Add regression test

Closes lampepfl/dotty-feature-requests#112

Author: nicolasstucki

tests/pos/f112.scala

@@ -0,0 +1,6 @@
+type X
+val x: X = ???
+extension (x: X) def unapply(arg: Any): Boolean = true
+def test =
+  ??? match
+    case x() =>