Make __has_unique_object_representations reject empty union types.

[EricWF]

Summary

Clang incorrectly reports empty unions as having a unique object representation. However, this is not correct since sizeof(EmptyUnion) == 1 AKA it has 8 bits of padding. Therefore it should be treated the same as an empty struct and report false.

@erichkeane also suggested this fix should be merged into the 6.0 release branch, so the initial release of __has_unique_object_representations is as bug-free as possible.

[erichkeane]

Yep, LGTM, thanks!

cfe/trunk/lib/AST/ASTContext.cpp

static bool unionHasUniqueObjectRepresentations(const ASTContext &Context,
 
   for (const auto *Field : RD->fields()) {
     if (!Context.hasUniqueObjectRepresentations(Field->getType()))
       return false;
     CharUnits FieldSize = Context.getTypeSizeInChars(Field->getType());
     if (FieldSize != UnionSize)
       return false;
   }
-  return true;
+  return !RD->field_empty();
 }
 
 static bool isStructEmpty(QualType Ty) {
   const RecordDecl *RD = Ty->castAs<RecordType>()->getDecl();
 
   if (!RD->field_empty())
     return false;
 

cfe/trunk/test/SemaCXX/type-traits.cpp

 static_assert(has_unique_object_representations<LLEnumClass>::value, "Enums are integrals, so unique!");
 
 // because references aren't trivially copyable.
 static_assert(!has_unique_object_representations<int &>::value, "No references!");
 static_assert(!has_unique_object_representations<const int &>::value, "No references!");
 static_assert(!has_unique_object_representations<volatile int &>::value, "No references!");
 static_assert(!has_unique_object_representations<const volatile int &>::value, "No references!");
 static_assert(!has_unique_object_representations<Empty>::value, "No empty types!");
+static_assert(!has_unique_object_representations<EmptyUnion>::value, "No empty types!");
 
 class Compressed : Empty {
   int x;
 };
 
 static_assert(has_unique_object_representations<Compressed>::value, "But inheriting from one is ok");
 
 class EmptyInheritor : Compressed {};