No static check for pattern match on Identifiers

[bishabosha]

Minimized code

object UnitTest extends App {
  def foo(m: Unit) = m match {
    case runtime.BoxedUnit.UNIT => println("ok")
  }
  foo(())
}

Output

compiles and prints "ok"

Expectation

error like in Scala 2

UnitTest.scala:4: error: type mismatch;
 found   : scala.runtime.BoxedUnit
 required: Unit
    case runtime.BoxedUnit.UNIT => println("ok")
                           ^
1 error

[nicolasstucki]

The code seems to be technically correct. runtime.BoxedUnit.UNIT is the only value of type Unit. Not sure if we should disallow this.

[nicolasstucki]

Scala 2 compact issues are for those that scala 2 supports but scala 3.0 does not. Here it is the opposite.

[bishabosha]

ok I guess it is a philosophical debate whether or not to allow a case that is a supertype because potentially it could match but maybe its a useless branch

trait Trait

object UnitTest extends App {
  val Foo: AnyRef = ""
  def foo(m: Trait) = m match {
    case Foo => println("ok")
  }
  foo(Foo)
}

Scala 2 has the same behaviour here.
This is a silly example Im thinking now anyway because its the same argument when you match on Any

[nicolasstucki]

That should not compile. There should be an error in foo(Foo).

[bishabosha]

Sorry yeah the confusion comes from if it is a type error to pass the value Foo to expected type Trait then why can you match on the value Foo, and then the answer would be that the runtime class of Foo could be, unless its not even a concern because == is defined on Any

[dwijnand]

I think this can be closed. The BoxedUnit.Unit specific example isn't worth considering, as it's scala.runtime. The more general trait Trait; val Foo: AnyVal; (m: Trait) match { case Foo => } example is fine as there's nothing to suggest that m isn't <: Trait & Foo.type. I learnt this week that that check is called isPopulated in nsc.