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

[Erich Keane 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 {
----------------
erichkeane wrote:

Did the FIXME ever get a core issue?  Perhaps we need to run that down a bit?  @Endilll or @shafik , any chance the 'FIXME" that we just 'fixed' looks familiar?

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