Is comparing a BOOL against YES dangerous?

Question

I found a comment today in a source file:

//  - no longer compare BOOL against YES (dangerous!)

Is comparing BOOL against

Answer 1

The problem is that BOOL is not a native type, but a typedef:

typedef signed char      BOOL;

#define YES             (BOOL)1
#define NO              (BOOL)0

As a char, its values aren't constrained to TRUE and FALSE. What happens with another value?

BOOL b = 42;
if (b)
{
    // true
}
if (b != YES)
{
    // also true
}