[Clang] Profile singly-resolved UnresolvedLookupExpr with the declaration

For a dependent variable template specialization, we don't build a
dependent Decl node or a DeclRefExpr to represent it. Instead, we
preserve the UnresolvedLookupExpr until instantiation.

However, this approach isn't ideal for constraint normalization. We
consider the qualifier during profiling, but since that's based on the
written code, it can introduce confusing differences, even when the
expressions resolve to the same declaration.

This change ensures that, if possible, we profile the resolved
declaration instead of its qualifier. For expressions that resolve
to more than one declarations, we still profile its qualifier, as
otherwise it would make us depend on the order of lookup results.

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -545,6 +545,7 @@ Bug Fixes to C++ Support
 - Clang no longer crashes when establishing subsumption between some constraint expressions. (#GH122581)
 - Clang now issues an error when placement new is used to modify a const-qualified variable
   in a ``constexpr`` function. (#GH131432)
+- Fixed a function declaration mismatch that caused inconsistencies between concepts and variable template declarations. (#GH139476)
 - Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
 
 Bug Fixes to AST Handling

clang/lib/AST/StmtProfile.cpp

@@ -2189,7 +2189,10 @@ StmtProfiler::VisitCXXPseudoDestructorExpr(const CXXPseudoDestructorExpr *S) {
 
 void StmtProfiler::VisitOverloadExpr(const OverloadExpr *S) {
   VisitExpr(S);
-  VisitNestedNameSpecifier(S->getQualifier());
+  if (S->getNumDecls() == 1)
+    VisitDecl(*S->decls_begin());
+  else
+    VisitNestedNameSpecifier(S->getQualifier());
   VisitName(S->getName(), /*TreatAsDecl*/ true);
   ID.AddBoolean(S->hasExplicitTemplateArgs());
   if (S->hasExplicitTemplateArgs())

clang/test/SemaTemplate/concepts-out-of-line-def.cpp

@@ -779,3 +779,18 @@ template <typename T>
 consteval void S::mfn() requires (bool(&fn)) {}
 
 }
+
+namespace GH139476 {
+
+namespace moo {
+  template <typename T>
+  constexpr bool baa = true;
+
+  template <typename T> requires baa<T>
+  void caw();
+}
+
+template <typename T> requires moo::baa<T>
+void moo::caw() {}
+
+}