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

[Krystian Stasiowski via cfe-commits]

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@@ -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 {
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sdkrystian wrote:

The FIXME wasn't fixed, it's just broken in a different way that's consistent with other "current instantiation broken-ness" related things. I'm going to submit a patch (probably this week) which diagnoses cases where the current instantiation is used in a context which requires a complete type.

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