Allow SAM types to contain match alias refinements

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

@@ -5892,7 +5892,7 @@ object Types extends TypeUtils {
 
       /** Copy type aliases refinements to `toTp` from `fromTp` */
       def withRefinements(toType: Type, fromTp: Type): Type = fromTp.dealias match
-        case RefinedType(fromParent, name, info: TypeAlias) if tp0.member(name).exists =>
+        case RefinedType(fromParent, name, info: AliasingBounds) if tp0.member(name).exists =>
           val parent1 = withRefinements(toType, fromParent)
           RefinedType(toType, name, info)
         case _ => toType

tests/pos/i20080.scala

@@ -0,0 +1,32 @@
+
+trait Zippable[-A, -B]:
+  type Out
+  def zip(left: A, right: B): Out
+
+object Zippable extends ZippableLowPrio:
+  given append[A <: Tuple, B]: (Zippable[A, B] { type Out = Tuple.Append[A, B] }) =
+    (left, right) => left :* right
+
+trait ZippableLowPrio:
+  given pair[A, B]: (Zippable[A, B] { type Out = (A, B) }) =
+    (left, right) => (left, right)
+
+
+object Minimization:
+
+  trait Fun1:
+    type Out
+    def apply(x: Any): Out
+
+  type M[X] = X match
+    case String => X
+
+  def test[A] =
+
+    val _: Fun1 { type Out = M[A] } = new Fun1:
+      type Out = M[A]
+      def apply(x: Any): Out = ???
+
+    val _: Fun1 { type Out = M[A] } = x => ???
+
+    val _: Fun1 { type Out = A match {case String => A} } = x => ???