[Clang][Sema] Retain the expanding index for unevaluated type constraints

clang/docs/ReleaseNotes.rst

@@ -407,6 +407,8 @@ Bug Fixes to C++ Support
 - Clang now uses the correct set of template argument lists when comparing the constraints of
   out-of-line definitions and member templates explicitly specialized for a given implicit instantiation of
   a class template. (#GH102320)
+- Fixed an issue in constraint evaluation, where type constraints on the lambda expression
+  containing outer unexpanded parameters were not correctly expanded. (#GH101754)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -11252,6 +11252,7 @@ class Sema final : public SemaBase {
                             ConceptDecl *NamedConcept, NamedDecl *FoundDecl,
                             const TemplateArgumentListInfo *TemplateArgs,
                             TemplateTypeParmDecl *ConstrainedParameter,
+                            QualType ConstrainedType,
                             SourceLocation EllipsisLoc);
 
   bool AttachTypeConstraint(AutoTypeLoc TL,

clang/lib/Sema/SemaTemplate.cpp

@@ -1134,7 +1134,8 @@ bool Sema::BuildTypeConstraint(const CXXScopeSpec &SS,
       SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
       ConceptName, CD, /*FoundDecl=*/USD ? cast<NamedDecl>(USD) : CD,
       TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
-      ConstrainedParameter, EllipsisLoc);
+      ConstrainedParameter, Context.getTypeDeclType(ConstrainedParameter),
+      EllipsisLoc);
 }
 
 template <typename ArgumentLocAppender>
@@ -1191,6 +1192,7 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
                                 ConceptDecl *NamedConcept, NamedDecl *FoundDecl,
                                 const TemplateArgumentListInfo *TemplateArgs,
                                 TemplateTypeParmDecl *ConstrainedParameter,
+                                QualType ConstrainedType,
                                 SourceLocation EllipsisLoc) {
   // C++2a [temp.param]p4:
   //     [...] If Q is of the form C<A1, ..., An>, then let E' be
@@ -1199,7 +1201,7 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
     TemplateArgs ? ASTTemplateArgumentListInfo::Create(Context,
                                                        *TemplateArgs) : nullptr;
 
-  QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
+  QualType ParamAsArgument = ConstrainedType;
 
   ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
       *this, NS, NameInfo, NamedConcept, FoundDecl,

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1646,6 +1646,21 @@ namespace {
                                            SubstTemplateTypeParmPackTypeLoc TL,
                                            bool SuppressObjCLifetime);
 
+    QualType
+    TransformSubstTemplateTypeParmType(TypeLocBuilder &TLB,
+                                       SubstTemplateTypeParmTypeLoc TL) {
+      if (SemaRef.CodeSynthesisContexts.back().Kind !=
+          Sema::CodeSynthesisContext::ConstraintSubstitution)
+        return inherited::TransformSubstTemplateTypeParmType(TLB, TL);
+
+      auto PackIndex = TL.getTypePtr()->getPackIndex();
+      std::optional<Sema::ArgumentPackSubstitutionIndexRAII> SubstIndex;
+      if (SemaRef.ArgumentPackSubstitutionIndex == -1 && PackIndex)
+        SubstIndex.emplace(SemaRef, *PackIndex);
+
+      return inherited::TransformSubstTemplateTypeParmType(TLB, TL);
+    }
+
     CXXRecordDecl::LambdaDependencyKind
     ComputeLambdaDependency(LambdaScopeInfo *LSI) {
       if (auto TypeAlias = getEnclosingTypeAliasTemplateDecl(getSema());
@@ -3056,6 +3071,58 @@ namespace {
 
 } // namespace
 
+namespace {
+
+struct ExpandPackedTypeConstraints
+    : TreeTransform<ExpandPackedTypeConstraints> {
+
+  using inherited = TreeTransform<ExpandPackedTypeConstraints>;
+
+  ExpandPackedTypeConstraints(Sema &SemaRef) : inherited(SemaRef) {}
+
+  using inherited::TransformTemplateTypeParmType;
+
+  QualType TransformTemplateTypeParmType(TypeLocBuilder &TLB,
+                                         TemplateTypeParmTypeLoc TL, bool) {
+    const TemplateTypeParmType *T = TL.getTypePtr();
+    if (!T->isParameterPack()) {
+      TemplateTypeParmTypeLoc NewTL =
+          TLB.push<TemplateTypeParmTypeLoc>(TL.getType());
+      NewTL.setNameLoc(TL.getNameLoc());
+      return TL.getType();
+    }
+
+    assert(SemaRef.ArgumentPackSubstitutionIndex != -1);
+
+    QualType Result = SemaRef.Context.getSubstTemplateTypeParmType(
+        TL.getType(), T->getDecl(), T->getIndex(),
+        SemaRef.ArgumentPackSubstitutionIndex);
+    SubstTemplateTypeParmTypeLoc NewTL =
+        TLB.push<SubstTemplateTypeParmTypeLoc>(Result);
+    NewTL.setNameLoc(TL.getNameLoc());
+    return Result;
+  }
+
+  QualType TransformSubstTemplateTypeParmType(TypeLocBuilder &TLB,
+                                              SubstTemplateTypeParmTypeLoc TL) {
+    const SubstTemplateTypeParmType *T = TL.getTypePtr();
+    if (T->getPackIndex()) {
+      SubstTemplateTypeParmTypeLoc TypeLoc =
+          TLB.push<SubstTemplateTypeParmTypeLoc>(TL.getType());
+      TypeLoc.setNameLoc(TL.getNameLoc());
+      return TypeLoc.getType();
+    }
+    return inherited::TransformSubstTemplateTypeParmType(TLB, TL);
+  }
+
+  bool SubstTemplateArguments(ArrayRef<TemplateArgumentLoc> Args,
+                              TemplateArgumentListInfo &Out) {
+    return inherited::TransformTemplateArguments(Args.begin(), Args.end(), Out);
+  }
+};
+
+} // namespace
+
 bool Sema::SubstTypeConstraint(
     TemplateTypeParmDecl *Inst, const TypeConstraint *TC,
     const MultiLevelTemplateArgumentList &TemplateArgs,
@@ -3064,9 +3131,62 @@ bool Sema::SubstTypeConstraint(
       TC->getTemplateArgsAsWritten();
 
   if (!EvaluateConstraints) {
-      Inst->setTypeConstraint(TC->getConceptReference(),
-                              TC->getImmediatelyDeclaredConstraint());
-      return false;
+    bool ShouldExpandExplicitTemplateArgs =
+        TemplArgInfo && ArgumentPackSubstitutionIndex != -1 &&
+        llvm::any_of(TemplArgInfo->arguments(), [](auto &Arg) {
+          return Arg.getArgument().containsUnexpandedParameterPack();
+        });
+
+    // We want to transform the packs into Subst* nodes for type constraints
+    // inside a pack expansion. For example,
+    //
+    //  template <class... Ts> void foo() {
+    //    bar([](C<Ts> auto value) {}...);
+    //  }
+    //
+    // As we expand Ts in the process of instantiating foo(), and retain
+    // the original template depths of Ts until the constraint evaluation, we
+    // would otherwise have no chance to expand Ts by the time of evaluating
+    // C<auto, Ts>.
+    //
+    // So we form a Subst* node for Ts along with a proper substitution index
+    // here, and substitute the node with a complete MLTAL later in evaluation.
+    if (ShouldExpandExplicitTemplateArgs) {
+      TemplateArgumentListInfo InstArgs;
+      InstArgs.setLAngleLoc(TemplArgInfo->LAngleLoc);
+      InstArgs.setRAngleLoc(TemplArgInfo->RAngleLoc);
+      if (ExpandPackedTypeConstraints(*this).SubstTemplateArguments(
+              TemplArgInfo->arguments(), InstArgs))
+        return true;
+
+      // The type of the original parameter.
+      auto *ConstraintExpr = TC->getImmediatelyDeclaredConstraint();
+      QualType ConstrainedType;
+
+      if (auto *FE = dyn_cast<CXXFoldExpr>(ConstraintExpr)) {
+        assert(FE->getLHS());
+        ConstraintExpr = FE->getLHS();
+      }
+      auto *CSE = cast<ConceptSpecializationExpr>(ConstraintExpr);
+      assert(!CSE->getTemplateArguments().empty() &&
+             "Empty template arguments?");
+      ConstrainedType = CSE->getTemplateArguments()[0].getAsType();
+      assert(!ConstrainedType.isNull() &&
+             "Failed to extract the original ConstrainedType?");
+
+      return AttachTypeConstraint(
+          TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
+          TC->getNamedConcept(),
+          /*FoundDecl=*/TC->getConceptReference()->getFoundDecl(), &InstArgs,
+          Inst, ConstrainedType,
+          Inst->isParameterPack()
+              ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
+                    ->getEllipsisLoc()
+              : SourceLocation());
+    }
+    Inst->setTypeConstraint(TC->getConceptReference(),
+                            TC->getImmediatelyDeclaredConstraint());
+    return false;
   }
 
   TemplateArgumentListInfo InstArgs;
@@ -3082,6 +3202,7 @@ bool Sema::SubstTypeConstraint(
       TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
       TC->getNamedConcept(),
       /*FoundDecl=*/TC->getConceptReference()->getFoundDecl(), &InstArgs, Inst,
+      Context.getTypeDeclType(Inst),
       Inst->isParameterPack()
           ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
                 ->getEllipsisLoc()

clang/lib/Sema/SemaType.cpp

@@ -3035,7 +3035,9 @@ InventTemplateParameter(TypeProcessingState &state, QualType T,
             AutoLoc.getNestedNameSpecifierLoc(), AutoLoc.getConceptNameInfo(),
             AutoLoc.getNamedConcept(), /*FoundDecl=*/AutoLoc.getFoundDecl(),
             AutoLoc.hasExplicitTemplateArgs() ? &TAL : nullptr,
-            InventedTemplateParam, D.getEllipsisLoc());
+            InventedTemplateParam,
+            S.Context.getTypeDeclType(InventedTemplateParam),
+            D.getEllipsisLoc());
       }
     } else {
       // The 'auto' appears in the decl-specifiers; we've not finished forming
@@ -3072,7 +3074,9 @@ InventTemplateParameter(TypeProcessingState &state, QualType T,
             /*FoundDecl=*/
             USD ? cast<NamedDecl>(USD) : CD,
             TemplateId->LAngleLoc.isValid() ? &TemplateArgsInfo : nullptr,
-            InventedTemplateParam, D.getEllipsisLoc());
+            InventedTemplateParam,
+            S.Context.getTypeDeclType(InventedTemplateParam),
+            D.getEllipsisLoc());
       }
     }
   }

clang/test/SemaCXX/fold_lambda_with_variadics.cpp

@@ -179,3 +179,57 @@ void foo() {
 }
 
 } // namespace GH99877
+
+namespace GH101754 {
+
+template <typename... Ts> struct Overloaded : Ts... {
+  using Ts::operator()...;
+};
+
+template <typename... Ts> Overloaded(Ts...) -> Overloaded<Ts...>;
+
+template <class T, class U>
+concept same_as = __is_same(T, U);  // #same_as
+
+template <typename... Ts> constexpr auto foo() {
+  return Overloaded{[](same_as<Ts> auto value) { return value; }...}; // #lambda
+}
+
+static_assert(foo<int, double>()(123) == 123);
+static_assert(foo<int, double>()(2.718) == 2.718);
+
+static_assert(foo<int, double>()('c'));
+// expected-error@-1 {{no matching function}}
+
+// expected-note@#lambda {{constraints not satisfied}}
+// expected-note@#lambda {{'same_as<char, int>' evaluated to false}}
+// expected-note@#same_as {{evaluated to false}}
+
+// expected-note@#lambda {{constraints not satisfied}}
+// expected-note@#lambda {{'same_as<char, double>' evaluated to false}}
+// expected-note@#same_as {{evaluated to false}}
+
+template <class T, class U, class V>
+concept C = same_as<T, U> && same_as<U, V>; // #C
+
+template <typename... Ts> constexpr auto bar() {
+  return ([]<class Up>() {
+    return Overloaded{[](C<Up, Ts> auto value) { // #bar
+      return value;
+    }...};
+  }.template operator()<Ts>(), ...);
+}
+static_assert(bar<int, float>()(3.14f)); // OK, bar() returns the last overload i.e. <float>.
+
+static_assert(bar<int, float>()(123));
+// expected-error@-1 {{no matching function}}
+// expected-note@#bar {{constraints not satisfied}}
+// expected-note@#bar {{'C<int, float, int>' evaluated to false}}
+// expected-note@#C {{evaluated to false}}
+
+// expected-note@#bar {{constraints not satisfied}}
+// expected-note@#bar {{'C<int, float, float>' evaluated to false}}
+// expected-note@#C {{evaluated to false}}
+// expected-note@#same_as 2{{evaluated to false}}
+
+} // namespace GH101754