Fix #3588: Do not emit switch when matching on Any

[nicolasstucki]

No description provided.

[nicolasstucki]

As done scalac.

[allanrenucci]

I think it should be part of collectSwitchCases and maybe use the already defined isSwitchableType function

[allanrenucci]

Scalac does not emit switch for String. Is it a limitation we can lift? Java supports switch for String since JDK 7

[vitorsvieira]

https://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.11 and how achieve the same with dotty's enum?

[nicolasstucki]

We have code testing that strings are switchable. I saw that failure in the first commit.

[smarter]

Is support for switching on strings actually implemented somewhere in the compiler? The Java support is implemented by desugaring them into two switch statements on integers, there is no bytecode-level support for switching on strings.

[nicolasstucki]

We currenly do exactly the same as in scalac. We desugar it into if/then/else

[allanrenucci]

What's the warning emitted?

The test actually expect something of type Map[Nothing, Nothing].

[allanrenucci]

I tried with isSwitchableType and it seems to work if you widen before calling it:

isSwitchableType(scrutinee.tpe.widen)

[nicolasstucki]

done

[allanrenucci]

I would add two more tests:

• a pos test compiled with -Xfatal-warnings:

class Test {
  def test(x: 1) = (x: @annotation.switch) match {
    case 1 => 1
    case 2 => 2
    case 3 => 3
  }
}

• a neg test compiled with -Xfatal-warnings:

object Test {
  case class IntAnyVal(x: Int) extends AnyVal

  def test(x: IntAnyVal) = (x: @annotation.switch) match { //error: warning: could not emit switch
    case IntAnyVal(1) => 1
    case IntAnyVal(2) => 2
    case IntAnyVal(3) => 3
    case _ => 4
  }
}

[nicolasstucki]

Done

[allanrenucci]

LGTM. I think you can squash the commits since the history is not meaningful.