[Bug c++/106784] Add __is_convertible built-in

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106784

--- Comment #4 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:8a7bcf95a82c3dd68bd4bcfbd8432eb970575bc2

commit r13-2822-g8a7bcf95a82c3dd68bd4bcfbd8432eb970575bc2
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Sep 20 15:48:52 2022 -0400

    c++: Implement __is_{nothrow_,}convertible [PR106784]

    To improve compile times, the C++ library could use compiler built-ins
    rather than implementing std::is_convertible (and _nothrow) as class
    templates.  This patch adds the built-ins.  We already have
    __is_constructible and __is_assignable, and the nothrow forms of those.

    Microsoft (and clang, for compatibility) also provide an alias called
    __is_convertible_to.  I did not add it, but it would be trivial to do
    so.

    I noticed that our __is_assignable doesn't implement the "Access checks
    are performed as if from a context unrelated to either type" requirement,
    therefore std::is_assignable / __is_assignable give two different results
    here:

      class S {
        operator int();
        friend void g(); // #1
      };

      void
      g ()
      {
        // #1 doesn't matter
        static_assert(std::is_assignable<int&, S>::value, "");
        static_assert(__is_assignable(int&, S), "");
      }

    This is not a problem if __is_assignable is not meant to be used by
    the users.

    This patch doesn't make libstdc++ use the new built-ins, but I had to
    rename a class otherwise its name would clash with the new built-in.

            PR c++/106784

    gcc/c-family/ChangeLog:

            * c-common.cc (c_common_reswords): Add __is_convertible and
            __is_nothrow_convertible.
            * c-common.h (enum rid): Add RID_IS_CONVERTIBLE and
            RID_IS_NOTHROW_CONVERTIBLE.

    gcc/cp/ChangeLog:

            * constraint.cc (diagnose_trait_expr): Handle CPTK_IS_CONVERTIBLE
            and CPTK_IS_NOTHROW_CONVERTIBLE.
            * cp-objcp-common.cc (names_builtin_p): Handle RID_IS_CONVERTIBLE
            RID_IS_NOTHROW_CONVERTIBLE.
            * cp-tree.h (enum cp_trait_kind): Add CPTK_IS_CONVERTIBLE and
            CPTK_IS_NOTHROW_CONVERTIBLE.
            (is_convertible): Declare.
            (is_nothrow_convertible): Likewise.
            * cxx-pretty-print.cc (pp_cxx_trait_expression): Handle
            CPTK_IS_CONVERTIBLE and CPTK_IS_NOTHROW_CONVERTIBLE.
            * method.cc (is_convertible): New.
            (is_nothrow_convertible): Likewise.
            * parser.cc (cp_parser_primary_expression): Handle
RID_IS_CONVERTIBLE
            and RID_IS_NOTHROW_CONVERTIBLE.
            (cp_parser_trait_expr): Likewise.
            * semantics.cc (trait_expr_value): Handle CPTK_IS_CONVERTIBLE and
            CPTK_IS_NOTHROW_CONVERTIBLE.
            (finish_trait_expr): Likewise.

    libstdc++-v3/ChangeLog:

            * include/std/type_traits: Rename __is_nothrow_convertible to
            __is_nothrow_convertible_lib.
            * testsuite/20_util/is_nothrow_convertible/value_ext.cc: Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/has-builtin-1.C: Enhance to test __is_convertible and
            __is_nothrow_convertible.
            * g++.dg/ext/is_convertible1.C: New test.
            * g++.dg/ext/is_convertible2.C: New test.
            * g++.dg/ext/is_nothrow_convertible1.C: New test.
            * g++.dg/ext/is_nothrow_convertible2.C: New test.