I would say, GCC's behavior shows commitment to the standard. It's exactly he same logic as the promotion of char/signed char/unsigned char, or int and unsigned

I would say, GCC's behavior shows commitment to the standard. It's exactly he same logic as the promotion of char/signed char/unsigned char, or int and unsigned int. That is to say, if we work this example with bitfields that are 8 bits wide, like uint32_t uf2 : 8; bf.uf2 = 255; and use a shift of 24: u2 = bf.uf2 << 24; the behavior observed on GCC will not change if we edit the declaration of the member to unsigned char: unsigned char uf2; I.e. an unsigned bitfield that is 8 bits wide is basically like unsigned char and promotes that way. From that the reasoning follows for other widths.