Fix scala#10349: Add regression test

Author: OlivierBlanvillain

tests/neg/10349.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