deadalnix writes:

class A {}
final class B : A {}

auto foo(A a) {
    return cast(B) a;
}

This generates a call into the runtime, making things completely opaque to the optimizer. The algorithm used by the runtime is linear. But there is a dumb constant time solution. because B is final, a is either an instance of B, or it is not (as in, it cannot be an instance of a subclass of B). Therefore, we expect the codegen to look like the following instead:

auto foo(A a) {
    return typeid(a) is typeid(B) ? a : null;
}

This obviously won't compile but you get the idea. Not only this is constant time, but very simple too, and visible to the optimizer, which means the check can be folded by the opitimizer after other transforms, for instance inlining.

https://forum.dlang.org/post/hteuczyclxajakrisxjd@forum.dlang.org

Here we go: https://github.com/snazzy-d/sdc/blob/master/test/llvm/downcast.d

Except the interface part.