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.)