[Bug c/88662] New: Document trap representations of _Bool

[gonzalobg88 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88662

            Bug ID: 88662
           Summary: Document trap representations of _Bool
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gonzalobg88 at gmail dot com
  Target Milestone: ---

Compiling

unsigned int foo(unsigned int x, _Bool b) {
    return x - (unsigned int)b;
}

only produces correct results if the value of `_Bool` is either `0` or `1` [0],
see https://gcc.godbolt.org/z/l0DPjc:

foo:
        movzx   esi, sil
        mov     eax, edi
        sub     eax, esi
        ret

This probably means that all other representations of `_Bool` are trap
representations, but this does not appear to be documented anywhere. 

>From my reading of the C standard, the role that padding bits play for `_Bool`
is unclear.

[0] one can construct such a _Bool by writing to it via a char* .