Keep opaques in match type scrutinee

Fixes scala#19326

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -3488,7 +3488,7 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
                 false
 
           case MatchTypeCasePattern.AbstractTypeConstructor(tycon, argPatterns) =>
-            scrut.dealias match
+            scrut.dealiasKeepOpaques match
               case scrutDealias @ AppliedType(scrutTycon, args) if scrutTycon =:= tycon =>
                 matchArgs(argPatterns, args, tycon.typeParams, scrutIsWidenedAbstract)
               case _ =>

tests/pos/i19326.scala

@@ -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
+