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

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`.  

Reviewers: rsmith, majnemer, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25430

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283755 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaDeclCXX.cpp

@@ -9470,11 +9470,13 @@ bool Sema::CheckUsingDeclQualifier(SourceLocation UsingLoc,
         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;
     }

test/SemaCXX/using-decl-templates.cpp

@@ -92,3 +92,12 @@ namespace aliastemplateinst {
 
   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