The obvious use case is the interface between something that’s not type-checked by the compiler and something that is. It’d be useful to be able to do `runtime_cast<Foo>(JSON.parse(foo_json))` on, say, the response to an API call and know that the result was definitely a valid Foo without having to write a bunch of boilerplate to validate that by hand.

(Reading through the thread, there’s apparently some libraries that can do this; I’ll have to try them out next time I need this.)

Because there's no guarantee that compiler did that. Even if you do this in your code, the moment it gets called from pure JS, all bets are off. With proper runtime checks, you could get consistent behavior out of that.