Widen skolem in scrutinee when normalizing match aliase

Co-Authored-By: Dale Wijnand <dale.wijnand@gmail.com>

Author: mbovel

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

@@ -4478,7 +4478,15 @@ object Types {
           case MatchAlias(alias) =>
             trace(i"normalize $this", typr, show = true) {
               MatchTypeTrace.recurseWith(this) {
-                alias.applyIfParameterized(args.map(_.normalized)).tryNormalize
+                alias.applyIfParameterized(args.map(_.normalized)) match
+                  case mt @ MatchType(bound, scrutinee, cases) =>
+                    val scrutinee1 = scrutinee.widenSkolem
+                    if scrutinee1 ne scrutinee then
+                      mt.derivedMatchType(bound, scrutinee.widenSkolem, cases).normalized
+                    else
+                      mt.tryNormalize
+                  case mt =>
+                    mt.tryNormalize
               }
             }
           case _ =>

tests/pos/16583.scala

@@ -0,0 +1,15 @@
+import scala.compiletime.constValueTuple
+
+val ll0: Tuple3["one", "two", "three"] = constValueTuple[("one", "two", "three")]
+val ll1 = constValueTuple[("one", "two", "three")].toList
+val ll3: List["one" | ("two" | ("three" | Nothing))] = constValueTuple[("one", "two", "three")].toList
+val ll4: List["one" | ("two" | "three")] = constValueTuple[("one", "two", "three")].toList
+
+inline def labels[Labels <: Tuple](using ev: Tuple.Union[Labels] <:< String): List[String] =
+  val tmp = constValueTuple[Labels].toList
+  ev.substituteCo( tmp )
+
+def test = labels[("one", "two", "three")]
+
+def toList(x: Tuple): List[Tuple.Union[x.type]] = ???
+def test2[Labels <: Tuple] = toList((???): Labels)

tests/pos/16654.scala

@@ -0,0 +1,7 @@
+def toCsvFlat[A <: Product](a: A)(using m: scala.deriving.Mirror.ProductOf[A]) = {
+  def flatTuple(any: Any): Tuple = any match
+    case p: Product => p.productIterator.map(flatTuple).foldLeft(EmptyTuple: Tuple)(_ ++ _)
+    case a          => Tuple1(a)
+
+  val tuple = flatTuple(Tuple.fromProductTyped(a)).toList
+}