struct a {unsigned b : 2;};
int foo (struct a *p) {return 2u < p->b + 1;}
int bar (struct a *p) {return 2u < (unsigned)p->b + 1;}

gcc -W -Wall  gives
bitfield.c:2: warning: comparison between signed and unsigned
bitfield.c:3: warning: comparison between signed and unsigned

1) is it correct to promote an unsigned bitfield smaller than an int to
signed int? (I think so, but am not 100% sure)
2) (unsigned)p->b + 1, should have unsigned type, hence the 2nd warning is spurious