Implicit conversion from zero_t do not work.

[pabuhr]

struct foo;
void ?{}( foo **, zero_t z ) {}
int fred() {
    foo * h;
    if (h) {} 
}

should convert to:

if (h != ?{}(temp, 0), temp ) { ... }

Instead converts to:

if ( ((int )(__h__P4sfoo_2!=((struct foo *)0))) ) {}