Match type on Map crashes the compiler

[weihsiu]

Minimized code

Scala 3.0.0-M1

object Firsts:

  type First[X] = X match
    case Map[_, v] => First[Option[v]]

  def first[X](x: X): First[X] = x match
    case x: Map[_, _] => first(x.values.headOption)

  @main
  def runFirsts2(): Unit =
    assert(first(Map.empty[Int, Int]) == None)

Output (click arrow to expand)

compiler crashes with the following exception.

[error] java.lang.NullPointerException [error] dotty.tools.dotc.core.NameKinds$UniqueNameKind.fresh(NameKinds.scala:222) [error] dotty.tools.dotc.typer.ProtoTypes$.newTypeVar(ProtoTypes.scala:560) [error] dotty.tools.dotc.core.TypeOps$.$anonfun$4(TypeOps.scala:755) [error] scala.collection.immutable.List.map(List.scala:246) [error] dotty.tools.dotc.core.TypeOps$.instantiateToSubType(TypeOps.scala:755) [error] dotty.tools.dotc.core.TypeOps$.refineUsingParent(TypeOps.scala:657) [error] dotty.tools.dotc.core.TypeComparer.decompose$2$$anonfun$1(TypeComparer.scala:2382) [error] scala.collection.immutable.List.map(List.scala:250) [error] dotty.tools.dotc.core.TypeComparer.decompose$1(TypeComparer.scala:2382) [error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2405) [error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2475) [error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2475) [error] dotty.tools.dotc.core.TrackingTypeComparer.matchCase$1(TypeComparer.scala:2768) [error] dotty.tools.dotc.core.TrackingTypeComparer.recur$2(TypeComparer.scala:2777) [error] dotty.tools.dotc.core.TrackingTypeComparer.op$proxy44$1(TypeComparer.scala:2798) [error] dotty.tools.dotc.core.TrackingTypeComparer.matchCases(TypeComparer.scala:2799) [error] dotty.tools.dotc.core.Types$MatchType.liftedTree1$2(Types.scala:4402) [error] dotty.tools.dotc.core.Types$MatchType.matchCases$1(Types.scala:4405) [error] dotty.tools.dotc.core.Types$MatchType.reduced$$anonfun$1(Types.scala:4406) [error] dotty.tools.dotc.core.TypeComparer.inSubComparer(TypeComparer.scala:2491) [error] dotty.tools.dotc.core.TypeComparer.tracked(TypeComparer.scala:2501) ...

[griggt]

With 3.1.0-RC2 it no longer crashes:

$ scalac -3.1.0-RC2 i10349.scala
-- Error: i10349.scala:7:30 ----------------------------------------------------
7 |    case x: Map[_, _] => first(x.values.headOption)
  |                              ^
  |                     Match type reduction failed since selector  Option[_]
  |                     matches none of the cases
  |
  |                         case Map[_, v] => Firsts.First[Option[v]]
-- Error: i10349.scala:11:16 ---------------------------------------------------
11 |    assert(first(Map.empty[Int, Int]) == None)
   |                ^
   |                Match type reduction failed since selector  Option[Int]
   |                matches none of the cases
   |
   |                    case Map[_, v] => Firsts.First[Option[v]]
2 errors found