cast.m

int main(int argc, const char * argv[]) {
    @autoreleasepool {
      
        Queue<Sub *> *subQueue = [[Queue alloc] init];
        Queue<Sup *> *supQueue = subQueue; // Warning: Incompatible pointer types initializing 'Queue<Base *>' with an expression of type 'Queue<Sub *>'
        
        [supQueue enqueue:[Sub new]];
    }
  
    return 0;
}