[Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables)

["in-gcc at baka dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213

            Bug ID: 95213
           Summary: GCC -Werror=conversion error when assigning to a
                    bitfield (when mixing constants and variables)
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: in-gcc at baka dot org
  Target Milestone: ---

When trying to assign to a bitfield with -Werror=conversion, having a constant
mix with a variable causes a "conversion from 'unsigned int' to 'unsigned
int:24' may change value", where as alternate formulations with two constants
or two variables work fine.  This error seems to be present on every version of
gcc ever (the ones I tried since 4.9.1, at least one in each major version, all
did the same thing).

Watch it fail on godbolt: https://gcc.godbolt.org/z/vavr-e


struct foo { unsigned int a:8; unsigned int b:24; };
void bar(struct foo num, unsigned int x) {
    num.b = (5U << 1) | (1 & 1);
    unsigned int z = 5U;
    num.b = ((z << 1) | (x & 1)) & 0xffffffU;
    num.b = ((5U << 1) | (x & 1)) & 0xffffffU;
    num.a = (unsigned char)x;
}


In the above example, the last assignment of num.b fails to compile where the
other two succeed.  Note in the last example, casting the bit (x&1) to _Bool
will fix the problem, but is obviously not portable to larger expressions (e.g.
x&7).