Closed
Description
In the below code, pattern matching silently upcasts invariant type Exp, from Exp[Double] to Exp[Any] even though Exp is invariant.
case class Man[T]()
implicit def man[T] = Man[T]()
abstract class Exp[T:Man] {
def mT = man[T]
}
case class Const[A:Man](x: A) extends Exp[A]
case class Node[A:Man, B:Man](base: Exp[A]) extends Exp[B] {
def mA = man[A]
def mB = man[B]
}
val m = Node[Int, String](Node[Double, Int](Const(3.14)))
val n = m match {
case Node(Node(t)) => t
}
//At the REPL we get
//n: Exp[Any] = Const(3.14)
//scala> n.mT
//res0: Man[Any] = Man()This example was shown to me by my colleague Cai Yufei; the above typing issue occurs when trying to write a type-correct map fusion in LMS-like scenarios - this explains the strange type for Node above.
The use of primitive types above is irrelevant, as shown below:
scala> val m = Node[java.io.File, StringBuffer](Node[String, java.io.File](Const("a string")))
m: Node[java.io.File,java.lang.StringBuffer] = Node(Node(Const(a string)))
scala> val n = m match {
| case Node(Node(t)) => t
| }
n: Exp[Any] = Const(a string)