Missing unchecked warning when pattern matching on a type parameter

[jedesah]

def remove[S](a: S | Int, f: Int => S):S = a match {
  case a: S => a
  case a: Int => f(a)
}

val t: Int | String = 5
val t1 = remove[String](t, _.toString)

This compiles fine, but at runtime, the call to remove will throw a java.lang.ClassCastException (Integer cannot be cast to String)

[jedesah]

It seems it has something to do with the use of a generic type because if I were to hardcode remove to String as follows:

def remove(a: String | Int):String = a match {
  case a: String => a
  case a: Int => a.toString
}

val t: Int | String = 5
val t1 = remove(t)

then no ClassCastException

[jedesah]

I may be able to have a look at this myself if someone can give a few pointers to where the problem might lie.

[senia-psm]

@jedesah in scala 2.12.x you'll get a warning:

scala> def unsafeCast[T](a: Any) = a match { case x: T => x; case _ => ??? }
<console>:11: warning: abstract type pattern T is unchecked since it is eliminated by erasure
       def unsafeCast[T](a: Any) = a match { case x: T => x; case _ => ??? }
                                                     ^
unsafeCast: [T](a: Any)T

scala> unsafeCast[String](1)
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

There is actually no typecheck here: case a: S => a.

[smarter]

Yes, pattern-matching on type parameters is unsafe and we're missing the warning, thanks for pointing that out

[jedesah]

@smarter Haha! Glad I could help!