You need flow typing when you do pattern matching on generalized abstract data type (GADT). Here is an example in Java sealed interface Foo<T> permits Impl

You need flow typing when you do pattern matching on generalized abstract data type (GADT). Here is an example in Java sealed interface Foo<T> permits Impl {} // union type record Impl() implements Foo<String> {} // product type <T> T m(Foo<T> foo) { return switch(foo) { case Impl impl -> "hello"; // flow typing T=String }; }