[Bug libstdc++/115522] [13/14 Regression] std::to_array no longer works for struct which is trivial but not default constructible

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115522

--- Comment #14 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>:

https://gcc.gnu.org/g:9690fb3a43e5cf26a5fb853283d4200df312a640

commit r15-2146-g9690fb3a43e5cf26a5fb853283d4200df312a640
Author: Marek Polacek <pola...@redhat.com>
Date:   Tue Jun 18 16:49:24 2024 -0400

    c++: implement DR1363 and DR1496 for __is_trivial [PR85723]

    is_trivial was introduced in
    <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2230.html>
    which split POD into is_trivial and is_standard_layout.

    Later came CWG 1363.  Since

      struct A {
        A() = default;
        A(int = 42) {}
      };

    cannot be default-initialized, it should not be trivial, so the definition
    of what is a trivial class changed.

    Similarly, CWG 1496 concluded that

      struct B {
        B() = delete;
      }:

    should not be trivial either.

    P0848 adjusted the definition further to say "eligible".  That means
    that

      template<typename T>
      struct C {
        C() requires false = default;
      };

    should not be trivial, either, since C::C() is not eligible.

    Bug 85723 reports that we implement none of the CWGs.

    I chose to fix this by using type_has_non_deleted_trivial_default_ctor
    which uses locate_ctor which uses build_new_method_call, which would
    be used by default-initialization as well.  With that, all __is_trivial
    problems I could find in the Bugzilla are fixed, except for PR96288,
    which may need changes to trivially-copyable, so I'm not messing with
    that now.

    I hope this has no ABI implications.  There's effort undergoing to
    remove "trivial class" from the core language as it's not really
    meaningful.  So the impact of this change should be pretty low except
    to fix a few libstdc++ problems.

            PR c++/108769
            PR c++/58074
            PR c++/115522
            PR c++/85723

    gcc/cp/ChangeLog:

            * class.cc (type_has_non_deleted_trivial_default_ctor): Fix
formatting.
            * tree.cc (trivial_type_p): Instead of TYPE_HAS_TRIVIAL_DFLT, use
            type_has_non_deleted_trivial_default_ctor.

    gcc/testsuite/ChangeLog:

            * g++.dg/warn/Wclass-memaccess.C: Add dg-warning.
            * g++.dg/ext/is_trivial1.C: New test.
            * g++.dg/ext/is_trivial2.C: New test.
            * g++.dg/ext/is_trivial3.C: New test.
            * g++.dg/ext/is_trivial4.C: New test.
            * g++.dg/ext/is_trivial5.C: New test.
            * g++.dg/ext/is_trivial6.C: New test.