https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109680
--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Not sure about the from case, but what do I know about C++?
While To type is directly returned from a function, std::declval doesn't return
>From but
>From &&.
#include <type_traits>
using F1 = int (int);
using T1 = int (*) (int);
static_assert (std::is_convertible_v<F1, T1>);
using F2 = int (int) const;
using T2 = int (*) (int);
static_assert (!std::is_convertible_v<F2, T2>);
using F3 = int (*) (int);
using T3 = int (int);
static_assert (!std::is_convertible_v<F3, T3>);
using F4 = int (*) (int);
using T4 = int (int) const;
static_assert (!std::is_convertible_v<F4, T4>);
passes in both g++ 12 and clang++.
And, on
using F1 = int (int);
using T1 = int (*) (int);
static_assert (__is_convertible(F1, T1));
using F2 = int (int) const;
using T2 = int (*) (int);
static_assert (!__is_convertible(F2, T2));
using F3 = int (*) (int);
using T3 = int (int);
static_assert (!__is_convertible(F3, T3));
using F4 = int (*) (int);
using T4 = int (int) const;
static_assert (!__is_convertible(F4, T4));
clang++ emits:
/tmp/2a.C:6:34: error: non-member function of type 'F2' (aka 'int (int) const')
cannot have 'const' qualifier
static_assert (!__is_convertible(F2, T2));
^
/tmp/2a.C:6:1: error: static_assert failed due to requirement
'!__is_convertible(int (int), int (*)(int))'
static_assert (!__is_convertible(F2, T2));
^ ~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/2a.C:12:38: error: non-member function of type 'T4' (aka 'int (int)
const') cannot have 'const' qualifier
static_assert (!__is_convertible(F4, T4));
^
while g++ trunk
/tmp/2a.C:6:16: error: static assertion failed
6 | static_assert (!__is_convertible(F2, T2));
| ^~~~~~~~~~~~~~~~~~~~~~~~~
