Pattern matching on sealed abstract classes causes spurious warning

[Jacoby6000]

In some situations, matching on a sealed abstract class using a type annotation will cause the compiler to warn about unreachable code

  sealed trait Foo
  class Bar(val s: String) extends Foo
  sealed abstract class Baz(val s: String) extends Foo
  
  val f: Foo => String = { 
    case bar: Bar => bar.s
    case baz: Baz => baz.s
  }

This warning does not happen if the case for the sealed abstract class comes first.

[allanrenucci]

Can be reproduced with sealed trait too:

sealed trait Foo
class Bar extends Foo
sealed trait Baz extends Foo

class Test {
  def f(x: Foo) = x match { 
    case bar: Bar => 1
    case baz: Baz => 2
  }
}

[liufengyun]

This seems to be the intended behaviour, because Baz cannot be instantiated. Scalac 2.12.3 generates the same warning.

[allanrenucci]

What about this example:

sealed trait Foo
class Bar extends Foo
sealed trait Baz extends Foo

class Test {
  def f = {
    val x = new Baz {}
    x match { 
      case bar: Bar => 1
      case baz: Baz => 2
    }
  }
}

scalac emits no warning here.

[liufengyun]

The last example above is due to the problem of anonymous class extending sealed class/trait.

@smarter suggested to simply disallow anonymous class extending sealed class/trait long time ago, see below:

#1917 (comment)

Currently, the exhaustivity check doesn't have the information to know whether a sealed trait has anonymous children or not.