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From: "mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113360] [13/14 Regression] Truncated constexpr error
 messages with -std=c++23/26
Date: Mon, 15 Jan 2024 23:38:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113360

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The "deducing from brace-enclosed initializer" error is emitted during pars=
ing
from listify.  The body of init_list is not potentially-constant (due to
erroneous FOR_COND), so we register_constexpr_fundef it with result =3D=3D
error_mark_node.

require_rvalue_constant_expression in finish_static_assert thinks init_list=
 is
potentially-constant, so we call cxx_constant_value.  new_call.fundef->resu=
lt
is error_mark_node so we don't bother evaluating the body.  We call
explain_invalid_constexpr_fn but that has nothing to say, so we end up with=
 a
missing reason.
But we already said what went wrong, so we shouldn't repeat it here anyway.

The body is:

{
  int total =3D 0;

  <<cleanup_point   int total =3D 0;>>;
  {
    int x =3D <<< error >>>;
    <<< error >>> __for_range;
    <<< error >>> __for_begin;
    <<< error >>> __for_end;

    <<< Unknown tree: for_stmt

      <<< error >>>
      <<< error >>>
      <<cleanup_point       int x =3D <<< error >>>;>>;
      <<cleanup_point <<< Unknown tree: expr_stmt
        (void) (total =3D total + VIEW_CONVERT_EXPR<int>(x)) >>>>>;
       >>>;
  }
  return <retval> =3D VIEW_CONVERT_EXPR<int>(total) =3D=3D 6;
}

Can be reproduced in C++20 with -Wno-invalid-constexpr.=