https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78939
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
No, no change. I think we need to change the tuple_size<cv T> specializations
to be incomplete when tuple_size<T> is incomplete, even though that appears to
contradict the changes made by lwg 2770.
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -88,24 +88,23 @@ _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, 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>
But this also affects C++11 mode, so I'm still trying to understand all the
consequences of such a change. At the least, some tests need changing:
FAIL: 20_util/pair/astuple/astuple.cc (test for excess errors)
FAIL: 20_util/tuple/cv_tuple_size.cc (test for excess errors)
UNRESOLVED: 20_util/tuple/cv_tuple_size.cc compilation failed to produce
executable
FAIL: 20_util/tuple/tuple_size.cc (test for excess errors)
FAIL: 23_containers/array/tuple_interface/tuple_size.cc (test for excess
errors)
These all fail for the same reason, and I think it's a FE bug.
