[nly]

> If you're expecting one type and get another, you have to handle it, right?

Depends. You could debate the same philosophy wrt C and null pointers. Should you check all your arguments to see if they're null, if the documentation/specification says they should never be null, or accept undefined behaviour and segfault?

In practice, most code assumes everything is checked, sanitized and used correctly at the outer most API boundary.