[Bug c++/113141] [13/14 Regression] ICE on conversion to reference in aggregate initialization

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113141

--- Comment #10 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:df7bfdb7dbf2d46aa5768a0280d4dcfcc868b7f9

commit r14-9947-df7bfdb7dbf2d46aa5768a0280d4dcfcc868b7f9
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Apr 10 15:12:26 2024 -0400

    c++: reference cast, conversion fn [PR113141]

    The second testcase in 113141 is a separate issue: we first decide that the
    conversion is ill-formed, but then when recalculating the special c_cast_p
    handling makes us think it's OK.  We don't want that, it should continue to
    fall back to the reinterpret_cast interpretation.  And while we're here,
    let's warn that we're not using the conversion function.

    Note that the standard seems to say that in this case we should
    treat (Matrix &) as const_cast<Matrix &>(static_cast<const Matrix &>(X)),
    which would use the conversion operator, but that doesn't match existing
    practice, so let's resolve that another day.  I've raised this issue with
    CWG; at the moment I lean toward never binding a temporary in a C-style
cast
    to reference type, which would also be a change from existing practice.

            PR c++/113141

    gcc/c-family/ChangeLog:

            * c.opt: Add -Wcast-user-defined.

    gcc/ChangeLog:

            * doc/invoke.texi: Document -Wcast-user-defined.

    gcc/cp/ChangeLog:

            * call.cc (reference_binding): For an invalid cast, warn and don't
            recalculate.

    gcc/testsuite/ChangeLog:

            * g++.dg/conversion/ref12.C: New test.

    Co-authored-by: Patrick Palka <ppalka@redhat.com>