Unsoundness in the virtual pattern matcher

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This is my clumsy attempt to exploit the fact that the new pattern matcher does not store case class members in local variables, but instead it goes through getters each time it's referenced in the rhs.

For a while I thought it is only annoying when debugging, until I realized that case classes can have mutable members. So here is a ClassCastException based on the fact that the type checker assumes pattern-bound identifiers to be stable:

abstract class Bomb {
	type T
	val x: T

	def size(that: T): Int
}

class StringBomb extends Bomb {
	type T = String
	val x = "abc"
	def size(that: String): Int = that.length
}

class IntBomb extends Bomb { 
	type T = Int
	val x = 10

	def size(that: Int) = x + that
}

case class Mean(var bomb: Bomb)

object Main extends App {
	def foo(x: Mean) = x match {
		case Mean(b) => 
			// b is assumed to be a stable identifier, but it can actually be mutated
			println(b.size({ mutate(); b.x }))
	}

	def mutate() {
	   	m.bomb = new IntBomb
	}

	val m = Mean(new StringBomb)
	foo(m)
}

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Imported From: https://issues.scala-lang.org/browse/SI-6070?orig=1
Reporter: @dragos
Affected Versions: 2.10.0
See #5158

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@adriaanm said (edited on Jul 20, 2012 8:51:17 AM UTC):
scala/scala#937