[Bug c++/109680] [13/14 Regression] is_convertible<int() const, int(*)()> incorrectly true

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109680

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:4c2ffb02fd4104d77c5d907662f04434dc4c3fe8

commit r14-669-g4c2ffb02fd4104d77c5d907662f04434dc4c3fe8
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue May 2 17:36:00 2023 -0400

    c++: wrong std::is_convertible with cv-qual fn [PR109680]

    This PR points out that std::is_convertible has given the wrong answer
    in

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

    since r13-2822 implemented __is_{,nothrow_}convertible.

    std::is_convertible uses the imaginary

      To test() { return std::declval<From>(); }

    to do its job.  Here, From is 'int () const'.  std::declval is defined as:

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

    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, this is pretty much like:

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

    I think is_convertible_helper needs to simulate std::declval better.
    To that end, I'm introducing build_trait_object, to be used where
    a declval is needed.

            PR c++/109680

    gcc/cp/ChangeLog:

            * method.cc (build_trait_object): New.
            (assignable_expr): Use it.
            (ref_xes_from_temporary): Likewise.
            (is_convertible_helper): Likewise.  Check FUNC_OR_METHOD_TYPE_P.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/is_convertible6.C: New test.