Rework MatchType recursion in collectParts

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -661,15 +661,9 @@ trait ImplicitRunInfo:
             case t: TypeLambda =>
               for p <- t.paramRefs do partSeen += p
               traverseChildren(t)
-            case t: MatchType =>
-              traverseChildren(t)
-              traverse(t.normalized)
-            case MatchType.InDisguise(mt)
-                if !t.isInstanceOf[LazyRef] // skip recursive applications (eg. Tuple.Map)
-            =>
-              traverse(mt)
             case t =>
               traverseChildren(t)
+              traverse(t.normalized)
       catch case ex: Throwable => handleRecursive("collectParts of", t.show, ex)
 
       def apply(tp: Type): collection.Set[Type] =

tests/pos/i17395-spec.ordered.scala

@@ -0,0 +1,17 @@
+trait ThingWithPart { type Part }
+type   PartField[A] = ThingWithPart { type Part = A }
+type ExtractPart[B] = B match { case PartField[a] => a }
+
+trait TC[C]
+object TC:
+  def tcForOptionPart[D](implicit tc: TC[ExtractPart[D]]): TC[Option[ExtractPart[D]]] = new {}
+
+class Value
+object Value:
+  implicit val tcValue: TC[Value] = new {}
+
+class ValuePartHolder extends ThingWithPart { type Part = Value }
+
+class Test:
+  def t1: Unit =
+    val tc = TC.tcForOptionPart[ValuePartHolder]

tests/pos/i19857.scala

@@ -0,0 +1,23 @@
+sealed trait DFTypeAny
+
+sealed trait DFTuple[T <: NonEmptyTuple] extends DFTypeAny
+
+sealed trait DFBit extends DFTypeAny
+
+sealed trait DFValOf[T]
+
+type Of[T] <: DFTypeAny = T match
+  case DFTypeAny => T & DFTypeAny
+  case Product   => FromProduct[T]
+
+type JUSTVAL[T] = DFValOf[Of[T]]
+
+type FromProduct[T <: Product] <: DFTypeAny = T match
+  case NonEmptyTuple => DFTuple[Tuple.Map[T, JUSTVAL]]
+
+trait Width2[T]
+
+object Width2:
+  inline given [T]: Width2[T] = new Width2[T] {}
+
+val x = summon[Width2[Of[(DFBit, DFBit)]]]