[Bug c++/125674] [13/14/15 Regression] ICE in cxx_eval_constant_expression with invalid bit-field width in template

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125674

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-14 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:a11b76ce4db284c4071901a3fe283be655bc74b5

commit r14-12743-ga11b76ce4db284c4071901a3fe283be655bc74b5
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jun 12 22:43:06 2026 +0200

    c++: Diagnose invalid type of bitfield widths in templates [PR125674]

    As the first testcase shows, outside of templates or when
    the bitfield width is not type dependent, we diagnose it
    in grokbitfield:
      if (width != error_mark_node)
        {
          /* The width must be an integer type.  */
          if (!type_dependent_expression_p (width)
              && !INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (TREE_TYPE (width)))
            error ("width of bit-field %qD has non-integral type %qT", value,
                   TREE_TYPE (width));
          else if (!check_for_bare_parameter_packs (width))
            {
              /* Temporarily stash the width in DECL_BIT_FIELD_REPRESENTATIVE.
                 check_bitfield_decl picks it from there later and sets
DECL_SIZE
                 accordingly.  */
              DECL_BIT_FIELD_REPRESENTATIVE (value) = width;
              SET_DECL_C_BIT_FIELD (value);
            }
        }
    Later on in check_bitfield_decl we verify it is a constant expression,
    folded into INTEGER_CST, non-negative etc.
    But during instantiation, we don't repeat that check, so only call
    check_bitfield_decl later on which can sometimes emit different diagnostics
    (so e.g.
    bit-field âD<1.0e+0>::dâ width not an integer constant
    instead of
    width of bit-field âD<N>::dâ has non-integral type âdoubleâ
    ) but what the second testcase shows, we can ICE during cxx_constant_value
    even before that if the type is even more problematic.

    The following patch fixes that by repeating the test from grokbitfield
    during tsubst_decl.

    2026-06-12  Jakub Jelinek  <jakub@redhat.com>

            PR c++/125674
            * pt.cc (tsubst_decl): Diagnose bit-field widths
            with invalid type.

            * g++.dg/template/bitfield5.C: New test.
            * g++.dg/template/bitfield6.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit 50ba4a8ca7a51a72514aae55d9fb81732ef7a3bd)