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

llvm-svn: 324134

diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index c81d578..061af1c 100644 (file)
--- a/clang/lib/AST/ASTContext.cpp
+++ b/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) {

diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 8cf6880..ca96503 100644 (file)
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -2566,6 +2566,7 @@ 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;