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, 50 insertions, 0 deletions

diff --git a/test/files/pos/t7200b.scala b/test/files/pos/t7200b.scala
new file mode 100644
index 0000000000..9d579c6ef9
--- /dev/null
+++ b/test/files/pos/t7200b.scala
@@ -0,0 +1,50 @@
+import language.higherKinds
+
+trait T {
+  def t = 0
+}
+trait Foo {
+  def coflatMap[A <: T](f: A): A
+}
+
+object O extends Foo {
+  def coflatMap[A <: T](f: A) = {
+    val f2 = coflatMap(f) // inferred in 2.9.2 / 2.10.0 as [Nothing]
+    f2.t                  // so this does't type check.
+    f2
+  }
+}
+
+// Why? When a return type is inherited, the derived method
+// symbol first gets a preliminary type assigned, based on the
+//   1) method type of a unique matching super member
+//   2) viewed as a member type of the inheritor (to substitute,
+//      e.g. class type parameters)
+//   3) substituted to replace the super-method's type parameters
+//      with those of the inheritor
+//   4) dissected to take just the return type wrapped in thisMethodType().
+//
+// In Scala 2.10.0 and earlier, this preliminary method type
+//
+//   1) [A#11329 <: <empty>#3.this.T#7068](<param> f#11333: A#11329)A#11329
+//   2) [A#11329 <: <empty>#3.this.T#7068](<param> f#11333: A#11329)A#11329
+//   3) (<param> f#12556: A#11336)A#11336
+//   4) [A#11336 <: <empty>#3.this.T#7068](<param> f#12552: A#11337&0)A#11336
+//
+// The type #4 from the old version is problematic: the parameter is typed with
+// a skolem for the type parameter `A`. It won't be considered to match the
+// method it overrides, instead they are seen as being overloaded, and type inference
+// goes awry (Nothing is inferred as the type argument for the recursive call
+// to coflatMap.
+//
+// The Namers patch adds one step here: it subsitutes the type parameter symbols
+// for the skolems:
+//
+//  https://github.com/scala/scala/commit/b74c33eb#L2R1014
+//
+// So we end up with a method symbol info:
+//
+//   5) [A#11336 <: <empty>#3.this.T#7068](<param> f#12505: A#11336)A#11336
+//
+// This *does* match the method in the super class, and type inference
+// chooses the correct type argument.
\ No newline at end of file