[Clang] Implement CWG2369 "Ordering between constraints and substitution"

clang/include/clang/Sema/Sema.h

@@ -13007,11 +13007,11 @@ class Sema final : public SemaBase {
   /// instantiation arguments.
   ///
   /// \param DC In the event we don't HAVE a declaration yet, we instead provide
-  ///  the decl context where it will be created.  In this case, the `Innermost`
-  ///  should likely be provided.  If ND is non-null, this is ignored.
+  ///  the decl context where it will be created.  In this case, the \p
+  ///  Innermost should likely be provided.  If \p ND is non-null, this is ignored.
   ///
   /// \param Innermost if non-NULL, specifies a template argument list for the
-  /// template declaration passed as ND.
+  /// template declaration passed as \p ND.
   ///
   /// \param RelativeToPrimary true if we should get the template
   /// arguments relative to the primary template, even when we're
@@ -13302,6 +13302,7 @@ class Sema final : public SemaBase {
   ExprResult
   SubstConstraintExpr(Expr *E,
                       const MultiLevelTemplateArgumentList &TemplateArgs);
+
   // Unlike the above, this does not evaluates constraints.
   ExprResult SubstConstraintExprWithoutSatisfaction(
       Expr *E, const MultiLevelTemplateArgumentList &TemplateArgs);
@@ -14416,10 +14417,10 @@ class Sema final : public SemaBase {
       const MultiLevelTemplateArgumentList &TemplateArgs,
       SourceRange TemplateIDRange);
 
-  bool CheckInstantiatedFunctionTemplateConstraints(
-      SourceLocation PointOfInstantiation, FunctionDecl *Decl,
-      ArrayRef<TemplateArgument> TemplateArgs,
-      ConstraintSatisfaction &Satisfaction);
+  bool CheckFunctionTemplateConstraints(SourceLocation PointOfInstantiation,
+                                        FunctionDecl *Decl,
+                                        ArrayRef<TemplateArgument> TemplateArgs,
+                                        ConstraintSatisfaction &Satisfaction);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.

clang/include/clang/Sema/Template.h

@@ -522,6 +522,9 @@ enum class TemplateSubstitutionKind : char {
     llvm::PointerUnion<Decl *, DeclArgumentPack *> *
     findInstantiationOf(const Decl *D);
 
+    llvm::PointerUnion<Decl *, DeclArgumentPack *> *
+    findInstantiationUnsafe(const Decl *D);
+
     void InstantiatedLocal(const Decl *D, Decl *Inst);
     void InstantiatedLocalPackArg(const Decl *D, VarDecl *Inst);
     void MakeInstantiatedLocalArgPack(const Decl *D);

clang/lib/Sema/SemaConcept.cpp

@@ -847,7 +847,7 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
                                     bool ForOverloadResolution) {
   // 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
+  // CheckInstantiatedFunctionTemplateConstraints after this will check it
   // better.
   if (FD->isDependentContext() ||
       FD->getTemplatedKind() ==
@@ -1109,12 +1109,50 @@ bool Sema::EnsureTemplateArgumentListConstraints(
   return false;
 }
 
-bool Sema::CheckInstantiatedFunctionTemplateConstraints(
+static bool CheckFunctionConstraintsWithoutInstantiation(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    FunctionTemplateDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
+    ConstraintSatisfaction &Satisfaction) {
+  SmallVector<const Expr *, 3> TemplateAC;
+  Template->getAssociatedConstraints(TemplateAC);
+  if (TemplateAC.empty()) {
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  LocalInstantiationScope Scope(SemaRef);
+
+  FunctionDecl *FD = Template->getTemplatedDecl();
+  // 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.
+  MultiLevelTemplateArgumentList MLTAL =
+      SemaRef.getTemplateInstantiationArgs(FD, /*DC=*/nullptr,
+                                           /*Final=*/false,
+                                           /*Innermost=*/TemplateArgs,
+                                           /*RelativeToPrimary=*/true,
+                                           /*ForConstraintInstantiation=*/true);
+
+  std::optional<Sema::CXXThisScopeRAII> ThisScope;
+  if (auto *Method = dyn_cast<CXXMethodDecl>(FD))
+    ThisScope.emplace(SemaRef, /*Record=*/Method->getParent(),
+                      /*ThisQuals=*/Method->getMethodQualifiers());
+  return SemaRef.CheckConstraintSatisfaction(
+      Template, TemplateAC, MLTAL, PointOfInstantiation, Satisfaction);
+}
+
+bool Sema::CheckFunctionTemplateConstraints(
     SourceLocation PointOfInstantiation, FunctionDecl *Decl,
     ArrayRef<TemplateArgument> TemplateArgs,
     ConstraintSatisfaction &Satisfaction) {
   // In most cases we're not going to have constraints, so check for that first.
   FunctionTemplateDecl *Template = Decl->getPrimaryTemplate();
+
+  if (!Template)
+    return ::CheckFunctionConstraintsWithoutInstantiation(
+        *this, PointOfInstantiation, Decl->getDescribedFunctionTemplate(),
+        TemplateArgs, Satisfaction);
+
   // Note - code synthesis context for the constraints check is created
   // inside CheckConstraintsSatisfaction.
   SmallVector<const Expr *, 3> TemplateAC;
@@ -1517,7 +1555,7 @@ substituteParameterMappings(Sema &S, NormalizedConstraint &N,
 static bool substituteParameterMappings(Sema &S, NormalizedConstraint &N,
                                         const ConceptSpecializationExpr *CSE) {
   MultiLevelTemplateArgumentList MLTAL = S.getTemplateInstantiationArgs(
-      CSE->getNamedConcept(), CSE->getNamedConcept()->getLexicalDeclContext(),
+      CSE->getNamedConcept(), CSE->getNamedConcept()->getDeclContext(),
       /*Final=*/false, CSE->getTemplateArguments(),
       /*RelativeToPrimary=*/true,
       /*ForConstraintInstantiation=*/true);

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -3982,18 +3982,6 @@ TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
       Result != TemplateDeductionResult::Success)
     return Result;
 
-  // C++ [temp.deduct.call]p10: [DR1391]
-  //   If deduction succeeds for all parameters that contain
-  //   template-parameters that participate in template argument deduction,
-  //   and all template arguments are explicitly specified, deduced, or
-  //   obtained from default template arguments, remaining parameters are then
-  //   compared with the corresponding arguments. For each remaining parameter
-  //   P with a type that was non-dependent before substitution of any
-  //   explicitly-specified template arguments, if the corresponding argument
-  //   A cannot be implicitly converted to P, deduction fails.
-  if (CheckNonDependent())
-    return TemplateDeductionResult::NonDependentConversionFailure;
-
   // Form the template argument list from the deduced template arguments.
   TemplateArgumentList *SugaredDeducedArgumentList =
       TemplateArgumentList::CreateCopy(Context, SugaredBuilder);
@@ -4008,6 +3996,38 @@ TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
     Owner = FunctionTemplate->getLexicalDeclContext();
   FunctionDecl *FD = FunctionTemplate->getTemplatedDecl();
 
+  // C++20 [temp.deduct.general]p5: (CWG2369)
+  // If the function template has associated constraints, those constraints are
+  // checked for satisfaction. If the constraints are not satisfied, type
+  // deduction fails.
+  // FIXME: We haven't implemented CWG2369 for lambdas yet, because we need
+  // to figure out how to instantiate lambda captures to the scope without
+  // first instantiating the lambda.
+  bool IsLambda = isLambdaCallOperator(FD) || isLambdaConversionOperator(FD);
+  if (!IsLambda && !IsIncomplete) {
+    if (CheckFunctionTemplateConstraints(
+            Info.getLocation(),
+            FunctionTemplate->getCanonicalDecl()->getTemplatedDecl(),
+            CanonicalBuilder, Info.AssociatedConstraintsSatisfaction))
+      return TemplateDeductionResult::MiscellaneousDeductionFailure;
+    if (!Info.AssociatedConstraintsSatisfaction.IsSatisfied) {
+      Info.reset(Info.takeSugared(),
+                 TemplateArgumentList::CreateCopy(Context, CanonicalBuilder));
+      return TemplateDeductionResult::ConstraintsNotSatisfied;
+    }
+  }
+  // C++ [temp.deduct.call]p10: [DR1391]
+  //   If deduction succeeds for all parameters that contain
+  //   template-parameters that participate in template argument deduction,
+  //   and all template arguments are explicitly specified, deduced, or
+  //   obtained from default template arguments, remaining parameters are then
+  //   compared with the corresponding arguments. For each remaining parameter
+  //   P with a type that was non-dependent before substitution of any
+  //   explicitly-specified template arguments, if the corresponding argument
+  //   A cannot be implicitly converted to P, deduction fails.
+  if (CheckNonDependent())
+    return TemplateDeductionResult::NonDependentConversionFailure;
+
   MultiLevelTemplateArgumentList SubstArgs(
       FunctionTemplate, CanonicalDeducedArgumentList->asArray(),
       /*Final=*/false);
@@ -4042,8 +4062,8 @@ TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
   //   ([temp.constr.decl]), those constraints are checked for satisfaction
   //   ([temp.constr.constr]). If the constraints are not satisfied, type
   //   deduction fails.
-  if (!IsIncomplete) {
-    if (CheckInstantiatedFunctionTemplateConstraints(
+  if (IsLambda && !IsIncomplete) {
+    if (CheckFunctionTemplateConstraints(
             Info.getLocation(), Specialization, CanonicalBuilder,
             Info.AssociatedConstraintsSatisfaction))
       return TemplateDeductionResult::MiscellaneousDeductionFailure;

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -891,10 +891,12 @@ Expr *buildIsDeducibleConstraint(Sema &SemaRef,
       Context.getTrivialTypeSourceInfo(
           Context.getDeducedTemplateSpecializationType(
               TemplateName(AliasTemplate), /*DeducedType=*/QualType(),
-              /*IsDependent=*/true)), // template specialization type whose
-                                      // arguments will be deduced.
+              /*IsDependent=*/true),
+          AliasTemplate->getLocation()), // template specialization type whose
+                                         // arguments will be deduced.
       Context.getTrivialTypeSourceInfo(
-          ReturnType), // type from which template arguments are deduced.
+          ReturnType, AliasTemplate->getLocation()), // type from which template
+                                                     // arguments are deduced.
   };
   return TypeTraitExpr::Create(
       Context, Context.getLogicalOperationType(), AliasTemplate->getLocation(),

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1365,6 +1365,20 @@ namespace {
     // Whether an incomplete substituion should be treated as an error.
     bool BailOutOnIncomplete;
 
+  private:
+    bool isSubstitutingConstraints() const {
+      return llvm::any_of(
+          llvm::reverse(SemaRef.CodeSynthesisContexts), [](auto &Context) {
+            return Context.Kind ==
+                   Sema::CodeSynthesisContext::ConstraintSubstitution;
+          });
+    }
+
+    // CWG2770: Function parameters should be instantiated when they are
+    // needed by a satisfaction check of an atomic constraint or
+    // (recursively) by another function parameter.
+    bool maybeInstantiateFunctionParameterToScope(ParmVarDecl *OldParm);
+
   public:
     typedef TreeTransform<TemplateInstantiator> inherited;
 
@@ -1378,9 +1392,7 @@ namespace {
     void setEvaluateConstraints(bool B) {
       EvaluateConstraints = B;
     }
-    bool getEvaluateConstraints() {
-      return EvaluateConstraints;
-    }
+    bool getEvaluateConstraints() const { return EvaluateConstraints; }
 
     /// Determine whether the given type \p T has already been
     /// transformed.
@@ -1421,12 +1433,19 @@ namespace {
                                  ArrayRef<UnexpandedParameterPack> Unexpanded,
                                  bool &ShouldExpand, bool &RetainExpansion,
                                  std::optional<unsigned> &NumExpansions) {
-      return getSema().CheckParameterPacksForExpansion(EllipsisLoc,
-                                                       PatternRange, Unexpanded,
-                                                       TemplateArgs,
-                                                       ShouldExpand,
-                                                       RetainExpansion,
-                                                       NumExpansions);
+      if (SemaRef.CurrentInstantiationScope && isSubstitutingConstraints()) {
+        for (UnexpandedParameterPack ParmPack : Unexpanded) {
+          NamedDecl *VD = ParmPack.first.dyn_cast<NamedDecl *>();
+          if (!isa_and_present<ParmVarDecl>(VD))
+            continue;
+          if (maybeInstantiateFunctionParameterToScope(cast<ParmVarDecl>(VD)))
+            return true;
+        }
+      }
+
+      return getSema().CheckParameterPacksForExpansion(
+          EllipsisLoc, PatternRange, Unexpanded, TemplateArgs, ShouldExpand,
+          RetainExpansion, NumExpansions);
     }
 
     void ExpandingFunctionParameterPack(ParmVarDecl *Pack) {
@@ -1916,9 +1935,33 @@ Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
     // template parameter.
   }
 
+  if (SemaRef.CurrentInstantiationScope) {
+    if (isSubstitutingConstraints() && isa<ParmVarDecl>(D) &&
+        maybeInstantiateFunctionParameterToScope(cast<ParmVarDecl>(D)))
+      return nullptr;
+  }
+
   return SemaRef.FindInstantiatedDecl(Loc, cast<NamedDecl>(D), TemplateArgs);
 }
 
+bool TemplateInstantiator::maybeInstantiateFunctionParameterToScope(
+    ParmVarDecl *OldParm) {
+  if (SemaRef.CurrentInstantiationScope->findInstantiationUnsafe(OldParm))
+    return false;
+  // The current context might have been changed in the process of transforming
+  // lambda expression. So resume it before we substitute into the parameter.
+  Sema::ContextRAII Context(SemaRef, OldParm->getDeclContext());
+
+  SmallVector<QualType> PTypes;
+  Sema::ExtParameterInfoBuilder TInfoBuilder;
+
+  return inherited::TransformFunctionTypeParams(
+      /*Loc=*/SourceLocation(), /*Params=*/OldParm, /*ParamTypes=*/nullptr,
+      /*ParamInfos=*/nullptr, /*PTypes=*/PTypes, /*PVars=*/nullptr,
+      TInfoBuilder, /*LastParamTransformed=*/nullptr,
+      /*IgnoreParameterIndex=*/true);
+}
+
 Decl *TemplateInstantiator::TransformDefinition(SourceLocation Loc, Decl *D) {
   Decl *Inst = getSema().SubstDecl(D, getSema().CurContext, TemplateArgs);
   if (!Inst)
@@ -4542,9 +4585,8 @@ static const Decl *getCanonicalParmVarDecl(const Decl *D) {
   return D;
 }
 
-
 llvm::PointerUnion<Decl *, LocalInstantiationScope::DeclArgumentPack *> *
-LocalInstantiationScope::findInstantiationOf(const Decl *D) {
+LocalInstantiationScope::findInstantiationUnsafe(const Decl *D) {
   D = getCanonicalParmVarDecl(D);
   for (LocalInstantiationScope *Current = this; Current;
        Current = Current->Outer) {
@@ -4569,6 +4611,14 @@ LocalInstantiationScope::findInstantiationOf(const Decl *D) {
       break;
   }
 
+  return nullptr;
+}
+
+llvm::PointerUnion<Decl *, LocalInstantiationScope::DeclArgumentPack *> *
+LocalInstantiationScope::findInstantiationOf(const Decl *D) {
+  auto *Result = findInstantiationUnsafe(D);
+  if (Result)
+    return Result;
   // If we're performing a partial substitution during template argument
   // deduction, we may not have values for template parameters yet.
   if (isa<NonTypeTemplateParmDecl>(D) || isa<TemplateTypeParmDecl>(D) ||

clang/lib/Sema/TreeTransform.h

@@ -713,7 +713,7 @@ class TreeTransform {
   /// variables vector are acceptable.
   ///
   /// LastParamTransformed, if non-null, will be set to the index of the last
-  /// parameter on which transfromation was started. In the event of an error,
+  /// parameter on which transformation was started. In the event of an error,
   /// this will contain the parameter which failed to instantiate.
   ///
   /// Return true on error.
@@ -722,7 +722,8 @@ class TreeTransform {
       const QualType *ParamTypes,
       const FunctionProtoType::ExtParameterInfo *ParamInfos,
       SmallVectorImpl<QualType> &PTypes, SmallVectorImpl<ParmVarDecl *> *PVars,
-      Sema::ExtParameterInfoBuilder &PInfos, unsigned *LastParamTransformed);
+      Sema::ExtParameterInfoBuilder &PInfos, unsigned *LastParamTransformed,
+      bool IgnoreParameterIndex = false);
 
   bool TransformFunctionTypeParams(
       SourceLocation Loc, ArrayRef<ParmVarDecl *> Params,
@@ -5997,16 +5998,16 @@ bool TreeTransform<Derived>::TransformFunctionTypeParams(
     const FunctionProtoType::ExtParameterInfo *ParamInfos,
     SmallVectorImpl<QualType> &OutParamTypes,
     SmallVectorImpl<ParmVarDecl *> *PVars,
-    Sema::ExtParameterInfoBuilder &PInfos,
-    unsigned *LastParamTransformed) {
+    Sema::ExtParameterInfoBuilder &PInfos, unsigned *LastParamTransformed,
+    bool IgnoreParameterIndex) {
   int indexAdjustment = 0;
 
   unsigned NumParams = Params.size();
   for (unsigned i = 0; i != NumParams; ++i) {
     if (LastParamTransformed)
       *LastParamTransformed = i;
     if (ParmVarDecl *OldParm = Params[i]) {
-      assert(OldParm->getFunctionScopeIndex() == i);
+      assert(IgnoreParameterIndex || OldParm->getFunctionScopeIndex() == i);
 
       std::optional<unsigned> NumExpansions;
       ParmVarDecl *NewParm = nullptr;

clang/test/CXX/drs/cwg23xx.cpp

@@ -528,3 +528,35 @@ namespace cwg2397 { // cwg2397: 17
 } // namespace cwg2397
 
 #endif
+
+#if __cplusplus >= 202002L
+
+namespace cwg2369 { // cwg2369: partial
+
+template <class T> struct Z {
+  typedef typename T::x xx;
+};
+
+template <class T>
+concept C = requires { typename T::A; };
+template <C T> typename Z<T>::xx f(void *, T); // #1
+template <class T> void f(int, T);             // #2
+
+struct A {
+} a;
+
+struct ZZ {
+  template <class T, class = typename Z<T>::xx> operator T *();
+  operator int();
+};
+
+void foo() {
+  ZZ zz;
+  f(1, a); // OK, deduction fails for #1 because there is no conversion from int
+           // to void*
+  f(zz, 42); // OK, deduction fails for #1 because C<int> is not satisfied
+}
+
+} // namespace cwg2369
+
+#endif