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Keep opaques in match type scrutinee #20033

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Keep opaques in match type scrutinee #20033

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -3488,7 +3488,7 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
false

case MatchTypeCasePattern.AbstractTypeConstructor(tycon, argPatterns) =>
scrut.dealias match
scrut.dealiasKeepOpaques match
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This change is direct contradiction with the spec, and will break as many things as it will fix (since another use case might be to precisely take private knowledge into account to be able to perform a reduction).

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If I understood correctly, the following

object OpaqueScope:
  opaque type Opaque[N <: Any] = Double
  val foo: StripOpaque[Opaque[Int]] = 1

  // Note that match types within OpaqueScope are not limited by the absence of dealiasing
  type NeedsRhs[D] = D match
    case Double => String
  val bar: NeedsRhs[Opaque[Float]] = "hello"

is an example of what you are mentioning right ?

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Yes, exactly.

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Then it still works, and is part of the test.

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Oh yes because you only changed the dealiasing rule for the AbstractTypeConstructor case. You have to complicate the test a bit more so that it would also fall into that case to observe the issue.

case scrutDealias @ AppliedType(scrutTycon, args) if scrutTycon =:= tycon =>
matchArgs(argPatterns, args, tycon.typeParams, scrutIsWidenedAbstract)
case _ =>
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37 changes: 37 additions & 0 deletions tests/pos/i19326.scala
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@@ -0,0 +1,37 @@

import OpaqueScope.Opaque

type MapSum[D1, D2] = D1 match
case Opaque[l1] => D2 match
case Opaque[l2] => Opaque[Sum[l1, l2]]

object OpaqueScope:
opaque type Opaque[N <: NatT] = Double

val foo: MapSum[Opaque[Succ[Zero]], Opaque[Zero]] = 1.0
end OpaqueScope

sealed trait NatT derives CanEqual
case class Zero() extends NatT
case class Succ[N <: NatT](n: N) extends NatT

type Sum[M <: NatT, N <: NatT] <: NatT = (M, N) match
case (_, Zero) => M
case (Zero, _) => N
case (Succ[predM], Succ[predN]) => Succ[Succ[Sum[predM, predN]]]


object Minimized:

object OpaqueScope:
opaque type Opaque[N <: Any] = Double
val foo: StripOpaque[Opaque[Int]] = 1

// Note that match types within OpaqueScope are not limited by the absence of dealiasing
type NeedsRhs[D] = D match
case Double => String
val bar: NeedsRhs[Opaque[Float]] = "hello"

type StripOpaque[D] = D match
case OpaqueScope.Opaque[x] => x