->getConstraintExpr()}, MLTAL,
           SourceRange(SS.isSet() ? SS.getBeginLoc() : ConceptNameInfo.getLoc(),
                       TemplateArgs->getRAngleLoc()),
           Satisfaction))
@@ -5567,7 +5623,8 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
 
     TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
     Params = SubstTemplateParams(Params, CurContext,
-                                 MultiLevelTemplateArgumentList(TemplateArgs));
+                                 MultiLevelTemplateArgumentList(TemplateArgs),
+                                 /*InstantiateConstraints*/ true);
     if (!Params)
       return true;
   }
@@ -5922,13 +5979,20 @@ bool Sema::CheckTemplateArgumentList(
   if (UpdateArgsWithConversions)
     TemplateArgs = std::move(NewArgs);
 
-  if (!PartialTemplateArgs &&
-      EnsureTemplateArgumentListConstraints(
-        Template, Converted, SourceRange(TemplateLoc,
-                                         TemplateArgs.getRAngleLoc()))) {
-    if (ConstraintsNotSatisfied)
-      *ConstraintsNotSatisfied = true;
-    return true;
+  if (!PartialTemplateArgs) {
+    TemplateArgumentList StackTemplateArgs(TemplateArgumentList::OnStack,
+                                           Converted);
+    MultiLevelTemplateArgumentList MLTAL = getTemplateInstantiationArgs(
+        Template, &StackTemplateArgs, /*RelativeToPrimary*/ true,
+        /*Pattern*/ nullptr,
+        /*LookBeyondLambda*/ true, /*IncludeContainingStruct*/ true);
+    if (EnsureTemplateArgumentListConstraints(
+            Template, MLTAL,
+            SourceRange(TemplateLoc, TemplateArgs.getRAngleLoc()))) {
+      if (ConstraintsNotSatisfied)
+        *ConstraintsNotSatisfied = true;
+      return true;
+    }
   }
 
   return false;
@@ -7458,7 +7522,9 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
       //   are not considered.
       if (ParamsAC.empty())
         return false;
+
       Template->getAssociatedConstraints(TemplateAC);
+
       bool IsParamAtLeastAsConstrained;
       if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
                                  IsParamAtLeastAsConstrained))

diff  --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 9ec33e8981983..fb7a13a21deb0 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -2783,6 +2783,20 @@ template<>
 struct IsPartialSpecialization<VarTemplatePartialSpecializationDecl> {
   static constexpr bool value = true;
 };
+template <typename TemplateDeclT>
+static bool DeducedArgsNeedReplacement(TemplateDeclT *Template) {
+  return false;
+}
+template <>
+bool DeducedArgsNeedReplacement<VarTemplatePartialSpecializationDecl>(
+    VarTemplatePartialSpecializationDecl *Spec) {
+  return !Spec->isClassScopeExplicitSpecialization();
+}
+template <>
+bool DeducedArgsNeedReplacement<ClassTemplatePartialSpecializationDecl>(
+    ClassTemplatePartialSpecializationDecl *Spec) {
+  return !Spec->isClassScopeExplicitSpecialization();
+}
 
 template<typename TemplateDeclT>
 static Sema::TemplateDeductionResult
@@ -2791,8 +2805,25 @@ CheckDeducedArgumentConstraints(Sema& S, TemplateDeclT *Template,
                                 TemplateDeductionInfo& Info) {
   llvm::SmallVector<const Expr *, 3> AssociatedConstraints;
   Template->getAssociatedConstraints(AssociatedConstraints);
-  if (S.CheckConstraintSatisfaction(Template, AssociatedConstraints,
-                                    DeducedArgs, Info.getLocation(),
+  MultiLevelTemplateArgumentList MLTAL;
+
+  bool NeedsReplacement = DeducedArgsNeedReplacement(Template);
+  TemplateArgumentList DeducedTAL{TemplateArgumentList::OnStack, DeducedArgs};
+
+  MLTAL = S.getTemplateInstantiationArgs(
+      Template, /*InnerMost*/ NeedsReplacement ? nullptr : &DeducedTAL,
+      /*RelativeToPrimary*/ true, /*Pattern*/
+      nullptr, /*LookBeyondLambda*/ true);
+
+  // getTemplateInstantiationArgs picks up the non-deduced version of the
+  // template args when this is a variable template partial specialization and
+  // not class-scope explicit specialization, so replace with Deduced Args
+  // instead of adding to inner-most.
+  if (NeedsReplacement)
+    MLTAL.replaceInnermostTemplateArguments(DeducedArgs);
+
+  if (S.CheckConstraintSatisfaction(Template, AssociatedConstraints, MLTAL,
+                                    Info.getLocation(),
                                     Info.AssociatedConstraintsSatisfaction) ||
       !Info.AssociatedConstraintsSatisfaction.IsSatisfied) {
     Info.reset(TemplateArgumentList::CreateCopy(S.Context, DeducedArgs));
@@ -4572,8 +4603,11 @@ CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
   if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
                                   /*PartialTemplateArgs=*/false, Converted))
     return Sema::DAR_FailedAlreadyDiagnosed;
+
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(Converted);
   if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
-                                    Converted, TypeLoc.getLocalSourceRange(),
+                                    MLTAL, TypeLoc.getLocalSourceRange(),
                                     Satisfaction))
     return Sema::DAR_FailedAlreadyDiagnosed;
   if (!Satisfaction.IsSatisfied) {

diff  --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index f09b3473c0749..4da1e0a2db3d3 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -57,9 +57,18 @@ using namespace sema;
 /// instantiating the definition of the given declaration, \p D. This is
 /// used to determine the proper set of template instantiation arguments for
 /// friend function template specializations.
+///
+/// \param LookBeyondLambda Indicates that this collection of arguments should
+/// continue looking when it encounters a lambda generic call operator.
+///
+/// \param IncludeContainingStructArgs Indicates that this collection of
+/// arguments should include arguments for any class template that this
+/// declaration is included inside of.
+
 MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
     const NamedDecl *D, const TemplateArgumentList *Innermost,
-    bool RelativeToPrimary, const FunctionDecl *Pattern) {
+    bool RelativeToPrimary, const FunctionDecl *Pattern, bool LookBeyondLambda,
+    bool IncludeContainingStructArgs) {
   // Accumulate the set of template argument lists in this structure.
   MultiLevelTemplateArgumentList Result;
 
@@ -155,11 +164,13 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
           break;
 
         // If this function is a generic lambda specialization, we are done.
-        if (isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
+        if (!LookBeyondLambda &&
+            isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
           break;
 
       } else if (Function->getDescribedFunctionTemplate()) {
-        assert(Result.getNumSubstitutedLevels() == 0 &&
+        assert((IncludeContainingStructArgs ||
+                Result.getNumSubstitutedLevels() == 0) &&
                "Outer template not instantiated?");
       }
 
@@ -176,10 +187,18 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
       }
     } else if (const auto *Rec = dyn_cast<CXXRecordDecl>(Ctx)) {
       if (ClassTemplateDecl *ClassTemplate = Rec->getDescribedClassTemplate()) {
-        assert(Result.getNumSubstitutedLevels() == 0 &&
+        assert((IncludeContainingStructArgs ||
+                Result.getNumSubstitutedLevels() == 0) &&
                "Outer template not instantiated?");
         if (ClassTemplate->isMemberSpecialization())
           break;
+        if (IncludeContainingStructArgs) {
+          QualType RecordType = Context.getTypeDeclType(Rec);
+          QualType Injected = cast<InjectedClassNameType>(RecordType)
+                                  ->getInjectedSpecializationType();
+          const auto *InjectedType = cast<TemplateSpecializationType>(Injected);
+          Result.addOuterTemplateArguments(InjectedType->template_arguments());
+        }
       }
     }
 
@@ -930,16 +949,17 @@ namespace {
     const MultiLevelTemplateArgumentList &TemplateArgs;
     SourceLocation Loc;
     DeclarationName Entity;
+    bool EvaluatingAConstraint = false;
 
   public:
     typedef TreeTransform<TemplateInstantiator> inherited;
 
     TemplateInstantiator(Sema &SemaRef,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
-                         SourceLocation Loc,
-                         DeclarationName Entity)
-      : inherited(SemaRef), TemplateArgs(TemplateArgs), Loc(Loc),
-        Entity(Entity) { }
+                         SourceLocation Loc, DeclarationName Entity,
+                         bool EvaluatingConstraint = false)
+        : inherited(SemaRef), TemplateArgs(TemplateArgs), Loc(Loc),
+          Entity(Entity), EvaluatingAConstraint(EvaluatingConstraint) {}
 
     /// Determine whether the given type \p T has already been
     /// transformed.
@@ -1194,6 +1214,18 @@ namespace {
       return DeclInstantiator.SubstTemplateParams(OrigTPL);
     }
 
+    TemplateParameterList *
+    TransformRequiresTemplateParameterList(TemplateParameterList *OrigTPL) {
+      if (!OrigTPL || !OrigTPL->size())
+        return OrigTPL;
+
+      DeclContext *Owner = OrigTPL->getParam(0)->getDeclContext();
+      TemplateDeclInstantiator DeclInstantiator(
+          getSema(),
+          /* DeclContext *Owner */ Owner, TemplateArgs, EvaluatingAConstraint);
+      return DeclInstantiator.SubstTemplateParams(OrigTPL);
+    }
+
     concepts::TypeRequirement *
     TransformTypeRequirement(concepts::TypeRequirement *Req);
     concepts::ExprRequirement *
@@ -1986,7 +2018,7 @@ TemplateInstantiator::TransformExprRequirement(concepts::ExprRequirement *Req) {
     if (TPLInst.isInvalid())
       return nullptr;
     TemplateParameterList *TPL =
-        TransformTemplateParameterList(OrigTPL);
+        TransformRequiresTemplateParameterList(OrigTPL);
     if (!TPL)
       TransRetReq.emplace(createSubstDiag(SemaRef, Info,
           [&] (llvm::raw_ostream& OS) {
@@ -2349,9 +2381,21 @@ namespace {
 
 bool Sema::SubstTypeConstraint(
     TemplateTypeParmDecl *Inst, const TypeConstraint *TC,
-    const MultiLevelTemplateArgumentList &TemplateArgs) {
+    const MultiLevelTemplateArgumentList &TemplateArgs,
+    bool isEvaluatingAConstraint) {
   const ASTTemplateArgumentListInfo *TemplArgInfo =
       TC->getTemplateArgsAsWritten();
+
+  // If we're not checking a constraint, we shouldn't be instantiating the type
+  // constraint, so we should just create a copy of the previous one.
+  if (!isEvaluatingAConstraint) {
+    Inst->setTypeConstraint(TC->getNestedNameSpecifierLoc(),
+                            TC->getConceptNameInfo(), TC->getNamedConcept(),
+                            TC->getNamedConcept(), TemplArgInfo,
+                            TC->getImmediatelyDeclaredConstraint());
+    return false;
+  }
+
   TemplateArgumentListInfo InstArgs;
 
   if (TemplArgInfo) {
@@ -2432,9 +2476,8 @@ ParmVarDecl *Sema::SubstParmVarDecl(ParmVarDecl *OldParm,
       // template's described function, but we might also get here later.
       // Make sure we do not instantiate the TypeConstraint more than once.
       if (Inst && !Inst->getTypeConstraint()) {
-        // TODO: Concepts: do not instantiate the constraint (delayed constraint
-        // substitution)
-        if (SubstTypeConstraint(Inst, TC, TemplateArgs))
+        if (SubstTypeConstraint(Inst, TC, TemplateArgs,
+                                /*isEvaluatingAConstraint*/ false))
           return nullptr;
       }
     }
@@ -3520,10 +3563,10 @@ Sema::SubstStmt(Stmt *S, const MultiLevelTemplateArgumentList &TemplateArgs) {
 bool Sema::SubstTemplateArguments(
     ArrayRef<TemplateArgumentLoc> Args,
     const MultiLevelTemplateArgumentList &TemplateArgs,
-    TemplateArgumentListInfo &Out) {
-  TemplateInstantiator Instantiator(*this, TemplateArgs,
-                                    SourceLocation(),
-                                    DeclarationName());
+    TemplateArgumentListInfo &Out, bool InstantiateConstraints) {
+  TemplateInstantiator Instantiator(
+      *this, TemplateArgs, SourceLocation(), DeclarationName(),
+      InstantiateConstraints);
   return Instantiator.TransformTemplateArguments(Args.begin(), Args.end(),
                                                  Out);
 }
@@ -3539,6 +3582,18 @@ Sema::SubstExpr(Expr *E, const MultiLevelTemplateArgumentList &TemplateArgs) {
   return Instantiator.TransformExpr(E);
 }
 
+ExprResult
+Sema::SubstConstraintExpr(Expr *E,
+                          const MultiLevelTemplateArgumentList &TemplateArgs) {
+  if (!E)
+    return E;
+
+  TemplateInstantiator Instantiator(*this, TemplateArgs, SourceLocation(),
+                                    DeclarationName(),
+                                    /*EvaluatingConstraint*/ true);
+  return Instantiator.TransformExpr(E);
+}
+
 ExprResult Sema::SubstInitializer(Expr *Init,
                           const MultiLevelTemplateArgumentList &TemplateArgs,
                           bool CXXDirectInit) {

diff  --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d7558017948a2..be099f5ccf6c2 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2062,19 +2062,7 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
       return nullptr;
   }
 
-  // FIXME: Concepts: Do not substitute into constraint expressions
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
-  if (TrailingRequiresClause) {
-    EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
-    ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
-                                           TemplateArgs);
-    if (SubstRC.isInvalid())
-      return nullptr;
-    TrailingRequiresClause = SubstRC.get();
-    if (!SemaRef.CheckConstraintExpression(TrailingRequiresClause))
-      return nullptr;
-  }
 
   // If we're instantiating a local function declaration, put the result
   // in the enclosing namespace; otherwise we need to find the instantiated
@@ -2182,6 +2170,11 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
     // definition. We don't want non-template functions to be marked as being
     // template instantiations.
     Function->setInstantiationOfMemberFunction(D, TSK_ImplicitInstantiation);
+  } else if (!isFriend) {
+    // If this is not a function template, and this is not a friend (that is,
+    // this is a locally declared function), save the instantiation relationship
+    // for the purposes of constraint instantiation.
+    Function->setInstantiatedFromDecl(D);
   }
 
   if (isFriend) {
@@ -2420,23 +2413,6 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
       return nullptr;
   }
 
-  // FIXME: Concepts: Do not substitute into constraint expressions
-  Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
-  if (TrailingRequiresClause) {
-    EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
-    auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(Owner);
-    Sema::CXXThisScopeRAII ThisScope(SemaRef, ThisContext,
-                                     D->getMethodQualifiers(), ThisContext);
-    ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
-                                           TemplateArgs);
-    if (SubstRC.isInvalid())
-      return nullptr;
-    TrailingRequiresClause = SubstRC.get();
-    if (!SemaRef.CheckConstraintExpression(TrailingRequiresClause))
-      return nullptr;
-  }
-
   DeclContext *DC = Owner;
   if (isFriend) {
     if (QualifierLoc) {
@@ -2454,6 +2430,9 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
     if (!DC) return nullptr;
   }
 
+  CXXRecordDecl *Record = cast<CXXRecordDecl>(DC);
+  Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
+
   DeclarationNameInfo NameInfo
     = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
 
@@ -2461,7 +2440,6 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
     adjustForRewrite(FunctionRewriteKind, D, T, TInfo, NameInfo);
 
   // Build the instantiated method declaration.
-  CXXRecordDecl *Record = cast<CXXRecordDecl>(DC);
   CXXMethodDecl *Method = nullptr;
 
   SourceLocation StartLoc = D->getInnerLocStart();
@@ -2769,10 +2747,8 @@ Decl *TemplateDeclInstantiator::VisitTemplateTypeParmDecl(
       // Invented template parameter type constraints will be instantiated with
       // the corresponding auto-typed parameter as it might reference other
       // parameters.
-
-      // TODO: Concepts: do not instantiate the constraint (delayed constraint
-      // substitution)
-      if (SemaRef.SubstTypeConstraint(Inst, TC, TemplateArgs))
+      if (SemaRef.SubstTypeConstraint(Inst, TC, TemplateArgs,
+                                      EvaluatingAConstraint))
         return nullptr;
     }
   }
@@ -4016,18 +3992,7 @@ TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
   if (Invalid)
     return nullptr;
 
-  // FIXME: Concepts: Substitution into requires clause should only happen when
-  // checking satisfaction.
-  Expr *InstRequiresClause = nullptr;
-  if (Expr *E = L->getRequiresClause()) {
-    EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
-    ExprResult Res = SemaRef.SubstExpr(E, TemplateArgs);
-    if (Res.isInvalid() || !Res.isUsable()) {
-      return nullptr;
-    }
-    InstRequiresClause = Res.get();
-  }
+  Expr *InstRequiresClause = L->getRequiresClause();
 
   TemplateParameterList *InstL
     = TemplateParameterList::Create(SemaRef.Context, L->getTemplateLoc(),
@@ -4038,8 +4003,10 @@ TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
 
 TemplateParameterList *
 Sema::SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
-                          const MultiLevelTemplateArgumentList &TemplateArgs) {
-  TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs);
+                          const MultiLevelTemplateArgumentList &TemplateArgs,
+                          bool InstantiateConstraints) {
+  TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs,
+                                        InstantiateConstraints);
   return Instantiator.SubstTemplateParams(Params);
 }
 

diff  --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 1b11f0bdb9b60..a8a7a6a3da844 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -13067,13 +13067,6 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
                                                         NewCallOpType);
   }
 
-  // Transform the trailing requires clause
-  ExprResult NewTrailingRequiresClause;
-  if (Expr *TRC = E->getCallOperator()->getTrailingRequiresClause())
-    // FIXME: Concepts: Substitution into requires clause should only happen
-    //                  when checking satisfaction.
-    NewTrailingRequiresClause = getDerived().TransformExpr(TRC);
-
   // Create the local class that will describe the lambda.
 
   // FIXME: DependencyKind below is wrong when substituting inside a templated
@@ -13108,7 +13101,7 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
       E->getCallOperator()->getEndLoc(),
       NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams(),
       E->getCallOperator()->getConstexprKind(),
-      NewTrailingRequiresClause.get());
+      E->getCallOperator()->getTrailingRequiresClause());
 
   LSI->CallOperator = NewCallOperator;
 

diff  --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index b4506da2bb2ba..d70e824224dfe 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -953,6 +953,10 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   case FunctionDecl::TK_NonTemplate:
     mergeRedeclarable(FD, Redecl);
     break;
+  case FunctionDecl::TK_DependentNonTemplate:
+    mergeRedeclarable(FD, Redecl);
+    FD->setInstantiatedFromDecl(readDeclAs<FunctionDecl>());
+    break;
   case FunctionDecl::TK_FunctionTemplate:
     // Merged when we merge the template.
     FD->setDescribedFunctionTemplate(readDeclAs<FunctionTemplateDecl>());

diff  --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index 01f692c9611b7..35b8db27bd0eb 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -585,6 +585,9 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   switch (D->getTemplatedKind()) {
   case FunctionDecl::TK_NonTemplate:
     break;
+  case FunctionDecl::TK_DependentNonTemplate:
+    Record.AddDeclRef(D->getInstantiatedFromDecl());
+    break;
   case FunctionDecl::TK_FunctionTemplate:
     Record.AddDeclRef(D->getDescribedFunctionTemplate());
     break;

diff  --git a/clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp b/clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
index dd6bfe8011ddf..15e00e4481e75 100644
--- a/clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
+++ b/clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
@@ -90,3 +90,24 @@ struct D { };
 
 static_assert(C<int>{}); // expected-note{{while checking constraint satisfaction for template 'C<int>' required here}}
 static_assert(D<int>{}); // expected-note{{while checking constraint satisfaction for template 'D<int>' required here}}
+
+// Test the delayed instantiation, the 'foo' implementation shouldn't cause the
+// constraint failure(or crash!) until the use to create 'y'.
+namespace DelayedInst {
+template <unsigned I>
+struct AAA {
+  template <typename T>
+    requires(sizeof(T) == I) // expected-note {{because 'sizeof(int) == 5U' (4 == 5) evaluated to false}}
+  struct B {
+    static constexpr int a = 0;
+  };
+
+  static constexpr auto foo() {
+    return B<int>::a; // expected-error{{constraints not satisfied for class template 'B' [with T = int]}}
+  }
+};
+
+constexpr auto x = AAA<4>::foo();
+constexpr auto y = AAA<5>::foo(); // expected-note {{in instantiation of member function 'DelayedInst::AAA<5>::foo' requested here}}
+
+} // namespace DelayedInst

diff  --git a/clang/test/CXX/temp/temp.constr/temp.constr.order/class-template-partial-specializations.cpp b/clang/test/CXX/temp/temp.constr/temp.constr.order/class-template-partial-specializations.cpp
index ceb5af87b8f02..7772eecc69be8 100644
--- a/clang/test/CXX/temp/temp.constr/temp.constr.order/class-template-partial-specializations.cpp
+++ b/clang/test/CXX/temp/temp.constr/temp.constr.order/class-template-partial-specializations.cpp
@@ -52,6 +52,30 @@ static_assert(F<unsigned>::value == 2);
 static_assert(F<char[10]>::value == 3);
 static_assert(F<char>::value == 1);
 
+template <unsigned I>
+struct S {
+  template <typename T>
+  struct F {
+    enum { value = 1 };
+  };
+
+  template <typename T>
+    requires C1<T> && C2<T>
+  struct F<T> {
+    enum { value = 2 };
+  };
+
+  template <typename T>
+    requires C1<T> || C2<T>
+  struct F<T> {
+    enum { value = 3 };
+  };
+};
+
+static_assert(S<1>::F<unsigned>::value == 2);
+static_assert(S<1>::F<char[10]>::value == 3);
+static_assert(S<1>::F<char>::value == 1);
+
 // Make sure atomic constraints subsume each other only if their parameter
 // mappings are identical.
 

diff  --git a/clang/test/CXX/temp/temp.constr/temp.constr.order/var-template-partial-specializations.cpp b/clang/test/CXX/temp/temp.constr/temp.constr.order/var-template-partial-specializations.cpp
index 98bfaead773aa..972a4fd0c5c82 100644
--- a/clang/test/CXX/temp/temp.constr/temp.constr.order/var-template-partial-specializations.cpp
+++ b/clang/test/CXX/temp/temp.constr/temp.constr.order/var-template-partial-specializations.cpp
@@ -51,5 +51,20 @@ static_assert(f<unsigned> == 2);
 static_assert(f<char[10]> == 3);
 static_assert(f<char> == 1);
 
-
-
+template <int I>
+struct S {
+  template <typename T>
+  static constexpr int f = 1;
+
+  template <typename T>
+    requires C1<T> && C2<T>
+  static constexpr int f<T> = 2;
+
+  template <typename T>
+    requires C1<T> || C2<T>
+  static constexpr int f<T> = 3;
+};
+
+static_assert(S<1>::f<unsigned> == 2);
+static_assert(S<1>::f<char[10]> == 3);
+static_assert(S<1>::f<char> == 1);

diff  --git a/clang/test/SemaTemplate/concepts-friends.cpp b/clang/test/SemaTemplate/concepts-friends.cpp
new file mode 100644
index 0000000000000..650376732339e
--- /dev/null
+++ b/clang/test/SemaTemplate/concepts-friends.cpp
@@ -0,0 +1,371 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+template <typename T>
+concept constraint = false;
+namespace temp_friend_9 {
+// A non-template friend declaration with a requires-clause shall be a
+// definition. ...Such a constrained friend function ... does not declare the
+// same function or function template as a declaration in any other scope.
+template <typename T>
+struct NonTemplateFriend {
+  friend void foo()
+    requires true
+  {}
+};
+
+// A friend function template with a constraint that depends on a template
+// parameter from an enclosing template shall be a definition.  Such a ...
+// function template declaration does not declare the same function or
+// function template as a declaration in any other scope.
+template <typename T>
+struct TemplateFromEnclosing {
+  template <typename U>
+  friend void foo()
+    requires constraint<T>
+  {}
+
+  T variable;
+  template <typename U>
+  friend void foo2()
+    requires constraint<decltype(variable)>
+  {}
+
+  template <typename U>
+  friend void foo3(T parmvar)
+    requires constraint<decltype(parmvar)>
+  {}
+
+  template <typename U>
+  friend void foo4()
+    requires requires(T &req) { (void)req; }
+  {}
+
+  using Alias = T;
+  template <typename U>
+  friend void foo5()
+    requires constraint<Alias>
+  {}
+
+  // All of these refer to a parent, so these are not duplicate definitions.
+  struct ChildOfEnclosing {
+    template <typename U>
+    friend void foo6()
+      requires constraint<T>
+    {}
+    template <typename U>
+    friend void foo7()
+      requires constraint<decltype(variable)>
+    {}
+    template <typename U>
+    friend void foo8(T parmvar)
+      requires constraint<decltype(parmvar)>
+    {}
+    // This is NOT a duplicate since it itself is not a template.
+    friend void foo9()
+      requires true
+    {}
+  };
+  template <typename T2>
+  struct TemplChildOfEnclosing {
+    template <typename U>
+    friend void foo10()
+      requires constraint<T>
+    {}
+  };
+};
+
+// Doesn't meet either of the requirements in the above as they don't refer to
+// an enclosing scope.
+template <typename T>
+struct Redefinition {
+  template <typename U>
+  friend void foo() // #REDEF
+    requires constraint<U>
+  {}
+
+  struct ChildOfRedef {
+    template <typename U>
+    friend void foo2() // #REDEF2
+      requires constraint<U>
+    {}
+  };
+  template <typename T2>
+  struct ChildOfRedef2 {
+    template <typename U>
+    friend void foo3() // #REDEF3
+      requires constraint<U>
+    {}
+  };
+};
+
+void bar() {
+  NonTemplateFriend<int> S1;
+  NonTemplateFriend<float> S2;
+  TemplateFromEnclosing<int> S3;
+  TemplateFromEnclosing<int>::ChildOfEnclosing S3b;
+  TemplateFromEnclosing<float> S4;
+  TemplateFromEnclosing<float>::ChildOfEnclosing S4b;
+  Redefinition<int> S5;
+  Redefinition<float> S6;
+  // expected-error@#REDEF {{redefinition of 'foo'}}
+  // expected-note at -2{{in instantiation of template class }}
+  // expected-note@#REDEF {{previous definition is here}}
+  Redefinition<int>::ChildOfRedef S7;
+  Redefinition<float>::ChildOfRedef S8;
+  // expected-error@#REDEF2 {{redefinition of 'foo2'}}
+  // expected-note at -2{{in instantiation of member class }}
+  // expected-note@#REDEF2 {{previous definition is here}}
+
+  Redefinition<int>::ChildOfRedef2<int> S9;
+  Redefinition<float>::ChildOfRedef2<float> S10;
+  // expected-error@#REDEF3 {{redefinition of 'foo3'}}
+  // expected-note at -2{{in instantiation of template class }}
+  // expected-note@#REDEF3 {{previous definition is here}}
+}
+} // namespace temp_friend_9
+
+namespace SameScopeRedefs {
+template <typename T>
+struct NonTemplateFriend {
+  friend void foo() // #NTF1
+    requires true
+  {}
+  friend void foo() // #NTF2
+    requires true
+  {}
+};
+
+template <typename T>
+struct TemplateFromEnclosing {
+  template <typename U>
+  friend void foo() // #TFE1
+    requires constraint<T>
+  {}
+  template <typename U>
+  friend void foo() // #TFE2
+    requires constraint<T>
+  {}
+};
+// Same as above, but doesn't require an instantiation pair to cause.
+template <typename T>
+struct Redefinition {
+  template <typename U>
+  friend void foo() // #RD1
+    requires constraint<U>
+  {}
+  template <typename U>
+  friend void foo() // #RD2
+    requires constraint<U>
+  {}
+};
+void bar() {
+  NonTemplateFriend<int> S1;
+  // expected-error@#NTF2 {{redefinition of 'foo'}}
+  // expected-note at -2{{in instantiation of template class}}
+  // expected-note@#NTF1 {{previous definition is here}}
+
+  TemplateFromEnclosing<int> S2;
+  // expected-error@#TFE2 {{redefinition of 'foo'}}
+  // expected-note at -2{{in instantiation of template class}}
+  // expected-note@#TFE1 {{previous definition is here}}
+
+  Redefinition<int> S3;
+  // expected-error@#RD2 {{redefinition of 'foo'}}
+  // expected-note at -2{{in instantiation of template class}}
+  // expected-note@#RD1 {{previous definition is here}}
+}
+} // namespace SameScopeRedefs
+
+namespace LibCXXOperatorRedef {
+template <typename T, typename U> struct is_same {
+  static constexpr bool value = false;
+};
+template <typename T> struct is_same<T, T> {
+  static constexpr bool value = false;
+};
+
+template <typename T, typename U>
+concept same_as = is_same<T, U>::value;
+
+// An issue found from libcxx when trying to commit the deferred concepts patch.
+// This caused an error of 'redefinition of funcN'.
+template <class _Tp> struct __range_adaptor_closure {
+  template <typename _View, typename _Closure>
+    requires same_as<_Tp, _Closure>
+  friend constexpr decltype(auto) R1func1(_View &&__view,
+                                          _Closure &&__closure){};
+  template <typename _View, typename _Closure>
+  friend constexpr decltype(auto) R1func2(_View &&__view,
+                                          _Closure &&__closure)
+    requires same_as<_Tp, _Closure>
+  {};
+  template <same_as<_Tp> _View, typename _Closure>
+  friend constexpr decltype(auto) R1func3(_View &&__view,
+                                          _Closure &&__closure){};
+};
+
+struct A : __range_adaptor_closure<A> {};
+struct B : __range_adaptor_closure<B> {};
+
+// These three fail because after the 1st pass of instantiation, they are still
+// identical.
+template <class _Tp> struct __range_adaptor_closure2 {
+  template <typename _View, typename _Closure>
+    requires same_as<_View, _Closure>
+  friend constexpr decltype(auto) R2func1(_View &&__view, // #FUNC1
+                                          _Closure &&__closure){};
+  template <typename _View, typename _Closure>
+  friend constexpr decltype(auto) R2func2(_View &&__view, // #FUNC2
+                                          _Closure &&__closure)
+    requires same_as<_View, _Closure>
+  {};
+  template <typename _View, same_as<_View> _Closure>
+  friend constexpr decltype(auto) R2func3(_View &&__view, // #FUNC3
+                                          _Closure &&__closure){};
+};
+
+struct A2 : __range_adaptor_closure2<A2> {};
+struct B2 : __range_adaptor_closure2<B2> {};
+// expected-error@#FUNC1{{redefinition of 'R2func1'}}
+// expected-note at -2{{in instantiation of template class}}
+// expected-note@#FUNC1{{previous definition is here}}
+// expected-error@#FUNC2{{redefinition of 'R2func2'}}
+// expected-note@#FUNC2{{previous definition is here}}
+// expected-error@#FUNC3{{redefinition of 'R2func3'}}
+// expected-note@#FUNC3{{previous definition is here}}
+
+// These three are fine, they all depend on the parent template parameter, so
+// are 
diff erent despite ::type not being valid.
+template <class _Tp> struct __range_adaptor_closure3 {
+  template <typename _View, typename _Closure>
+    requires same_as<typename _Tp::type, _Closure>
+  friend constexpr decltype(auto) R3func1(_View &&__view,
+                                          _Closure &&__closure){};
+  template <typename _View, typename _Closure>
+  friend constexpr decltype(auto) R3func2(_View &&__view,
+                                          _Closure &&__closure)
+    requires same_as<typename _Tp::type, _Closure>
+  {};
+  template <same_as<typename _Tp::type> _View, typename _Closure>
+  friend constexpr decltype(auto) R3func3(_View &&__view,
+                                          _Closure &&__closure){};
+};
+
+struct A3 : __range_adaptor_closure3<A3> {};
+struct B3 : __range_adaptor_closure3<B3> {};
+
+template <class _Tp> struct __range_adaptor_closure4 {
+  template <typename _View, typename _Closure>
+    requires same_as<_Tp, _View>
+  // expected-note at +1{{previous definition is here}}
+  void foo1(_View &&, _Closure &&) {}
+  template <typename _View, typename _Closure>
+    requires same_as<_Tp, _View>
+  // expected-error at +1{{class member cannot be redeclared}}
+  void foo1(_View &&, _Closure &&) {}
+
+  template <typename _View, typename _Closure>
+  // expected-note at +1{{previous definition is here}}
+  void foo2(_View &&, _Closure &&)
+    requires same_as<_Tp, _View>
+  {}
+  template <typename _View, typename _Closure>
+  // expected-error at +1{{class member cannot be redeclared}}
+  void foo2(_View &&, _Closure &&)
+    requires same_as<_Tp, _View>
+  {}
+
+  template <same_as<_Tp> _View, typename _Closure>
+  // expected-note at +1{{previous definition is here}}
+  void foo3(_View &&, _Closure &&) {}
+  template <same_as<_Tp> _View, typename _Closure>
+  // expected-error at +1{{class member cannot be redeclared}}
+  void foo3(_View &&, _Closure &&) {}
+};
+
+// Requires instantiation to fail, so no errors here.
+template <class _Tp> struct __range_adaptor_closure5 {
+  template <same_as<_Tp> U>
+  friend void foo() {}
+  template <same_as<_Tp> U>
+  friend void foo() {}
+};
+
+template <class _Tp> struct __range_adaptor_closure6 {
+  template <same_as<_Tp> U>
+  friend void foo() {} // #RAC6FOO1
+  template <same_as<_Tp> U>
+  friend void foo() {} // #RAC6FOO2
+};
+struct A6 : __range_adaptor_closure6<A6> {};
+// expected-error@#RAC6FOO2{{redefinition of 'foo'}}
+// expected-note at -2{{in instantiation of template class}}
+// expected-note@#RAC6FOO1{{previous definition is here}}
+
+template <class T> struct S1 {
+  template <typename U>
+  friend void dupe() {} // #S1DUPE
+
+  template <typename U>
+    requires same_as<U, U>
+  friend void dupe2() {} // #S1DUPE2
+};
+template <class T> struct S2 {
+  template <typename U>
+  friend void dupe() {} // #S2DUPE
+
+  template <typename U>
+    requires same_as<U, U>
+  friend void dupe2() {} // #S2DUPE2
+};
+
+template <class T> struct S3 {
+  template <typename U>
+    requires same_as<T, U>
+  friend void dupe() {}
+};
+template <class T> struct S4 {
+  template <typename U>
+    requires same_as<T, U>
+  friend void dupe() {}
+};
+
+// Same as S3 and S4, but aren't instantiated with the same T.
+template <class T> struct S5 {
+  template <typename U>
+    requires same_as<T, U>
+  friend void not_dupe() {}
+};
+template <class T> struct S6 {
+  template <typename U>
+    requires same_as<T, U>
+  friend void not_dupe() {}
+};
+
+template <class T> struct S7 {
+  void not_dupe()
+    requires same_as<T, T>
+  {}
+};
+
+void useS() {
+  S1<int> s1;
+  S2<double> s2;
+  // expected-error@#S2DUPE{{redefinition}}
+  // expected-note at -2{{in instantiation of template class}}
+  // expected-note@#S1DUPE{{previous definition is here}}
+  // expected-error@#S2DUPE2{{redefinition}}
+  // expected-note@#S1DUPE2{{previous definition is here}}
+
+  // OK, they have 
diff erent 'scopes'.
+  S3<int> s3;
+  S4<int> s4;
+
+  // OK, because only instantiated with 
diff erent T.
+  S5<int> s5;
+  S6<double> s6;
+
+  S7<int> s7;
+}
+
+} // namespace LibCXXOperatorRedef

diff  --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index 23c79421bb6f8..390d0e4562725 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -256,3 +256,321 @@ C auto **j1 = g();   // expected-error {{deduced type 'int' does not satisfy 'C'
 C auto **&j2 = g();  // expected-error {{deduced type 'int' does not satisfy 'C'}}
 C auto **&&j3 = g(); // expected-error {{deduced type 'int' does not satisfy 'C'}}
 }
+
+namespace SubConstraintChecks {
+template <typename T>
+concept TrueConstraint = true;
+template <typename T>
+concept FalseConstraint = false;
+
+template <typename T, typename... Us>
+class ContainsConstrainedFuncTrue {
+public:
+  template <typename V, TrueConstraint Constrained>
+  static void func(V &&, Constrained &&C);
+};
+template <typename T, typename... Us>
+class ContainsConstrainedFuncFalse {
+public:
+  template <typename V, FalseConstraint Constrained>
+  static void func(V &&, Constrained &&C);
+};
+
+template <typename... Us>
+concept TrueConstraint2 =
+    requires(float &&t) {
+      ContainsConstrainedFuncTrue<float, Us...>::func(5, 0.0);
+    };
+template <typename... Us>
+concept FalseConstraint2 =
+    requires(float &&t) {
+      ContainsConstrainedFuncFalse<float, Us...>::func(5, 0.0); // #FC2_CONSTR
+    };
+
+template <typename T>
+void useTrue(int F)
+  requires TrueConstraint2<int>
+{}
+
+template <typename T>
+void useFalse(int F)             // #USE_FALSE
+  requires FalseConstraint2<int> // #USE_FALSE_CONSTR
+{}
+
+// Should only diagnose 'false' once instantiated.
+void UseUse() {
+  useTrue<int>(5);
+  useFalse<int>(5);
+  // expected-error at -1{{no matching function for call to 'useFalse'}}
+  // expected-note@#USE_FALSE{{constraints not satisfied}}
+  // expected-note@#USE_FALSE_CONSTR{{because 'int' does not satisfy 'FalseConstraint2'}}
+  // expected-note@#FC2_CONSTR {{would be invalid: no matching function for call to 'func'}}
+}
+} // namespace SubConstraintChecks
+
+namespace DeducedTemplateArgs {
+template <typename Itr> struct ItrTraits {
+  template <typename PtrItr> struct Ptr {
+  };
+  template <typename PtrItr>
+    requires requires { typename PtrItr::pointer; }
+  struct Ptr<PtrItr> {
+    using type = typename Itr::pointer;
+  };
+  using pointer = typename Ptr<Itr>::type; // #TRAITS_PTR
+};
+
+struct complete_itr {
+  using pointer = int;
+};
+
+template <typename T> class Complete {
+  using ItrType = ItrTraits<complete_itr>;
+  ItrType begin() noexcept { return ItrType(); }
+};
+
+// This version doesn't have 'pointer', so error confirms we are in the first
+// verison of 'Ptr'.
+struct not_complete_itr {
+};
+
+template <typename T> class NotComplete {
+  using ItrType = ItrTraits<not_complete_itr>;
+  ItrType begin() noexcept { return ItrType(); }
+  // expected-error@#TRAITS_PTR{{no type named 'type' in }}
+  // expected-note at -2{{in instantiation of template class }}
+};
+} // namespace DeducedTemplateArgs
+
+namespace DeferredInstantiationInstScope {
+template <typename T>
+struct remove_ref {
+  using type = T;
+};
+template <typename T>
+struct remove_ref<T &> {
+  using type = T;
+};
+template <typename T>
+struct remove_ref<T &&> {
+  using type = T;
+};
+
+template <typename T>
+constexpr bool IsInt = PR54443::is_same<typename remove_ref<T>::type,
+                                        int>::value;
+
+template <typename U>
+void SingleDepthReferencesTop(U &&u) {
+  struct lc {
+    void operator()()             // #SDRT_OP
+      requires IsInt<decltype(u)> // #SDRT_REQ
+    {}
+  };
+  lc lv;
+  lv(); // #SDRT_CALL
+}
+
+template <typename U>
+void SingleDepthReferencesTopNotCalled(U &&u) {
+  struct lc {
+    void operator()()
+      requires IsInt<typename decltype(u)::FOO>
+    {}
+  };
+  lc lv;
+}
+
+template <typename U>
+void SingleDepthReferencesTopCalled(U &&u) {
+  struct lc {
+    void operator()()                           // #CALLOP
+      requires IsInt<typename decltype(u)::FOO> // #CONSTR
+    {}
+  };
+  lc lv;
+  lv();
+  // expected-error at -1{{no matching function for call to object of type 'lc'}}
+  // expected-note@#SDRTC{{in instantiation of function template}}
+  // expected-note@#CALLOP{{constraints not satisfied}}
+  // expected-note@#CONSTR{{substituted constraint expression is ill-formed}}
+}
+
+template <typename U>
+void SingleDepthReferencesTopLambda(U &&u) {
+  []()
+    requires IsInt<decltype(u)>
+  {}();
+}
+
+template <typename U>
+void DoubleDepthReferencesTop(U &&u) {
+  struct lc { // #DDRT_STRCT
+    void operator()() {
+      struct lc2 {
+        void operator()()             // #DDRT_OP
+          requires IsInt<decltype(u)> // #DDRT_REQ
+        {}
+      };
+      lc2 lv2;
+      lv2(); // #DDRT_CALL
+    }
+  };
+  lc lv;
+  lv();
+}
+
+template <typename U>
+void DoubleDepthReferencesTopLambda(U &&u) {
+  []() { []()
+           requires IsInt<decltype(u)>
+         {}(); }();
+}
+
+template <typename U>
+void DoubleDepthReferencesAll(U &&u) {
+  struct lc { // #DDRA_STRCT
+    void operator()(U &&u2) {
+      struct lc2 {
+        void operator()(U &&u3)          // #DDRA_OP
+          requires IsInt<decltype(u)> && // #DDRA_REQ
+                   IsInt<decltype(u2)> && IsInt<decltype(u3)>
+        {}
+      };
+      lc2 lv2;
+      lv2(u2); // #DDRA_CALL
+    }
+  };
+  lc lv;
+  lv(u);
+}
+
+template <typename U>
+void DoubleDepthReferencesAllLambda(U &&u) {
+  [](U &&u2) {
+    [](U && u3)
+      requires IsInt<decltype(u)> &&
+               IsInt<decltype(u2)> && IsInt<decltype(u3)>
+    {}(u2);
+  }(u);
+}
+
+template <typename U>
+struct CausesFriendConstraint {
+  template <typename V>
+  friend void FriendFunc(CausesFriendConstraint, V) // #FF_DECL
+    requires IsInt<U> &&
+             IsInt<V> // #FF_REQ
+  {}
+};
+// FIXME: Re-enable this test when constraints are allowed to refer to captures.
+// template<typename T>
+// void ChecksCapture(T x) {
+//   [y = x]() requires(IsInt<decltype(y)>){}();
+// }
+
+template <typename T>
+void ChecksLocalVar(T x) {
+  T Local;
+  []()
+    requires(IsInt<decltype(Local)>)
+  {}();
+}
+
+template <typename T>
+void LocalStructMemberVar(T x) {
+  struct S {
+    T local;
+    void foo()
+      requires(IsInt<decltype(local)>) // #LSMV_REQ
+    {}
+  } s;
+  s.foo(); // #LSMV_CALL
+};
+
+template <typename T>
+struct ChecksMemberVar {
+  T t;
+  void foo()
+    requires(IsInt<decltype(t)>) // #CMV_FOO
+  {}
+  template <typename U>
+  void foo2()                    // #CMV_FOO2
+    requires(IsInt<decltype(t)>) // #CMV_FOO2_REQ
+  {}
+};
+
+void test_dependent() {
+  int v = 0;
+  float will_fail;
+  SingleDepthReferencesTop(v);
+  SingleDepthReferencesTop(will_fail);
+  // expected-error@#SDRT_CALL{{no matching function for call to object of type 'lc'}}
+  // expected-note at -2{{in instantiation of function template specialization}}
+  // expected-note@#SDRT_OP{{candidate function not viable}}
+  // expected-note@#SDRT_REQ{{'IsInt<decltype(u)>' evaluated to false}}
+
+  SingleDepthReferencesTopNotCalled(v);
+  // Won't error unless we try to call it.
+  SingleDepthReferencesTopNotCalled(will_fail);
+  SingleDepthReferencesTopCalled(v); // #SDRTC
+  SingleDepthReferencesTopLambda(v);
+  // FIXME: This should error on constraint failure! (Lambda!)
+  SingleDepthReferencesTopLambda(will_fail);
+  DoubleDepthReferencesTop(v);
+  DoubleDepthReferencesTop(will_fail);
+  // expected-error@#DDRT_CALL{{no matching function for call to object of type 'lc2'}}
+  // expected-note at -2{{in instantiation of function template specialization}}
+  // expected-note@#DDRT_STRCT{{in instantiation of member function}}
+  // expected-note@#DDRT_OP{{candidate function not viable}}
+  // expected-note@#DDRT_REQ{{'IsInt<decltype(u)>' evaluated to false}}
+
+  DoubleDepthReferencesTopLambda(v);
+  // FIXME: This should error on constraint failure! (Lambda!)
+  DoubleDepthReferencesTopLambda(will_fail);
+  DoubleDepthReferencesAll(v);
+  DoubleDepthReferencesAll(will_fail);
+  // expected-error@#DDRA_CALL{{no matching function for call to object of type 'lc2'}}
+  // expected-note at -2{{in instantiation of function template specialization}}
+  // expected-note@#DDRA_STRCT{{in instantiation of member function}}
+  // expected-note@#DDRA_OP{{candidate function not viable}}
+  // expected-note@#DDRA_REQ{{'IsInt<decltype(u)>' evaluated to false}}
+
+  DoubleDepthReferencesAllLambda(v);
+  // FIXME: This should error on constraint failure! (Lambda!)
+  DoubleDepthReferencesAllLambda(will_fail);
+
+  CausesFriendConstraint<int> CFC;
+  FriendFunc(CFC, 1);
+  FriendFunc(CFC, 1.0);
+  // expected-error at -1{{no matching function for call to 'FriendFunc'}}
+  // expected-note@#FF_DECL{{constraints not satisfied}}
+  // expected-note@#FF_REQ{{because 'IsInt<double>' evaluated to false}}
+
+  // FIXME: Re-enable this test when constraints are allowed to refer to captures.
+  // ChecksCapture(v);
+
+  ChecksLocalVar(v);
+  // FIXME: This should error on constraint failure! (Lambda!)
+  ChecksLocalVar(will_fail);
+
+  LocalStructMemberVar(v);
+  LocalStructMemberVar(will_fail);
+  // expected-error@#LSMV_CALL{{invalid reference to function 'foo'}}
+  // expected-note at -2{{in instantiation of function template specialization}}
+  // expected-note@#LSMV_REQ{{because 'IsInt<decltype(this->local)>' evaluated to false}}
+
+  ChecksMemberVar<int> CMV;
+  CMV.foo();
+  CMV.foo2<int>();
+
+  ChecksMemberVar<float> CMV2;
+  CMV2.foo();
+  // expected-error at -1{{invalid reference to function 'foo'}}
+  // expected-note@#CMV_FOO{{because 'IsInt<decltype(this->t)>' evaluated to false}}
+  CMV2.foo2<float>();
+  // expected-error at -1{{no matching member function for call to 'foo2'}}
+  // expected-note@#CMV_FOO2{{constraints not satisfied}}
+  // expected-note@#CMV_FOO2_REQ{{because 'IsInt<decltype(this->t)>' evaluated to false}}
+}
+} // namespace DeferredInstantiationInstScope

diff  --git a/clang/test/SemaTemplate/deferred-concept-inst.cpp b/clang/test/SemaTemplate/deferred-concept-inst.cpp
new file mode 100644
index 0000000000000..dd9a4086a42ed
--- /dev/null
+++ b/clang/test/SemaTemplate/deferred-concept-inst.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -std=c++2a -x c++ %s -verify -fsyntax-only -Wno-unused-value
+// expected-no-diagnostics
+
+namespace GithubBug44178 {
+template <typename D>
+struct CRTP {
+  void call_foo()
+    requires requires(D &v) { v.foo(); }
+  {
+    static_cast<D *>(this)->foo();
+  }
+};
+
+struct Test : public CRTP<Test> {
+  void foo() {}
+};
+
+int main() {
+  Test t;
+  t.call_foo();
+  return 0;
+}
+} // namespace GithubBug44178

diff  --git a/clang/test/SemaTemplate/instantiate-requires-clause.cpp b/clang/test/SemaTemplate/instantiate-requires-clause.cpp
index 73dd20d27d226..3f438d8885a20 100644
--- a/clang/test/SemaTemplate/instantiate-requires-clause.cpp
+++ b/clang/test/SemaTemplate/instantiate-requires-clause.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++2a -x c++ %s -verify
+// RUN: %clang_cc1 -std=c++2a -x c++ %s -Wno-unused-value -verify
 
 template <typename... Args> requires ((sizeof(Args) == 1), ...)
 // expected-note at -1 {{because '(sizeof(int) == 1) , (sizeof(char) == 1) , (sizeof(int) == 1)' evaluated to false}}
@@ -40,6 +40,20 @@ struct S {
 
 static_assert(S<void>::f(1));
 
+// Similar to the 'S' test, but tries to use 'U' in the requires clause.
+template <typename T2>
+struct S1 {
+  // expected-note at +3 {{candidate template ignored: constraints not satisfied [with U = int]}}
+  // expected-note at +3 {{because substituted constraint expression is ill-formed: type 'int' cannot be used prior to '::' because it has no members}}
+  template <typename U>
+  static constexpr auto f(U const index)
+    requires(U::foo)
+  { return true; }
+};
+
+// expected-error at +1 {{no matching function for call to 'f'}}
+static_assert(S1<void>::f(1));
+
 constexpr auto value = 0;
 
 template<typename T>

diff  --git a/clang/test/SemaTemplate/trailing-return-short-circuit.cpp b/clang/test/SemaTemplate/trailing-return-short-circuit.cpp
new file mode 100644
index 0000000000000..0d1c9b52b0e85
--- /dev/null
+++ b/clang/test/SemaTemplate/trailing-return-short-circuit.cpp
@@ -0,0 +1,62 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+template <class T>
+  requires(sizeof(T) > 2) || T::value // #FOO_REQ
+void Foo(T){};                        // #FOO
+
+template <class T>
+void TrailingReturn(T)       // #TRAILING
+  requires(sizeof(T) > 2) || // #TRAILING_REQ
+          T::value           // #TRAILING_REQ_VAL
+{};
+template <bool B>
+struct HasValue {
+  static constexpr bool value = B;
+};
+static_assert(sizeof(HasValue<true>) <= 2);
+
+template <bool B>
+struct HasValueLarge {
+  static constexpr bool value = B;
+  int I;
+};
+static_assert(sizeof(HasValueLarge<true>) > 2);
+
+void usage() {
+  // Passes the 1st check, short-circuit so the 2nd ::value is not evaluated.
+  Foo(1.0);
+  TrailingReturn(1.0);
+
+  // Fails the 1st check, but has a ::value, so the check happens correctly.
+  Foo(HasValue<true>{});
+  TrailingReturn(HasValue<true>{});
+
+  // Passes the 1st check, but would have passed the 2nd one.
+  Foo(HasValueLarge<true>{});
+  TrailingReturn(HasValueLarge<true>{});
+
+  // Fails the 1st check, fails 2nd because there is no ::value.
+  Foo(true);
+  // expected-error at -1{{no matching function for call to 'Foo'}}
+  // expected-note@#FOO{{candidate template ignored: constraints not satisfied [with T = bool]}}
+  // expected-note@#FOO_REQ{{because 'sizeof(_Bool) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#FOO_REQ{{because substituted constraint expression is ill-formed: type 'bool' cannot be used prior to '::' because it has no members}}
+
+  TrailingReturn(true);
+  // expected-error at -1{{no matching function for call to 'TrailingReturn'}}
+  // expected-note@#TRAILING{{candidate template ignored: constraints not satisfied [with T = bool]}}
+  // expected-note@#TRAILING_REQ{{because 'sizeof(_Bool) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#TRAILING_REQ_VAL{{because substituted constraint expression is ill-formed: type 'bool' cannot be used prior to '::' because it has no members}}
+
+  // Fails the 1st check, fails 2nd because ::value is false.
+  Foo(HasValue<false>{});
+  // expected-error at -1 {{no matching function for call to 'Foo'}}
+  // expected-note@#FOO{{candidate template ignored: constraints not satisfied [with T = HasValue<false>]}}
+  // expected-note@#FOO_REQ{{because 'sizeof(HasValue<false>) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#FOO_REQ{{and 'HasValue<false>::value' evaluated to false}}
+  TrailingReturn(HasValue<false>{});
+  // expected-error at -1 {{no matching function for call to 'TrailingReturn'}}
+  // expected-note@#TRAILING{{candidate template ignored: constraints not satisfied [with T = HasValue<false>]}}
+  // expected-note@#TRAILING_REQ{{because 'sizeof(HasValue<false>) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#TRAILING_REQ_VAL{{and 'HasValue<false>::value' evaluated to false}}
+}


        


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