Does it (Dart, C#) support exhaustivity checking for such matching over subclasses?
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
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);
}
}
a1
A
/ \
B1 B2
/ \ / \
C1 C2 C3
a2 +----+----+----+
C1| 1 |
B1< +----+----+----+
A< C2| | 2 | |
B2< + +----+ 3 +
C3| 4 | |
+----+----+----+
Figuring that all out was quite the challenge! The full design is here:
https://github.com/dart-lang/language/blob/main/accepted/3.0...
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.
that first paragraph, i believe, addresses your question, with excellent example from munificent in sibling comment.
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).