[Clang][Concepts] Correct the CurContext for friend declarations (#106890)

`FindInstantiatedDecl()` relies on the `CurContext` to find the
corresponding class template instantiation for a class template
declaration.

Previously, we pushed the semantic declaration context for constraint
comparison, which is incorrect for constraints on friend declarations.
In issue #78101, the semantic context of the friend is the TU, so we
missed the implicit template specialization `Template<void, 4>` when
looking for the instantiation of the primary template `Template` at the
time of checking the member instantiation; instead, we mistakenly picked
up the explicit specialization `Template<float, 5>`, hence the error.

As a bonus, this also fixes a crash when diagnosing constraints. The
DeclarationName is not necessarily an identifier, so it's incorrect to
call `getName()` on e.g. overloaded operators. Since the
DiagnosticBuilder has correctly handled Decl printing, we don't need to
find the printable name ourselves.

Fixes https://github.com/llvm/llvm-project/issues/78101

1 files changed, 8 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 86d6f30..6a1b325 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1012,7 +1012,14 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   // possible that e.g. constraints involving C<Class<T>> and C<Class> are
   // perceived identical.
   std::optional<Sema::ContextRAII> ContextScope;
-  if (auto *RD = dyn_cast<CXXRecordDecl>(DeclInfo.getDeclContext())) {
+  const DeclContext *DC = [&] {
+    if (!DeclInfo.getDecl())
+      return DeclInfo.getDeclContext();
+    return DeclInfo.getDecl()->getFriendObjectKind()
+               ? DeclInfo.getLexicalDeclContext()
+               : DeclInfo.getDeclContext();
+  }();
+  if (auto *RD = dyn_cast<CXXRecordDecl>(DC)) {
     ThisScope.emplace(S, const_cast<CXXRecordDecl *>(RD), Qualifiers());
     ContextScope.emplace(S, const_cast<DeclContext *>(cast<DeclContext>(RD)),
                          /*NewThisContext=*/false);