Merge pull request #2483 from adriaanm/merge-2.10.x

Merge 2.10.x

1 files changed, 34 insertions, 0 deletions

diff --git a/test/files/run/t7200.scala b/test/files/run/t7200.scala
new file mode 100644
index 0000000000..ba342df14d
--- /dev/null
+++ b/test/files/run/t7200.scala
@@ -0,0 +1,34 @@
+import language.higherKinds
+
+object Test extends App {
+
+  // Slice of comonad is where this came up
+  trait Foo[F[_]] {
+    def coflatMap[A, B](f: F[A] => B): F[A] => F[B]
+  }
+
+  // A non-empty list
+  case class Nel[A](head: A, tail: List[A])
+
+  object NelFoo extends Foo[Nel] {
+
+    // It appears that the return type for recursive calls is not inferred
+    // properly, yet no warning is issued. Providing a return type or
+    // type arguments for the recursive call fixes the problem.
+
+    def coflatMap[A, B](f: Nel[A] => B) = // ok w/ return type
+      l => Nel(f(l), l.tail match {
+        case Nil => Nil
+        case h :: t => {
+          val r = coflatMap(f)(Nel(h, t)) // ok w/ type args
+          r.head :: r.tail
+        }
+      })
+  }
+
+  // Without a recursive call all is well, but with recursion we get a
+  // ClassCastException from Integer to Nothing
+  NelFoo.coflatMap[Int, Int](_.head + 1)(Nel(1, Nil)) // Ok
+  NelFoo.coflatMap[Int, Int](_.head + 1)(Nel(1, List(2))) // CCE
+
+}