I'm a heavy user of exhaustive pattern matching with sealed classes, definitely a great feature. You can achieve the same outcome as union types, but it's a lot more leg work, e.g.:
```
sealed class MyParam {}
class Bar extends MyParam { String val; }
class Baz extends MyParam { int val; }
void foo(MyParam param) {
switch(myParam) {
case Baz(val: final val):
case Bar(val: final val):
}
}
```
compared to:
```
void foo(Bar | Baz myVar) {
switch(myVar) {
case Bar:
case Baz:
}
}
```
I hadn't seen the extension_type_unions package, though, I'll check it out.
