Make `__is_base_of` more friendly with unions

[mclow.lists]

Summary

Unions are never base classes, and never have base classes.
It doesn't matter if they are complete or not. See http://llvm.org/PR41843

[mclow.lists]

Note: There are tabs in clang/test/SemaCXX/type-traits.cpp that I didn't remove because it would have cluttered up the diff.
I can de-tabify the file when it is committed if people want.

[ldionne]

Shouldn't we also add unit tests for PR41843 in libc++?

[mclow.lists]

In D61858#1500270, @ldionne wrote:

Shouldn't we also add unit tests for PR41843 in libc++?

Yes. But I want to do that later.
After this has landed (and probably wait a week for all the bots that are running trunk to update).

[ldionne]

I'm not a frontend expert, but this looks reasonable to me.

[rsmith]

Yeah, this seems to match the library wording. (I think it's short-sighted and over-fitting -- this is baking into the library specification that the language happens to not allow unions to have base classes today -- but this isn't the venue for that discussion.)

[mclow.lists]

Committed as revision 360614

clang/lib/Sema/SemaExprCXX.cpp

     assert(Self.Context.hasSameUnqualifiedType(LhsT, RhsT)
              == (lhsRecord == rhsRecord));
 
+    // Unions are never base classes, and never have base classes.
+    // It doesn't matter if they are complete or not. See PR#41843
+    if (lhsRecord && lhsRecord->getDecl()->isUnion())
+      return false;
+    if (rhsRecord && rhsRecord->getDecl()->isUnion())
+      return false;
+
     if (lhsRecord == rhsRecord)
-      return !lhsRecord->getDecl()->isUnion();
+      return true;
 
     // C++0x [meta.rel]p2:
     //   If Base and Derived are class types and are different types

clang/test/SemaCXX/type-traits.cpp

 enum Enum { EV };
 struct POD { Enum e; int i; float f; NonPOD* p; };
 struct Empty {};
+struct IncompleteStruct;
 typedef Empty EmptyAr[10];
 typedef Empty EmptyArNB[];
 typedef Empty EmptyArMB[1][2];

   { int arr[F(__is_base_of(Base, NonderivedTemp<int>))]; }
   { int arr[F(__is_base_of(Base, UndefinedTemp<int>))]; } // expected-error {{implicit instantiation of undefined template 'UndefinedTemp<int>'}}
 
+  { int arr[F(__is_base_of(IncompleteUnion, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(Union, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, Union))]; }
+  { int arr[F(__is_base_of(IncompleteStruct, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, IncompleteStruct))]; }
+  { int arr[F(__is_base_of(Empty, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, Empty))]; }
+  { int arr[F(__is_base_of(int, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, int))]; }
+  { int arr[F(__is_base_of(Empty, Union))]; }
+  { int arr[F(__is_base_of(Union, Empty))]; }
+  { int arr[F(__is_base_of(int, Empty))]; }
+  { int arr[F(__is_base_of(Union, int))]; }
+
   isBaseOfT<Base, Derived>();
   isBaseOfF<Derived, Base>();