Merging r324134:

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r324134 | ericwf | 2018-02-02 21:30:39 +0100 (Fri, 02 Feb 2018) | 14 lines

Make __has_unique_object_representations reject empty union types.

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. 

Reviewers: erichkeane, rsmith, aaron.ballman, majnemer

Reviewed By: erichkeane

Subscribers: cfe-commits, erichkeane

Differential Revision: https://reviews.llvm.org/D42863
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llvm-svn: 324213

clang/lib/AST/ASTContext.cpp

@@ -2145,7 +2145,7 @@ static bool unionHasUniqueObjectRepresentations(const ASTContext &Context,
     if (FieldSize != UnionSize)
       return false;
   }
-  return true;
+  return !RD->field_empty();
 }
 
 static bool isStructEmpty(QualType Ty) {

clang/test/SemaCXX/type-traits.cpp

@@ -2524,6 +2524,7 @@ static_assert(!has_unique_object_representations<const int &>::value, "No refere
 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;