https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78939
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oops, that was the previous diff, I meant to paste this (which constrains the
__enable_if_has_tuple_size to only be used when _Tp has no cv-quals, so we
avoid ambiguities):
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -88,24 +88,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
struct tuple_size;
// _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 2313. tuple_size should always derive from integral_constant<size_t, N>
// 2770. tuple_size<const T> specialization is not SFINAE compatible
- template<typename _Tp, typename = void>
- struct __tuple_size_cv_impl { };
- template<typename _Tp>
- struct __tuple_size_cv_impl<_Tp,
__void_t<decltype(tuple_size<_Tp>::value)>>
- : integral_constant<size_t, tuple_size<_Tp>::value> { };
+ template<typename _Tp,
+ typename _Up = typename remove_cv<_Tp>::type,
+ typename = typename enable_if<is_same<_Tp, _Up>::value>::type,
+ size_t = tuple_size<_Tp>::value>
+ using __enable_if_has_tuple_size = _Tp;
- // _GLIBCXX_RESOLVE_LIB_DEFECTS
- // 2313. tuple_size should always derive from integral_constant<size_t, N>
template<typename _Tp>
- struct tuple_size<const _Tp> : __tuple_size_cv_impl<_Tp> { };
+ struct tuple_size<const __enable_if_has_tuple_size<_Tp>>
+ : public tuple_size<_Tp> { };
template<typename _Tp>
- struct tuple_size<volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+ struct tuple_size<volatile __enable_if_has_tuple_size<_Tp>>
+ : public tuple_size<_Tp> { };
template<typename _Tp>
- struct tuple_size<const volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+ struct tuple_size<const volatile __enable_if_has_tuple_size<_Tp>>
+ : public tuple_size<_Tp> { };
/// Gives the type of the ith element of a given tuple type.
template<std::size_t __i, typename _Tp>
