GADT matching with variant type parameters is not supported

[Jacoby6000]

This works fine if you make Expr invariant. It will not work with Covariance or Invariance. They both fail with the same error.

  trait Value

  sealed trait Expr[-I]
  case class Lit(i: Int) extends Expr[Value]

  def matchExpr[I](expr: Expr[I]): Expr[I] = 
    expr match {
      case Lit(i) => Lit(i)
    }

Error:

-- [E007] Type Mismatch Error: /tmp/scastie6519525892041936582/src/main/scala/main.scala:11:24 
11 |      case Lit(i) => Lit(i)
   |                     ^^^^^^
   |                     found:    Main.Lit
   |                     required: Main.Expr[I]

[smarter]

Our current implementation of GADTs is limited to invariant type parameters, which is hopefully a limitation we can lift (though this has to be done carefully to avoid issues like in section 3 of http://lampwww.epfl.ch/%7Ehmiller/scala2013/resources/pdfs/paper5.pdf), from Typer.scala:

    /** gadtSyms = "all type parameters of enclosing methods that appear
     *              non-variantly in the selector type" todo: should typevars
     *              which appear with variances +1 and -1 (in different
     *              places) be considered as well?
     */
    val gadtSyms: Set[Symbol] = ctx.traceIndented(i"GADT syms of $selType", gadts) {
      val accu = new TypeAccumulator[Set[Symbol]] {
        def apply(tsyms: Set[Symbol], t: Type): Set[Symbol] = {
          val tsyms1 = t match {
            case tr: TypeRef if (tr.symbol is TypeParam) && tr.symbol.owner.isTerm && variance == 0 =>
              tsyms + tr.symbol
            case _ =>
              tsyms
          }
          foldOver(tsyms1, t)
        }
      }
      accu(Set.empty, selType)
    }

[abgruszecki]

Fixed in PR mentioned above.