libc++ vs libstdc++ std::is_move_assignable: Which is the most correct?

Question

I'm trying to get a deeper understanding of C++ by reading the C++14 standard along with the source of libc++ and libstdc++. The implementation of various type_traits items varies between the two, particularly is_move_assignable, and I'm trying to figure out which of them is "more correct."

libc++:

template <class _Tp> struct is_move_assignable
    : public is_assignable<typename add_lvalue_reference<_Tp>::type,
                           const typename add_rvalue_reference<_Tp>::type> {};

libstdc++:

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
  struct __is_move_assignable_impl;

template<typename _Tp>
  struct __is_move_assignable_impl<_Tp, false>
  : public false_type { };

template<typename _Tp>
  struct __is_move_assignable_impl<_Tp, true>
  : public is_assignable<_Tp&, _Tp&&>
  { };

template<typename _Tp>
  struct is_move_assignable
  : public __is_move_assignable_impl<_Tp>
  { };

The standard states:

For a referenceable type T, the same result as is_assignable<T&, T&&>::value, otherwise false.

The first thing I noted is that libc++ applies const to the second template parameter, which doesn't seem right since the move assignment operator takes a non-const rvalue. libstdc++ also uses __is_referenceable, which follows the wording of the standard, but libc++ doesn't. Is that requirement covered by libc++'s use of add_lvalue_reference and add_rvalue_reference, which both enforce __is_referenceable on their own?

I would really appreciate any insight into why each project chose their solutions!

Answer 1

Thanks! Any idea why the authors might have added const, then?

My best guess is temporary (hopefully) insanity:

https://github.com/llvm-mirror/libcxx/commit/6063ec176d5056683d6ddd310c2e3a8f1c7e1b46#diff-48f5ee43879b5ad38888f0a6ead10113R1245

;-)

I removed the const and ran the current unit tests and nothing failed.