https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109680
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
template <typename T, typename U = T&&>
U __declval (int);
template <typename T>
T __declval (long);
template <typename T>
auto declval () noexcept -> decltype (__declval <T> (0));
using To = int () const;
using From = int (*) ();
To foo () { return declval <From> (); }
>From bar () { return declval <To> (); }
static_assert (!__is_convertible (To, From), "");
static_assert (!__is_convertible (From, To), "");
should show that both assertions should succeed. Dunno if in this case the
reason why it isn't convertible is simply the fact that functions can't be
declared to return function/method/array types or some other reason as well.
