Oh, I forgot that HitResult->Actor is actually a TWeakObjectPtr<AActor>, not a raw pointer, so the argument type is wrong and the compiler is telling you about it. You should use Cast<ADoorTrigger>(HitResult->Actor->Get()). UE4 sometimes uses wrappers around raw pointers, like TSharedPtr or TWeakObjectPtr for safer handling. These pointer wrappers have accessor methods like Get(), or * to give you the raw pointer. You could have used the code suggestions to look at the types of FHitResult::Actor and the arguments that Cast expects and figured this out for yourself though.