Partial Function: Prevent double evaluation of pattern matchers and guards

[allanrenucci]

The documentation of PartialFunction::applyOrElse says:

For all partial function literals the compiler generates an applyOrElse implementation which avoids double evaluation of pattern matchers and guards.

However this is not currently the case in Dotty:

object Test {
  private[this] var count = 0

  def test(x: Int) = { count += 1; true }

  object Foo {
    def unapply(x: Int): Option[Int] = { count += 1; Some(x) }
  }

  def main(args: Array[String]): Unit = {
    val res = List(1, 2).collect { case x if test(x) => x }
    println(count) // prints 4 but should be 2

    count = 0
    val res2 = List(1, 2).collect { case Foo(x) => x }
    println(count) // prints 4 but should be 2
  }
}

[allanrenucci]

Needed to make collections-strawman tests pass