To add to what others have said: Kotlin is a language that does path sensitive (re-)typing of variables. This is often very convenient in the presence of OO-sty

To add to what others have said: Kotlin is a language that does path sensitive (re-)typing of variables. This is often very convenient in the presence of OO-style polymorphism or in cases where the language supports ad-hoc unions. Personally it doesn't feel complicated to use/understand (at least the kind of thing that these languages do)