Incorrect union type exhaustivity warning (Nested pattern)

Compiler version

3.0.1-RC1

Minimized code

import scala.reflect.Typeable

case class Err1()
case class Err2()

def handleError[A: Typeable, B: Typeable](x: Either[A | B, Nothing]): Unit =
  x match // false alarm warning: It would fail on pattern case: Left(_)
    case Left(e: A)  => println("A")
    case Left(_: B) => println("B")
    case Right(_)    => println("Nothing")

handleError[Err1, Err2](Left(Err1())) // prints A
handleError[Err1, Err2](Left(Err2())) // prints B

Output

match may not be exhaustive.

It would fail on pattern case: Left(_)

Expectation

As all cases are handled no warnings need to be generated.

Unrelated question, but shouldn’t removing Right be also fine? since it is of Nothing type? If you remove the last, the compiler gives out warning for Right pattern as well. (Looks like scala 2 has the same issue for this)

[liufengyun]

Unrelated question, but shouldn’t removing Right be also fine? since it is of Nothing type? If you remove the last, the compiler gives out warning for Right pattern as well. (Looks like scala 2 has the same issue for this)

Yes, conceptually it’s possible to remove a pattern if it only matches Nothing. Currently, we didn’t optimize for that, as it seems there is no need for it in practice.

Yep would be nice just for the sake of completeness and demo’ing scala

A PR is welcome: you can tweak the following two methods following the example for Unit and Boolean:

https://github.com/lampepfl/dotty/blob/229ec59c3b803c7004c988905a06c07b4f5e67d1/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala#L594-L670

[liufengyun]

@liufengyun this seems another case related to #12279 ?

@Swoorup Thanks for reporting. This seems to be fixed in master