[Bug c++/101073] [10/11/12/13 Regression] ICE in cxx_eval_constant_expression, at cp/constexpr.c:6941

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101073

--- Comment #6 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:b2287a4d9a640fdc2caef6a067830ea65044deb7

commit r13-6145-gb2287a4d9a640fdc2caef6a067830ea65044deb7
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Feb 8 19:13:18 2023 -0500

    c++: ICE with -fno-elide-constructors and trivial fn [PR101073]

    In constexpr-nsdmi3.C, with -fno-elide-constructors, we don't elide
    the Y::Y(const Y&) call used to initialize o.c.  So store_init_value
    -> cxx_constant_init must constexpr-evaluate the call to Y::Y(const Y&)
    in cxx_eval_call_expression.  It's a trivial function, so we do the
    "Shortcut trivial constructor/op=" code and rather than evaluating
    the function, we just create an assignment

      o.c = *(const struct Y &) (const struct Y *) &(&<PLACEHOLDER_EXPR struct
X>)->b

    which is a MODIFY_EXPR, so the preeval code in cxx_eval_store_expression
    clears .ctor and .object, therefore we can't replace the PLACEHOLDER_EXPR
    whereupon we crash at

          /* A placeholder without a referent.  We can get here when
             checking whether NSDMIs are noexcept, or in massage_init_elt;
             just say it's non-constant for now.  */
          gcc_assert (ctx->quiet);

    The PLACEHOLDER_EXPR can also be on the LHS as in constexpr-nsdmi10.C.
    I don't think we can do much here, but I noticed that the whole
    trivial_fn_p (fun) block is only entered when -fno-elide-constructors.
    This is true since GCC 9; it wasn't easy to bisect what changes made it
    so, but r240845 is probably one of them.  -fno-elide-constructors is an
    option for experiments only so it's not clear to me why we'd still want
    to shortcut trivial constructor/op=.  I propose to remove the code and
    add a checking assert to make sure we're not getting a trivial_fn_p
    unless -fno-elide-constructors.

            PR c++/101073

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_eval_call_expression): Replace shortcutting
trivial
            constructor/op= with a checking assert.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-nsdmi3.C: New test.
            * g++.dg/cpp1y/constexpr-nsdmi10.C: New test.