[Sema] Prevent using member declaration diagnostic if the base class is invalid.

[EricWF]

Summary

Once a base class has been made invalid (by a static_assert for example) all using-member declarations in the derived classes will result in a "not a base class" diagnostic. This diagnostic is very misleading and should not be emitted.

This change is needed to help libc++ produce reasonable diagnostics in std::optional and std::variant.

[EricWF]

Simplify test case.

[aaron.ballman]

LGTM!

[EricWF]

Thanks @aaron.ballman!

lib/Sema/SemaDeclCXX.cpp

                                  cast<CXXRecordDecl>(NamedContext))) {
       if (CurContext == NamedContext) {
         Diag(NameLoc,
              diag::err_using_decl_nested_name_specifier_is_current_class)
           << SS.getRange();
         return true;
       }
 
-      Diag(SS.getRange().getBegin(),
-           diag::err_using_decl_nested_name_specifier_is_not_base_class)
-        << SS.getScopeRep()
-        << cast<CXXRecordDecl>(CurContext)
-        << SS.getRange();
+      if (!cast<CXXRecordDecl>(NamedContext)->isInvalidDecl()) {
+        Diag(SS.getRange().getBegin(),
+             diag::err_using_decl_nested_name_specifier_is_not_base_class)
+          << SS.getScopeRep()
+          << cast<CXXRecordDecl>(CurContext)
+          << SS.getRange();
+      }
       return true;
     }
 
     return false;
   }
 
   // C++03 [namespace.udecl]p4:
   //   A using-declaration used as a member-declaration shall refer

test/SemaCXX/using-decl-templates.cpp

 };
 
 namespace aliastemplateinst {
   template<typename T> struct A { };
   template<typename T> using APtr = A<T*>; // expected-note{{previous use is here}}
 
   template struct APtr<int>; // expected-error{{elaborated type refers to a type alias template}}
 }
+
+namespace DontDiagnoseInvalidTest {
+template <bool Value> struct Base {
+  static_assert(Value, ""); // expected-error {{static_assert failed}}
+};
+struct Derived : Base<false> { // expected-note {{requested here}}
+  using Base<false>::Base; // OK. Don't diagnose that 'Base' isn't a base class of Derived.
+};
+} // namespace DontDiagnoseInvalidTest