[Bug c++/111895] error: invalid operands of types 'unsigned char:2' and 'int' to binary 'operator!=' on enum class bitfield cast to bool

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111895

--- Comment #4 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:6fa7284e2824310bb7204d41b5243c677ecb62d3

commit r14-4902-g6fa7284e2824310bb7204d41b5243c677ecb62d3
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Oct 23 17:06:45 2023 -0400

    c++: error with bit-fields and scoped enums [PR111895]

    Here we issue a bogus error: invalid operands of types 'unsigned char:2'
    and 'int' to binary 'operator!=' when casting a bit-field of scoped enum
    type to bool.

    In build_static_cast_1, perform_direct_initialization_if_possible returns
    NULL_TREE, because the invented declaration T t(e) fails, which is
    correct.  So we go down to ocp_convert, which has code to deal with this
    case:
              /* We can't implicitly convert a scoped enum to bool, so convert
                 to the underlying type first.  */
              if (SCOPED_ENUM_P (intype) && (convtype & CONV_STATIC))
                e = build_nop (ENUM_UNDERLYING_TYPE (intype), e);
    but the SCOPED_ENUM_P is false since intype is <unnamed-unsigned:2>.
    This could be fixed by using unlowered_expr_type.  But then
    c_common_truthvalue_conversion/CASE_CONVERT has a similar problem, and
    unlowered_expr_type is a C++-only function.

    Rather than adding a dummy unlowered_expr_type to C, I think we should
    follow [expr.static.cast]p3: "the lvalue-to-rvalue conversion is applied
    to the bit-field and the resulting prvalue is used as the operand of the
    static_cast."  There are no prvalue bit-fields, so the l-to-r conversion
    performed in decay_conversion will get us an expression whose type is the
    enum.

            PR c++/111895

    gcc/cp/ChangeLog:

            * typeck.cc (build_static_cast_1): Call decay_conversion.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/scoped_enum12.C: New test.