Fix #2578 Part 1: Tighten type checking of pattern bindings

[odersky]

Check that patterns in val/var defs are irrefutable, unless the pattern is followed by : @unchecked.

For the moment, this requires -strict mode. We plan to turn this on by default once the bootstrap compiler supports : @unchecked in patterns (i.e. once this PR is part of the bootstrap compiler).

Edit: In fact, we probably can do this only in 3.1, for cross compilation reasons with Scala 2.

[smarter]

Edit: In fact, we probably can do this only in 3.1, for cross compilation reasons with Scala 2.

We can do it earlier if we get some of the parser changes into Scala 2, right ? What's the minimum set of changes needed ?

[abgruszecki]

Otherwise LGTM.

[abgruszecki]

For UnApply, we should also check if the extractor itself is irrefutable, i.e. if it returns Some, or true, or a product. The logic to do that is in SpaceEngine#irrefutable.

Additionally, for some reason unapplies are simply rejected as val definitions:

scala> {
     |   object Positive { def unapply(i: Int): Option[Int] = Some(i).filter(_ > 0) }
     |   val Positive(p) = 5
     |   5 match { case Positive(p) => p }
     | }
3 |  val Positive(p) = 5
  |      ^^^^^^^^^^^
  | ((i: Int): Option[Int])(Positive.unapply) is not a valid result type of an unapply method of an extractor.