[clang] Reapply "[Clang][Sema] Use the correct lookup context when building overloaded 'operator->' in the current instantiation (#104458)" (PR #109422)

[Erich Keane via cfe-commits]

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@@ -484,16 +484,19 @@ namespace N4 {
   template<typename T>
   struct A {
     void not_instantiated(A a, A<T> b, T c) {
-      a->x;
-      b->x;
+      a->x; // expected-error {{member reference type 'A<T>' is not a pointer; did you mean to use '.'?}}
+      b->x; // expected-error {{member reference type 'A<T>' is not a pointer; did you mean to use '.'?}}
       c->x;
     }
 
     void instantiated(A a, A<T> b, T c) {
-      a->x; // expected-error {{member reference type 'A<int>' is not a pointer; did you mean to use '.'?}}
-            // expected-error at -1 {{no member named 'x' in 'N4::A<int>'}}
-      b->x; // expected-error {{member reference type 'A<int>' is not a pointer; did you mean to use '.'?}}
-            // expected-error at -1 {{no member named 'x' in 'N4::A<int>'}}
+      // FIXME: We should only emit a single diagnostic suggesting to use '.'!
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erichkeane wrote:

This is unfortunate, it looks like we're having this problem diagnosing in both phases here, and can get worse with partial instantiations.  Typically we'd want to solve this by replacing the expression with a RecoveryExpr or something to suppress it in the instantiated version, can you look into something like that?

https://github.com/llvm/llvm-project/pull/109422