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From: "mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109680] [13/14 Regression] is_convertible<int() const,
 int(*)()> incorrectly true
Date: Tue, 02 May 2023 19:22:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109680

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Ah, I think I see what's going on here.  Once again, the problem is that th=
is
assert no longer passes:

#include <utility>
static_assert (!std::is_convertible_v <int () const, int (*) ()>, "");

std::is_convertible does
To test() { return std::declval<From>(); }
here, From is 'int () const'.

std::declval is defined as:

template<class T>
typename std::add_rvalue_reference<T>::type declval() noexcept;

Now, std::add_rvalue_reference is defined as "If T is a function type that =
has
no cv- or ref- qualifier or an object type, provides a member typedef type
which is T&&, otherwise type is T."

In our case, T is cv-qualified, so the result is T, so we end up with

int () const declval() noexcept;

which is invalid.  In other words:

using T =3D int () const;
T fn1(); // bad, fn returning a fn
T& fn2(); // bad, cannot declare reference to qualified function type
T* fn3(); // bad, cannot declare pointer to qualified function type

using U =3D int ();
U fn4(); // bad, fn returning a fn
U& fn5(); // OK
U* fn6(); // OK

So the check we're looking for is probably

if (TREE_CODE (type) =3D=3D FUNCTION_TYPE
    && (type_memfn_quals (type) !=3D TYPE_UNQUALIFIED
        || type_memfn_rqual (type) !=3D REF_QUAL_NONE))

but I think it should be put wherever we simulate declval().=