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

[Jason Merrill <jason@redhat.com>]

On 10/24/23 12:18, Marek Polacek wrote:
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >8 --
> 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
> will get us an expression whose type is the enum.  (I thought we didn't
> need decay_conversion because that does a whole lot more but using it
> would make sense to me too.)

It's possible that we might want some of that more, particularly 
mark_rvalue_use; decay_conversion seems like the right answer.  OK with 
that change.

rvalue() would also make sense, though that seems to be missing a call 
to unlowered_expr_type at the moment.  In fact, after "otherwise, it's 
the lvalue-to-rvalue conversion" in decay_conv should probably just be a 
call to rvalue, with missing bits added to the latter function.

Jason