r324134 - Make __has_unique_object_representations reject empty union types.

[Hans Wennborg via cfe-commits]

Merged to 6.0 in r324213.

On Fri, Feb 2, 2018 at 9:30 PM, Eric Fiselier via cfe-commits
<cfe-commits at lists.llvm.org> wrote:
> Author: ericwf
> Date: Fri Feb  2 12:30:39 2018
> New Revision: 324134
>
> URL: http://llvm.org/viewvc/llvm-project?rev=324134&view=rev
> Log:
> 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
>
> Modified:
>     cfe/trunk/lib/AST/ASTContext.cpp
>     cfe/trunk/test/SemaCXX/type-traits.cpp
>
> Modified: cfe/trunk/lib/AST/ASTContext.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/ASTContext.cpp?rev=324134&r1=324133&r2=324134&view=diff
> ==============================================================================
> --- cfe/trunk/lib/AST/ASTContext.cpp (original)
> +++ cfe/trunk/lib/AST/ASTContext.cpp Fri Feb  2 12:30:39 2018
> @@ -2145,7 +2145,7 @@ static bool unionHasUniqueObjectRepresen
>      if (FieldSize != UnionSize)
>        return false;
>    }
> -  return true;
> +  return !RD->field_empty();
>  }
>
>  static bool isStructEmpty(QualType Ty) {
>
> Modified: cfe/trunk/test/SemaCXX/type-traits.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/type-traits.cpp?rev=324134&r1=324133&r2=324134&view=diff
> ==============================================================================
> --- cfe/trunk/test/SemaCXX/type-traits.cpp (original)
> +++ cfe/trunk/test/SemaCXX/type-traits.cpp Fri Feb  2 12:30:39 2018
> @@ -2566,6 +2566,7 @@ static_assert(!has_unique_object_represe
>  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;
>
>
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