Bug when pattern matching on existential values

[scabug]

Existentials as arguments to unapply result in a compilation error:

Minimal example:

  trait Match {
    type X
    def unapply(x: X): Option[String]
  }

  def foo(m: Match): m.X => String = {
    case m(s) => s
  }

yields

an unapply result must have a member `def isEmpty: Boolean
[error]     case m(s) => s
[error]          ^
[error] one error found
[error] (compile:compile) Compilation failed

In contrast, a variant using universal types works just fine:

  trait Match[X] {
    def unapply(x: X): Option[String]
  }

  def foo[X](m: Match[X]): X => String = {
    case m(s) => s
  }

Might be related to the following issues:

• Unsoundness bug in pattern matcher when pattern reveals existentials #6680
• regression in extractors with existential-returning unapply/unapplySeq #8128

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9247?orig=1
Reporter: Jonathan Brachthäuser (b-studios)
Affected Versions: 2.11.3, 2.11.4, 2.11.5, 2.11.6

[scabug]

Julien Richard-Foy (julienrf) said (edited on Aug 11, 2016 11:21:42 AM UTC):
Is there a workaround for this issue?

[scabug]

@paulp said:
Workaround:

  def foo[A](m: Match { type X = A }): A => String = {
    case m(s) => s
  }

[scabug]

Jonathan Brachthäuser (b-studios) said:
Workaround 2: Use dotty.

Just tried with 0.1-SNAPSHOT and it works perfectly fine.