Code shared from run.dlang.io.

main.d

class C {
    int* result;

    
    ref T opCast(T: int*)(){
        return result;
    }
}

auto main() {
    auto c = new C();
    int y = 40;
    cast(int*) c = &y;
    assert(*c.result == y);
}

struct S {
    int* result;

    
    ref T opCast(T: int*)(){
        return result;
    }
}

auto f() {
    auto s = new S();
    int y = 40;
    //cast(int*) s = &y; // ERROR: cast(int*) s is not an lvalue and cannot be modified
    auto s2 = S();
    cast(int*) s2 = &y; //now it works
    //if(s2){ // ERROR: template instance opCast!bool not defined
              // For classes, I guess the default implicitly bool conversion just checks for null instead of trying to call opCast!bool
    
}