[concepts] Preserve the FoundDecl of ConceptReference properly (#85032)

The `ConceptReference`'s `FoundDecl` claims it "can differ from
`NamedConcept` when, for example, the concept was found through a
`UsingShadowDecl`", but such the contract was not previously respected.

Fixes #82628

clang-tools-extra/clangd/unittests/SelectionTests.cpp

@@ -10,6 +10,7 @@
 #include "SourceCode.h"
 #include "TestTU.h"
 #include "support/TestTracer.h"
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Decl.h"
 #include "llvm/Support/Casting.h"
 #include "gmock/gmock.h"
@@ -893,6 +894,33 @@ TEST(SelectionTest, DeclContextLambda) {
   EXPECT_TRUE(ST.commonAncestor()->getDeclContext().isFunctionOrMethod());
 }
 
+TEST(SelectionTest, UsingConcepts) {
+  llvm::Annotations Test(R"cpp(
+namespace ns {
+template <typename T>
+concept Foo = true;
+}
+
+using ns::Foo;
+
+template <Fo^o... T, Fo^o auto U>
+auto Func(Fo^o auto V) -> Fo^o decltype(auto) {
+  Fo^o auto W = V;
+  return W;
+}
+)cpp");
+  auto TU = TestTU::withCode(Test.code());
+  TU.ExtraArgs.emplace_back("-std=c++2c");
+  auto AST = TU.build();
+  for (auto Point : Test.points()) {
+    auto ST = SelectionTree::createRight(AST.getASTContext(), AST.getTokens(),
+                                         Point, Point);
+    auto *C = ST.commonAncestor()->ASTNode.get<ConceptReference>();
+    EXPECT_TRUE(C && C->getFoundDecl() &&
+                C->getFoundDecl()->getKind() == Decl::UsingShadow);
+  }
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang

clang-tools-extra/include-cleaner/unittests/WalkASTTest.cpp

@@ -548,9 +548,7 @@ TEST(WalkAST, Concepts) {
   testWalk(Concept, "template<typename T> requires ^Foo<T> void func() {}");
   testWalk(Concept, "template<typename T> void func() requires ^Foo<T> {}");
   testWalk(Concept, "void func(^Foo auto x) {}");
-  // FIXME: Foo should be explicitly referenced.
-  testWalk("template<typename T> concept Foo = true;",
-           "void func() { ^Foo auto x = 1; }");
+  testWalk(Concept, "void func() { ^Foo auto x = 1; }");
 }
 
 TEST(WalkAST, FriendDecl) {

clang/docs/ReleaseNotes.rst

@@ -379,6 +379,7 @@ Bug Fixes to C++ Support
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
+- Clang now properly preserves ``FoundDecls`` within a ``ConceptReference``. (#GH82628)
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -9215,7 +9215,7 @@ class Sema final {
 
   bool AttachTypeConstraint(NestedNameSpecifierLoc NS,
                             DeclarationNameInfo NameInfo,
-                            ConceptDecl *NamedConcept,
+                            ConceptDecl *NamedConcept, NamedDecl *FoundDecl,
                             const TemplateArgumentListInfo *TemplateArgs,
                             TemplateTypeParmDecl *ConstrainedParameter,
                             SourceLocation EllipsisLoc);

clang/lib/Sema/SemaDecl.cpp

@@ -1192,9 +1192,13 @@ Sema::NameClassification Sema::ClassifyName(Scope *S, CXXScopeSpec &SS,
     return ParsedType::make(T);
   }
 
-  if (isa<ConceptDecl>(FirstDecl))
+  if (isa<ConceptDecl>(FirstDecl)) {
+    // We want to preserve the UsingShadowDecl for concepts.
+    if (auto *USD = dyn_cast<UsingShadowDecl>(Result.getRepresentativeDecl()))
+      return NameClassification::Concept(TemplateName(USD));
     return NameClassification::Concept(
         TemplateName(cast<TemplateDecl>(FirstDecl)));
+  }
 
   if (auto *EmptyD = dyn_cast<UnresolvedUsingIfExistsDecl>(FirstDecl)) {
     (void)DiagnoseUseOfDecl(EmptyD, NameLoc);

clang/lib/Sema/SemaTemplate.cpp

@@ -1156,6 +1156,7 @@ bool Sema::BuildTypeConstraint(const CXXScopeSpec &SS,
 
   TemplateName TN = TypeConstr->Template.get();
   ConceptDecl *CD = cast<ConceptDecl>(TN.getAsTemplateDecl());
+  UsingShadowDecl *USD = TN.getAsUsingShadowDecl();
 
   DeclarationNameInfo ConceptName(DeclarationName(TypeConstr->Name),
                                   TypeConstr->TemplateNameLoc);
@@ -1174,15 +1175,15 @@ bool Sema::BuildTypeConstraint(const CXXScopeSpec &SS,
   }
   return AttachTypeConstraint(
       SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
-      ConceptName, CD,
+      ConceptName, CD, /*FoundDecl=*/USD ? cast<NamedDecl>(USD) : CD,
       TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
       ConstrainedParameter, EllipsisLoc);
 }
 
-template<typename ArgumentLocAppender>
+template <typename ArgumentLocAppender>
 static ExprResult formImmediatelyDeclaredConstraint(
     Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
-    ConceptDecl *NamedConcept, SourceLocation LAngleLoc,
+    ConceptDecl *NamedConcept, NamedDecl *FoundDecl, SourceLocation LAngleLoc,
     SourceLocation RAngleLoc, QualType ConstrainedType,
     SourceLocation ParamNameLoc, ArgumentLocAppender Appender,
     SourceLocation EllipsisLoc) {
@@ -1203,7 +1204,8 @@ static ExprResult formImmediatelyDeclaredConstraint(
   SS.Adopt(NS);
   ExprResult ImmediatelyDeclaredConstraint = S.CheckConceptTemplateId(
       SS, /*TemplateKWLoc=*/SourceLocation(), NameInfo,
-      /*FoundDecl=*/NamedConcept, NamedConcept, &ConstraintArgs);
+      /*FoundDecl=*/FoundDecl ? FoundDecl : NamedConcept, NamedConcept,
+      &ConstraintArgs);
   if (ImmediatelyDeclaredConstraint.isInvalid() || !EllipsisLoc.isValid())
     return ImmediatelyDeclaredConstraint;
 
@@ -1233,7 +1235,7 @@ static ExprResult formImmediatelyDeclaredConstraint(
 /// of arguments for the named concept).
 bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
                                 DeclarationNameInfo NameInfo,
-                                ConceptDecl *NamedConcept,
+                                ConceptDecl *NamedConcept, NamedDecl *FoundDecl,
                                 const TemplateArgumentListInfo *TemplateArgs,
                                 TemplateTypeParmDecl *ConstrainedParameter,
                                 SourceLocation EllipsisLoc) {
@@ -1246,24 +1248,24 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
 
   QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
 
-  ExprResult ImmediatelyDeclaredConstraint =
-      formImmediatelyDeclaredConstraint(
-          *this, NS, NameInfo, NamedConcept,
-          TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
-          TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
-          ParamAsArgument, ConstrainedParameter->getLocation(),
-          [&] (TemplateArgumentListInfo &ConstraintArgs) {
-            if (TemplateArgs)
-              for (const auto &ArgLoc : TemplateArgs->arguments())
-                ConstraintArgs.addArgument(ArgLoc);
-          }, EllipsisLoc);
+  ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
+      *this, NS, NameInfo, NamedConcept, FoundDecl,
+      TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
+      TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
+      ParamAsArgument, ConstrainedParameter->getLocation(),
+      [&](TemplateArgumentListInfo &ConstraintArgs) {
+        if (TemplateArgs)
+          for (const auto &ArgLoc : TemplateArgs->arguments())
+            ConstraintArgs.addArgument(ArgLoc);
+      },
+      EllipsisLoc);
   if (ImmediatelyDeclaredConstraint.isInvalid())
     return true;
 
   auto *CL = ConceptReference::Create(Context, /*NNS=*/NS,
                                       /*TemplateKWLoc=*/SourceLocation{},
                                       /*ConceptNameInfo=*/NameInfo,
-                                      /*FoundDecl=*/NamedConcept,
+                                      /*FoundDecl=*/FoundDecl,
                                       /*NamedConcept=*/NamedConcept,
                                       /*ArgsWritten=*/ArgsAsWritten);
   ConstrainedParameter->setTypeConstraint(CL,
@@ -1293,8 +1295,9 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
     return true;
   ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
       *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
-      TL.getNamedConcept(), TL.getLAngleLoc(), TL.getRAngleLoc(),
-      BuildDecltypeType(Ref), OrigConstrainedParm->getLocation(),
+      TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(), TL.getLAngleLoc(),
+      TL.getRAngleLoc(), BuildDecltypeType(Ref),
+      OrigConstrainedParm->getLocation(),
       [&](TemplateArgumentListInfo &ConstraintArgs) {
         for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
           ConstraintArgs.addArgument(TL.getArgLoc(I));
@@ -5415,7 +5418,7 @@ ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
 
   if (R.getAsSingle<ConceptDecl>()) {
     return CheckConceptTemplateId(SS, TemplateKWLoc, R.getLookupNameInfo(),
-                                  R.getFoundDecl(),
+                                  R.getRepresentativeDecl(),
                                   R.getAsSingle<ConceptDecl>(), TemplateArgs);
   }
 

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -2970,7 +2970,8 @@ bool Sema::SubstTypeConstraint(
   }
   return AttachTypeConstraint(
       TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
-      TC->getNamedConcept(), &InstArgs, Inst,
+      TC->getNamedConcept(),
+      /*FoundDecl=*/TC->getConceptReference()->getFoundDecl(), &InstArgs, Inst,
       Inst->isParameterPack()
           ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
                 ->getEllipsisLoc()

clang/lib/Sema/SemaType.cpp

@@ -3499,7 +3499,7 @@ InventTemplateParameter(TypeProcessingState &state, QualType T,
       if (!Invalid) {
         S.AttachTypeConstraint(
             AutoLoc.getNestedNameSpecifierLoc(), AutoLoc.getConceptNameInfo(),
-            AutoLoc.getNamedConcept(),
+            AutoLoc.getNamedConcept(), /*FoundDecl=*/AutoLoc.getFoundDecl(),
             AutoLoc.hasExplicitTemplateArgs() ? &TAL : nullptr,
             InventedTemplateParam, D.getEllipsisLoc());
       }
@@ -3525,11 +3525,17 @@ InventTemplateParameter(TypeProcessingState &state, QualType T,
         }
       }
       if (!Invalid) {
+        UsingShadowDecl *USD =
+            TemplateId->Template.get().getAsUsingShadowDecl();
+        auto *CD =
+            cast<ConceptDecl>(TemplateId->Template.get().getAsTemplateDecl());
         S.AttachTypeConstraint(
             D.getDeclSpec().getTypeSpecScope().getWithLocInContext(S.Context),
             DeclarationNameInfo(DeclarationName(TemplateId->Name),
                                 TemplateId->TemplateNameLoc),
-            cast<ConceptDecl>(TemplateId->Template.get().getAsTemplateDecl()),
+            CD,
+            /*FoundDecl=*/
+            USD ? cast<NamedDecl>(USD) : CD,
             TemplateId->LAngleLoc.isValid() ? &TemplateArgsInfo : nullptr,
             InventedTemplateParam, D.getEllipsisLoc());
       }
@@ -6423,9 +6429,12 @@ namespace {
       DeclarationNameInfo DNI = DeclarationNameInfo(
           TL.getTypePtr()->getTypeConstraintConcept()->getDeclName(),
           TemplateId->TemplateNameLoc);
+      auto TN = TemplateId->Template.get();
       auto *CR = ConceptReference::Create(
           Context, NNS, TemplateId->TemplateKWLoc, DNI,
-          /*FoundDecl=*/nullptr,
+          /*FoundDecl=*/TN.getKind() == TemplateName::NameKind::UsingTemplate
+              ? cast<NamedDecl>(TN.getAsUsingShadowDecl())
+              : cast_if_present<NamedDecl>(TN.getAsTemplateDecl()),
           /*NamedDecl=*/TL.getTypePtr()->getTypeConstraintConcept(),
           ASTTemplateArgumentListInfo::Create(Context, TemplateArgsInfo));
       TL.setConceptReference(CR);

clang/test/AST/ast-dump-concepts.cpp

@@ -57,3 +57,53 @@ struct Foo {
   template <variadic_concept<int>... Ts>
   Foo();
 };
+
+namespace GH82628 {
+namespace ns {
+
+template <typename T>
+concept C = true;
+
+} // namespace ns
+
+using ns::C;
+
+// CHECK:     ConceptDecl {{.*}} Foo
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.*}} typename depth 0 index 0 T
+// CHECK-NEXT: `-ConceptSpecializationExpr {{.*}} UsingShadow {{.*}} 'C'
+template <typename T>
+concept Foo = C<T>;
+
+// CHECK: TemplateTypeParmDecl {{.*}} Concept {{.*}} 'C' (UsingShadow {{.*}} 'C')
+template <C T>
+constexpr bool FooVar = false;
+
+// CHECK: ConceptSpecializationExpr {{.*}} UsingShadow {{.*}} 'C'
+template <typename T> requires C<T>
+constexpr bool FooVar2 = true;
+
+// CHECK: SimpleRequirement
+// CHECK-NEXT: `-ConceptSpecializationExpr {{.*}} UsingShadow {{.*}} 'C'
+template <typename T> requires requires (T) { C<T>; }
+constexpr bool FooVar3 = true;
+
+// CHECK: NonTypeTemplateParmDecl
+// CHECK-NEXT: `-ConceptSpecializationExpr {{.*}} UsingShadow {{.*}} 'C'
+template <C auto T>
+constexpr bool FooVar4 = bool(T());
+
+// CHECK: FunctionTemplateDecl
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.*}} Concept {{.*}} 'C' (UsingShadow {{.*}} 'C') depth 0 index 0 ... T
+// CHECK: NonTypeTemplateParmDecl {{.*}} depth 0 index 1 U
+// CHECK-NEXT: `-ConceptSpecializationExpr {{.*}} UsingShadow {{.*}} 'C'
+// CHECK: |-TemplateTypeParmDecl {{.*}} Concept {{.*}} 'C' (UsingShadow {{.*}} 'C') depth 0 index 2 V:auto
+
+template <C... T, C auto U>
+auto FooFunc(C auto V) -> C decltype(auto) {
+  // FIXME: TypeLocs inside of the function body cannot be dumped via -ast-dump for now.
+  // See clang-tools-extra/clangd/unittests/SelectionTests.cpp:SelectionTest.UsingConcepts for their checkings.
+  C auto W = V;
+  return W;
+}
+
+}