[clang] [Clang][Sema] Fix lookup of dependent operator= named by using-declaration (PR #91503)

[Vlad Serebrennikov via cfe-commits]

================
@@ -137,17 +137,16 @@ namespace InhCtor {
   int n = b.T(); // expected-error {{'T' is a protected member of 'InhCtor::A'}}
                  // expected-note at -15 {{declared protected here}}
 
-  // FIXME: EDG and GCC reject this too, but it's not clear why it would be
-  // ill-formed.
   template<typename T>
   struct S : T {
-    struct U : S { // expected-note 6{{candidate}}
+    // FIXME: S is incomplete here and we should diagnose this!
+    struct U : S {
----------------
Endilll wrote:

This rings a bell. I've put something similar in our DR tests, probably in a PR about complete-class context. I'm running out of time today, but I'll look into this tomorrow if Shafik doesn't beat me to it. 

https://github.com/llvm/llvm-project/pull/91503