Nothing toy-like about using ADTs to eliminate certain cases. When all cases are handled, your tests can move from a micro state to a macro state. Contraint types or linear types can be used to only allow certain sizes of inputs or require all file handles be closed when opened.

Naturally if your language’s type system is bad you can’t make these compile-time guarantees tho. Heck, a lot of developers are still using piss-poor languages with null or the infernce sucks with any.