A Switch over a sealed class can match unrelated types

[EArminjon]

Hello :) !

I didn't understand how the code bellow didn't show any warnings.

This can lead to very hard nested issue when you didn't understand why your switch isn't working as expected and you suddenly see that the value used in the switch can't match any case value...

In big entreprise team with different skill levels, it become a massive issue.

Bellow code of the issue, I deliberately put a dumb examples to be more explicit.

sealed class User {
  const User();
}

class Connected extends User {
  const Connected();
}

class Disconnected extends User {
  const Disconnected();
}

void test() {
  User user = const Connected();
  Widget child = AppBar();

  print(switch (user) {
    Connected() => true,
    Disconnected() => true,
    AppBar() => false, // not warning ? This type has nothing in common with User class
  });

  print(switch (child) {
    Connected() => true, // no warning ? This type has nothing in common with Widget class
    Disconnected() => true, // no warning ? This type has nothing in common with Widget class
    AppBar() => false,
    _ => throw UnimplementedError(),
  });

  print(switch (10) {
    10 => true,
    '10' => false, // warning well displayed !
    _ => throw UnimplementedError(),
  });
}

[mraleph]

You need to mark Connected and Disconnected as final. sealed does not imply finality of subclasses. As written now you can create a subtype of Connected and AppBar in another library.

If you mark all subclasses final you will get the warning.

[EArminjon]

Understood, as we can have something as bellow, the switch will well cover all cases.

class Test extends AppBar implements User {
  Test({super.key});
}

Thanks for your answer.

[eernstg]

@EArminjon, you did mention one case which could presumably be handled better. I created #60342 to clarify the matter.