Comment 2 for bug 291780

  /* Warn if two unsigned values are being compared in a size larger
     than their original size, and one (and only one) is the result of
     a `~' operator. This comparison will always fail.

     Also warn if one operand is a constant, and the constant does not
     have all bits set that are set in the ~ operand when it is
     extended. */

note that integer promotion is done on the operand(!) of ~. So u1 == (u8_t)(~u2)
is equal to

   (int)u1 == (int)(u8_t)(~(int)u2)

that we do not warn for the first case is because it is optimized to
u1 == ~u2 before.

Why do you think the warning is incorrect?