I don’t find that very convincing. It’s simply ambiguous, I might want this: *reg &= ~(uint64_t)x; or, and there’s no elegant way to even write this in C,

I don’t find that very convincing. It’s simply ambiguous, I might want this: *reg &= ~(uint64_t)x; or, and there’s no elegant way to even write this in C, I might want: *reg &=(uint64_t)(uint8_t)~x; The fact that I have to write two casts here to undo the damage of the auto-promotion is evidence of how broken this is.