cgo turns incomplete types into [0]byte behind the scenes. This means that a pointer to
an incomplete type can be assigned to a pointer to any other incomplete type.

Example:

package main

/*
typedef struct foo foo_t;
typedef struct bar bar_t;
foo_t* foofunc() {
    return 0;
}
bar_t* barfunc() {
    return 0;
}
*/
import "C"

func main() {
    var a *C.foo_t
    a = C.barfunc()
    _ = a
}