[Clang] Profile singly-resolved UnresolvedLookupExpr with the declaration (#140029)

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.

Fixes #139476

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -734,6 +734,7 @@ Bug Fixes to C++ Support
 - Fixed the handling of pack indexing types in the constraints of a member function redeclaration. (#GH138255)
 - Clang now correctly parses arbitrary order of ``[[]]``, ``__attribute__`` and ``alignas`` attributes for declarations (#GH133107)
 - Fixed a crash when forming an invalid function type in a dependent context. (#GH138657) (#GH115725) (#GH68852)
+- Fixed a function declaration mismatch that caused inconsistencies between concepts and variable template declarations. (#GH139476)
 - Clang no longer segfaults when there is a configuration mismatch between modules and their users (http://crbug.com/400353616).
 - Fix an incorrect deduction when calling an explicit object member function template through an overload set address.
 

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

@@ -853,3 +853,18 @@ template <int... Ts>
 requires C<Ts...[0]>
 auto TplClass<int>::buggy() -> void {}
 }
+
+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() {}
+
+}