Fix #13455: Also consider sealed classes closed sums

Author: OlivierBlanvillain

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

@@ -2528,7 +2528,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
     /** Can we enumerate all instantiations of this type? */
     def isClosedSum(tp: Symbol): Boolean =
-      tp.is(Sealed) && tp.isOneOf(AbstractOrTrait) && !tp.hasAnonymousChild
+      tp.is(Sealed) && !tp.hasAnonymousChild
 
     /** Splits a closed type into a disjunction of smaller types.
      *  It should hold that `tp` and `decompose(tp).reduce(_ or _)`

tests/pos/13455.scala

@@ -0,0 +1,10 @@
+sealed class R
+
+type X[T] = T match {
+  case R        => String
+  case (z => r) => Int
+}
+def x[T]: X[T] = ???
+
+def i(i0: Int): Unit = ???
+val a = i(x[Int => String])