I don't know about C#, but, yes Dart has quite sophisticated exhaustiveness checking over sealed class hiearchies, including hierarchies that are DAGs and not just trees. It also handles record/tuple types and generics, nested arbitrarily deeply. And patterns that call arbitrary getters on objects.
For example, if you have this class hierarchy:
sealed class A {}
sealed class B1 implements A {}
sealed class B2 implements A {}
class C1 implements B1 {}
class C2 implements B1, B2 {}
class C3 implements B2 {}
// A
// / \
// B1 B2
// / \ / \
// C1 C2 C3
Then it understands that this switch statement is exhaustive:
void test(A a1, A a2) {
switch ((a1, a2)) {
case (A(), C1()): print(1);
case (C2(), C2()): print(2);
case (C3(), B2()): print(3);
case (B1(), B2()): print(4);
}
}
Because they cover the Cartesian product of all possible combinations of subtypes like:
The real challenge is that pictures like this only work for simple two-element tuples. Once you have more fields in the tuple, or nested destructuring, the manifold of the value space the patterns have to cover quickly gets into higher dimensions that are basically impossible to visualize.
Not sure about Dart, but for C# I don't think it can (due to classes being open by default).
If anyone is interested in a deeper dive on C# and adding Discriminated Unions, this interview by Nick Chapsas with Mads Torgersen is a great discussion: https://youtu.be/Nuw3afaXLUc?t=3939 (note, I'm pretty sure this time stamp is the right part, but the discussion might start before this. I'm at work and can't listen too closely to the video atm).
