[PATCH] D42863: Make __has_unique_object_representations reject empty union types.

[Eric Fiselier via Phabricator via cfe-commits]

EricWF created this revision.
EricWF added reviewers: erichkeane, rsmith, aaron.ballman, majnemer.

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.


https://reviews.llvm.org/D42863

Files:
  lib/AST/ASTContext.cpp
  test/SemaCXX/type-traits.cpp


Index: test/SemaCXX/type-traits.cpp
===================================================================
--- test/SemaCXX/type-traits.cpp
+++ test/SemaCXX/type-traits.cpp
@@ -2566,6 +2566,7 @@
 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;
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -2145,7 +2145,7 @@
     if (FieldSize != UnionSize)
       return false;
   }
-  return true;
+  return !RD->field_empty();
 }
 
 static bool isStructEmpty(QualType Ty) {


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