Incorrect warning “type test cannot be checked at runtime”

[julienrf]

Minimized code

package bugreport

sealed trait Foo[+A]
case class Bar[A]() extends Foo[A]

def bugReport[A](foo: Foo[A]): Foo[A] =
  foo match {
    case bar: Bar[A] => bar
  }

Output

[warn] -- Warning:
[warn] 8 |    case bar: Bar[A] => bar
[warn]   |         ^^^^^^^^^^^
[warn]   |         the type test for bugreport.Bar[A] cannot be checked at runtime

Expectation

There should be no warning.

Additional information

• There was no warning in dotty 0.24.0-RC1
• There is no warning if type parameter A is covariant in Bar

[liufengyun]

This is actually correct, see the example below:

sealed trait Foo[+A]
case class Bar[A]() extends Foo[A]

class Dummy extends Bar[Nothing] with Foo[String]

def bugReport[A](foo: Foo[A]): Foo[A] =
  foo match {
    case bar: Bar[A] => bar
  }

def test = bugReport(new Dummy: Foo[String])

You need to mark Bar[A] as covariant, then the warning will disappear.

[julienrf]

Thank you Fengyun for the detailed explanation!