Merge pull request #13581 from dotty-staging/fix-10349

Fix #10349: Add regression test

Author: smarter

tests/neg-custom-args/allow-deep-subtypes/9849.scala

@@ -0,0 +1,11 @@
+object Firsts:
+
+  type First[X] = X match
+    case Map[_, v] => First[Option[v]]
+
+  def first[X](x: X): First[X] = x match
+    case x: Map[_, _] => first(x.values.headOption) // error
+
+  @main
+  def runFirsts2(): Unit =
+    assert(first(Map.empty[Int, Int]) == None) // error