SI-7200 Test case for fixed type inference error.

Broken in 2.9.2 and 2.10.0, but working in 2.10.1

--- sandbox/2.10.0.log
+++ sandbox/2.10.1.log
       def coflatMap[A >: Nothing <: Any, B >: Nothing <: Any](f: Test.Nel[A] => B): Test.Nel[A] => Test.Nel[B] = ((l: Test.Nel[A]) => Test.this.Nel.apply[B](f.apply(l), l.tail match {
         case immutable.this.Nil => immutable.this.Nil
-        case (hd: A, tl: List[A])scala.collection.immutable.::[A]((h @ _), (t @ _)) => {
-          val r: Test.Nel[Nothing] = NelFoo.this.coflatMap[A, Nothing](f).apply(Test.this.Nel.apply[A](h, t));
+        case (hd: A, tl: List[A])scala.collection.immutable.::[?A1]((h @ _), (t @ _)) => {
+          val r: Test.Nel[B] = NelFoo.this.coflatMap[A, B](f).apply(Test.this.Nel.apply[A](h, t));
           {
-            <synthetic> val x$1: Nothing = r.head;
-            r.tail.::[Nothing](x$1)
+            <synthetic> val x$1: B = r.head;
+            r.tail.::[B](x$1)
           }
         }
       }))

b74c33eb86 represents the exact moment of progression. Comments
in pos/t7200b.scala, a minimal test that demonstrates the problem
without type constructors or code execution, pinpoint the line of
code responsible for the fix.

Incidentally, I'm currently on a train somewhere between Solothurn
and Biel, and am consequently without the power of scala-bisector.
Undeterred, and inspired by a line I saw in Skyfall last night
("sometimes the olds ways are better"), I just pulled off a two-hop
bisection. Take that, O(log N)!

The one remaining worry is the appearance of the type variable
?A1 in the output of -Xprint:typer for run/t7200.scala.

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
+
+}