Add dependent function test

Author: odersky

tests/pos/depfun.scala

@@ -0,0 +1,17 @@
+// The following test is derived from scala/reflect/TypeTest.scala, but using
+// a dependent function instead of a dependent SAM. It shows that the special treatment
+// using a DependentTypeTree is not needed for plain function types.
+// But for SAM types, the treatment is needed, otherwise TypeTest.scala does
+// not typecheck. Todo: Figure out the reason for this difference.
+object Test:
+
+  type F[S, T] = (x: S) => Option[x.type & T]
+
+  /** Trivial type test that always succeeds */
+  def identity[T]: F[T, T] = Some(_)
+
+  val x: 1 = 1
+  val y = identity(x)
+  val z: Option[1] = y
+
+