Re-apply "Deferred Concept Instantiation Implementation"

This reverts commit 95d94a6.

This implements the deferred concepts instantiation, which should allow
the libstdc++ ranges to properly compile, and for the CRTP to work for
constrained functions.

Since the last attempt, this has fixed the issues from @wlei and
@mordante.

Differential Revision: https://reviews.llvm.org/D126907

clang/docs/ReleaseNotes.rst

@@ -345,6 +345,10 @@ C++20 Feature Support
   `Issue 50455 <https://github.com/llvm/llvm-project/issues/50455>`_,
   `Issue 54872 <https://github.com/llvm/llvm-project/issues/54872>`_,
   `Issue 54587 <https://github.com/llvm/llvm-project/issues/54587>`_.
+- 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>`_.
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/ASTContext.h

@@ -2669,6 +2669,11 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// template.
   bool hasSameTemplateName(const TemplateName &X, const TemplateName &Y) const;
 
+  /// Determine whether two Friend functions are different because constraints
+  /// that refer to an enclosing template, according to [temp.friend] p9.
+  bool FriendsDifferByConstraints(const FunctionDecl *X,
+                                  const FunctionDecl *Y) const;
+
   /// Determine whether the two declarations refer to the same entity.
   bool isSameEntity(const NamedDecl *X, const NamedDecl *Y) const;
 

clang/include/clang/AST/Decl.h

@@ -2474,6 +2474,19 @@ class FunctionDecl : public DeclaratorDecl,
     getCanonicalDecl()->FunctionDeclBits.IsMultiVersion = V;
   }
 
+  // Sets that this is a constrained friend where the constraint refers to an
+  // enclosing template.
+  void setFriendConstraintRefersToEnclosingTemplate(bool V = true) {
+    getCanonicalDecl()
+        ->FunctionDeclBits.FriendConstraintRefersToEnclosingTemplate = V;
+  }
+  // Indicates this function is a constrained friend, where the constraint
+  // refers to an enclosing template for hte purposes of [temp.friend]p9.
+  bool FriendConstraintRefersToEnclosingTemplate() const {
+    return getCanonicalDecl()
+        ->FunctionDeclBits.FriendConstraintRefersToEnclosingTemplate;
+  }
+
   /// Gets the kind of multiversioning attribute this declaration has. Note that
   /// this can return a value even if the function is not multiversion, such as
   /// the case of 'target'.

clang/include/clang/AST/DeclBase.h

@@ -1664,10 +1664,14 @@ class DeclContext {
 
     /// Indicates if the function uses Floating Point Constrained Intrinsics
     uint64_t UsesFPIntrin : 1;
+
+    // Indicates this function is a constrained friend, where the constraint
+    // refers to an enclosing template for hte purposes of [temp.friend]p9.
+    uint64_t FriendConstraintRefersToEnclosingTemplate : 1;
   };
 
   /// Number of non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = 28 };
+  enum { NumFunctionDeclBits = 29 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
@@ -1679,12 +1683,12 @@ class DeclContext {
     /// For the bits in FunctionDeclBitfields.
     uint64_t : NumFunctionDeclBits;
 
-    /// 23 bits to fit in the remaining available space.
+    /// 22 bits to fit in the remaining available space.
     /// Note that this makes CXXConstructorDeclBitfields take
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 20;
+    uint64_t NumCtorInitializers : 19;
     uint64_t IsInheritingConstructor : 1;
 
     /// Whether this constructor has a trail-allocated explicit specifier.

clang/include/clang/Sema/Sema.h

@@ -3673,6 +3673,31 @@ 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.
+  // TemplateDepth is the 'depth' of the friend function, which is used to
+  // compare whether a declaration reference is referring to a containing
+  // template, or just the current friend function. A 'lower' TemplateDepth in
+  // the AST refers to a 'containing' template. As the constraint is
+  // uninstantiated, this is relative to the 'top' of the TU.
+  bool ConstraintExpressionDependsOnEnclosingTemplate(unsigned TemplateDepth,
+                                                      const Expr *Constraint);
+
+  // Calculates whether the friend function depends on an enclosing template for
+  // the purposes of [temp.friend] p9.
+  bool FriendConstraintsDependOnEnclosingTemplate(const FunctionDecl *FD);
+
+  // Calculates whether two constraint expressions are equal irrespective of a
+  // difference in 'depth'. This takes a pair of optional 'NamedDecl's 'Old' and
+  // 'New', which are the "source" of the constraint, since this is necessary
+  // for figuring out the relative 'depth' of the constraint. The depth of the
+  // 'primary template' and the 'instantiated from' templates aren't necessarily
+  // the same, such as a case when one is a 'friend' defined in a class.
+  bool AreConstraintExpressionsEqual(const NamedDecl *Old,
+                                     const Expr *OldConstr,
+                                     const NamedDecl *New,
+                                     const Expr *NewConstr);
+
   enum class AllowedExplicit {
     /// Allow no explicit functions to be used.
     None,
@@ -7152,6 +7177,21 @@ 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(
@@ -7194,6 +7234,39 @@ class Sema final {
   bool CheckConstraintSatisfaction(
       const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
       const MultiLevelTemplateArgumentList &TemplateArgLists,
+      SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction) {
+    llvm::SmallVector<Expr *, 4> Converted;
+    return CheckConstraintSatisfaction(Template, ConstraintExprs, Converted,
+                                       TemplateArgLists, 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
@@ -7213,8 +7286,8 @@ class Sema final {
   /// \returns true if an error occurred, false otherwise.
   bool CheckFunctionConstraints(const FunctionDecl *FD,
                                 ConstraintSatisfaction &Satisfaction,
-                                SourceLocation UsageLoc = SourceLocation());
-
+                                SourceLocation UsageLoc = SourceLocation(),
+                                bool ForOverloadResolution = false);
 
   /// \brief Ensure that the given template arguments satisfy the constraints
   /// associated with the given template, emitting a diagnostic if they do not.
@@ -8222,12 +8295,19 @@ class Sema final {
     TPL_TemplateTemplateArgumentMatch
   };
 
-  bool TemplateParameterListsAreEqual(TemplateParameterList *New,
-                                      TemplateParameterList *Old,
-                                      bool Complain,
-                                      TemplateParameterListEqualKind Kind,
-                                      SourceLocation TemplateArgLoc
-                                        = SourceLocation());
+  bool TemplateParameterListsAreEqual(
+      const NamedDecl *NewInstFrom, TemplateParameterList *New,
+      const NamedDecl *OldInstFrom, TemplateParameterList *Old, bool Complain,
+      TemplateParameterListEqualKind Kind,
+      SourceLocation TemplateArgLoc = SourceLocation());
+
+  bool TemplateParameterListsAreEqual(
+      TemplateParameterList *New, TemplateParameterList *Old, bool Complain,
+      TemplateParameterListEqualKind Kind,
+      SourceLocation TemplateArgLoc = SourceLocation()) {
+    return TemplateParameterListsAreEqual(nullptr, New, nullptr, Old, Complain,
+                                          Kind, TemplateArgLoc);
+  }
 
   bool CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams);
 
@@ -8962,7 +9042,8 @@ 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
@@ -9670,23 +9751,21 @@ class Sema final {
                             const MultiLevelTemplateArgumentList &TemplateArgs,
                             SourceLocation Loc, DeclarationName Entity);
 
-  TypeSourceInfo *SubstFunctionDeclType(TypeSourceInfo *T,
-                            const MultiLevelTemplateArgumentList &TemplateArgs,
-                                        SourceLocation Loc,
-                                        DeclarationName Entity,
-                                        CXXRecordDecl *ThisContext,
-                                        Qualifiers ThisTypeQuals);
+  TypeSourceInfo *SubstFunctionDeclType(
+      TypeSourceInfo *T, const MultiLevelTemplateArgumentList &TemplateArgs,
+      SourceLocation Loc, DeclarationName Entity, CXXRecordDecl *ThisContext,
+      Qualifiers ThisTypeQuals, bool EvaluateConstraints = true);
   void SubstExceptionSpec(FunctionDecl *New, const FunctionProtoType *Proto,
                           const MultiLevelTemplateArgumentList &Args);
   bool SubstExceptionSpec(SourceLocation Loc,
                           FunctionProtoType::ExceptionSpecInfo &ESI,
                           SmallVectorImpl<QualType> &ExceptionStorage,
                           const MultiLevelTemplateArgumentList &Args);
-  ParmVarDecl *SubstParmVarDecl(ParmVarDecl *D,
-                            const MultiLevelTemplateArgumentList &TemplateArgs,
-                                int indexAdjustment,
-                                Optional<unsigned> NumExpansions,
-                                bool ExpectParameterPack);
+  ParmVarDecl *
+  SubstParmVarDecl(ParmVarDecl *D,
+                   const MultiLevelTemplateArgumentList &TemplateArgs,
+                   int indexAdjustment, Optional<unsigned> NumExpansions,
+                   bool ExpectParameterPack, bool EvaluateConstraints = true);
   bool SubstParmTypes(SourceLocation Loc, ArrayRef<ParmVarDecl *> Params,
                       const FunctionProtoType::ExtParameterInfo *ExtParamInfos,
                       const MultiLevelTemplateArgumentList &TemplateArgs,
@@ -9696,6 +9775,25 @@ class Sema final {
   ExprResult SubstExpr(Expr *E,
                        const MultiLevelTemplateArgumentList &TemplateArgs);
 
+  // A RAII type used by the TemplateDeclInstantiator and TemplateInstantiator
+  // to disable constraint evaluation, then restore the state.
+  template <typename InstTy> struct ConstraintEvalRAII {
+    InstTy &TI;
+    bool OldValue;
+
+    ConstraintEvalRAII(InstTy &TI)
+        : TI(TI), OldValue(TI.getEvaluateConstraints()) {
+      TI.setEvaluateConstraints(false);
+    }
+    ~ConstraintEvalRAII() { TI.setEvaluateConstraints(OldValue); }
+  };
+
+  // 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.
   ///
@@ -9725,7 +9823,6 @@ class Sema final {
                          const MultiLevelTemplateArgumentList &TemplateArgs,
                          TemplateArgumentListInfo &Outputs);
 
-
   Decl *SubstDecl(Decl *D, DeclContext *Owner,
                   const MultiLevelTemplateArgumentList &TemplateArgs);
 
@@ -9816,7 +9913,8 @@ class Sema final {
                     const MultiLevelTemplateArgumentList &TemplateArgs);
 
   bool SubstTypeConstraint(TemplateTypeParmDecl *Inst, const TypeConstraint *TC,
-                           const MultiLevelTemplateArgumentList &TemplateArgs);
+                           const MultiLevelTemplateArgumentList &TemplateArgs,
+                           bool EvaluateConstraint);
 
   bool InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                   ParmVarDecl *Param);

clang/include/clang/Sema/Template.h

@@ -503,6 +503,7 @@ enum class TemplateSubstitutionKind : char {
     const MultiLevelTemplateArgumentList &TemplateArgs;
     Sema::LateInstantiatedAttrVec* LateAttrs = nullptr;
     LocalInstantiationScope *StartingScope = nullptr;
+    bool EvaluateConstraints = true;
 
     /// A list of out-of-line class template partial
     /// specializations that will need to be instantiated after the
@@ -526,6 +527,13 @@ enum class TemplateSubstitutionKind : char {
           SubstIndex(SemaRef, SemaRef.ArgumentPackSubstitutionIndex),
           Owner(Owner), TemplateArgs(TemplateArgs) {}
 
+    void setEvaluateConstraints(bool B) {
+      EvaluateConstraints = B;
+    }
+    bool getEvaluateConstraints() {
+      return EvaluateConstraints;
+    }
+
 // Define all the decl visitors using DeclNodes.inc
 #define DECL(DERIVED, BASE) \
     Decl *Visit ## DERIVED ## Decl(DERIVED ## Decl *D);

clang/lib/AST/ASTContext.cpp

@@ -6452,6 +6452,31 @@ static bool hasSameOverloadableAttrs(const FunctionDecl *A,
   return true;
 }
 
+bool ASTContext::FriendsDifferByConstraints(const FunctionDecl *X,
+                                            const FunctionDecl *Y) const {
+  // If these aren't friends, then they aren't friends that differ by
+  // constraints.
+  if (!X->getFriendObjectKind() || !Y->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 (X->getLexicalDeclContext() == Y->getLexicalDeclContext())
+    return false;
+
+  if (!X->getDescribedFunctionTemplate()) {
+    assert(!Y->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 X->getTrailingRequiresClause();
+  }
+
+  return X->FriendConstraintRefersToEnclosingTemplate();
+}
+
 bool ASTContext::isSameEntity(const NamedDecl *X, const NamedDecl *Y) const {
   if (X == Y)
     return true;
@@ -6532,6 +6557,10 @@ bool ASTContext::isSameEntity(const NamedDecl *X, const NamedDecl *Y) const {
                               FuncY->getTrailingRequiresClause()))
       return false;
 
+    // Constrained friends are different in certain cases, see: [temp.friend]p9.
+    if (FriendsDifferByConstraints(FuncX, FuncY))
+      return false;
+
     auto GetTypeAsWritten = [](const FunctionDecl *FD) {
       // Map to the first declaration that we've already merged into this one.
       // The TSI of redeclarations might not match (due to calling conventions

clang/lib/AST/ASTImporter.cpp

@@ -3701,6 +3701,8 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
   ToFunction->setDefaulted(D->isDefaulted());
   ToFunction->setExplicitlyDefaulted(D->isExplicitlyDefaulted());
   ToFunction->setDeletedAsWritten(D->isDeletedAsWritten());
+  ToFunction->setFriendConstraintRefersToEnclosingTemplate(
+      D->FriendConstraintRefersToEnclosingTemplate());
   ToFunction->setRangeEnd(ToEndLoc);
 
   // Set the parameters.

clang/lib/AST/Decl.cpp

@@ -2970,6 +2970,7 @@ FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
   FunctionDeclBits.IsMultiVersion = false;
   FunctionDeclBits.IsCopyDeductionCandidate = false;
   FunctionDeclBits.HasODRHash = false;
+  FunctionDeclBits.FriendConstraintRefersToEnclosingTemplate = false;
   if (TrailingRequiresClause)
     setTrailingRequiresClause(TrailingRequiresClause);
 }