https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99692
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #10)
> template<typename _Ostream, typename _Tp>
> struct __is_insertable<_Ostream, _Tp,
> void_t<decltype(declval<_Ostream&>()
> << declval<const _Tp&>())>>
The problem is that this check produces a hard error (rather than substitution
failure) for declval<std::ostream&>() << declval<const N::A&>().
The relevant operator is not found by ADL, because it's not in the associated
namespaces of std::ostream or N::A (which is IMHO a design error in the code,
but nevermind). So the void_t expression should get a substitution error.
I think the problem is that PR 51577 causes GCC to look in the global scope and
so incorrectly find the declaration of:
std::ostream& operator<<(std::ostream& s, const N::A&)
which means that the __is_insertable check ... goes wrong somehow? I think it
makes the void_t expression ambiguous because of the global operator<< overload
which is not meant to be found.
The reduced example above can be reduced further:
namespace std
{
struct ostream { };
template<typename T> T&& declval();
template<typename...> using void_t = void;
struct false_type { static constexpr bool value = false; };
struct true_type { static constexpr bool value = true; };
template<typename _Ostream, typename _Tp, typename = void>
struct __is_insertable : false_type {};
template<typename _Ostream, typename _Tp>
struct __is_insertable<_Ostream, _Tp,
void_t<decltype(declval<_Ostream&>()
<< declval<const _Tp&>())>>
: true_type {};
}
struct CustomStream : std::ostream {};
namespace N {
class A{};
}
std::ostream& operator<<(std::ostream& s, const N::A&)
{
return s;
}
int main()
{
static_assert( ! std::__is_insertable<std::ostream, N::A>::value, "" );
}
This avoids the hard error in __is_insertable, but GCC fails the static_assert
(where as Clang passes it) which is PR 51577.
