Typeable instance not being discovered on generic type typetest

Compiler version

3.0.0 and nightly 3.0.1-RC1-bin-SNAPSHOT

Minimized code

import scala.reflect.Typeable

case class Env[T: Typeable](inner: T)

given [T](using Typeable[T]): Typeable[Env[T]] = new:
  def unapply(x: Any): Option[x.type & Env[T]] = x match
    case env: Env[?] => 
      env.inner match
        case _: T => Some(x.asInstanceOf[x.type & Env[T]])
        case _ => None
    case _ => None

@main def main = 
  val env: Env[?] = Env("test")
  env match
    case _: Env[Int] => println("Got Env[Int]")
    case _: Env[String] => println("Got Env[String]")
    case _ => println("Got Others")

Scastie: https://scastie.scala-lang.org/kacLqqBERuaF7qZ4vN2LTA

Output

the type test for Env[Int] cannot be checked at runtime
the type test for Env[String] cannot be checked at runtime
Got Env[Int]

Expectation

Got Env[String]

No warnings.

@liufengyun this appears to a bug?

[nicolasstucki]

Note that that warning has nothing to do with Typeable. It is just the consequence type erasure on classes.

[liufengyun]

Wondering if this is something that can be improved in the future?

I think for type applications where the constructor is an abstract type, there might be a possibility.

For classes, it’s difficult to specify when we should try the implicit search and it can be expensive if we always give it a try.

@nicolasstucki may have more thoughts on this.