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 
      };
    }