Merge pull request #5741 from dotty-staging/fix-#4566

Fix #4566: Add regression test

Author: odersky

compiler/test-resources/repl/i4566

@@ -0,0 +1,5 @@
+scala> object test { type ::[A, B]; def a: Int :: Int = ???; def b: Int = a }
+1 | object test { type ::[A, B]; def a: Int :: Int = ???; def b: Int = a }
+  |                                                                    ^
+  |                                                      Found:    Int :: Int
+  |                                                      Required: Int