Merge pull request #8446 from dotty-staging/fix-#6565

Fix #6565: Add test case

Author: odersky

tests/neg/i6565.scala

@@ -0,0 +1,17 @@
+class Err
+
+type Lifted[A] = Err | A
+
+def point[O](o: O): Lifted[O] = o
+def (o: Lifted[O]) map [O, U] (f: O => U): Lifted[U] = ???
+def (o: Lifted[O]) flatMap [O, U] (f: O => Lifted[U]): Lifted[U] = ???
+
+val error: Err = Err()
+
+lazy val ok: Lifted[String] = { // ok despite map returning a union
+  point("a").map(_ => if true then "foo" else error) // error
+}
+
+lazy val bad: Lifted[String] = { // found Lifted[Object]
+  point("a").flatMap(_ => point("b").map(_ => if true then "foo" else error)) // error
+}