Pre-DR: _Bool bit-fields
========================

Does a named bit-field declared as _Bool have a type which acts like
_Bool in terms of conversions on assignment to or initialization of
the bit-field, or one which acts like an ordinary unsigned integer
type of the specified width?

For example, given

  struct s { _Bool x : 3; } a;

does

  a.x = 3;

result in a.x having the value 1 or 3?  Does

  a.x = 0.4;

result in a.x having the value 1 or 0?  Does

  a.x = 100.0;

result in a.x having the value 1, or undefined behavior?  Does

  a.x = &a;

result in a.x having the value 1, or a violation of the constraints
for assignment?

(For previous discussion see reflector messages 10386-438.  I don't
think the issue of constraints for assignment was specifically
mentioned before.  It is necessary first to determine what the answer
ought to be - which behavior of conversions to _Bool is correct - and
then to determine what wording makes that answer clear, or to make
explicit that the choice of one of the two behaviors is unspecified.)