[backport] SI-6482, lost bounds in extension methods.

Squashed commit of the following:

commit 5c156185306ba797c0443d9dccae0ae7ce462a1f
Author: Paul Phillips <paulp@improving.org>
Date:   Sat Oct 6 15:42:50 2012 -0700

    A little more housecleaning in ExtensionMethods.

    The only real contribution is readability.
    (cherry picked from commit 61f12faacaaccf366f9211ba6493fb042a91f1d2)

    Conflicts:
    	src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala

commit 79f443edf584745d614e24fb9ca6644c6b18d439
Author: Paul Phillips <paulp@improving.org>
Date:   Sat Oct 6 14:22:19 2012 -0700

    Incorporated pull request feedback.
    (cherry picked from commit 153ccb4757718cceb219988f30381f73362e6075)

commit 707f580b0cdcb01e27ca4c76991dea427945b5bd
Author: Paul Phillips <paulp@improving.org>
Date:   Sat Oct 6 10:20:45 2012 -0700

    Fix for SI-6482, lost bounds in extension methods.

    That was a good one. How to create a new method with type
    parameters from multiple sources, herein.
    (cherry picked from commit ff9f60f420c090b6716c927ab0359b082f2299de)

commit 8889c7a13f74bc175e48aa2209549089a974c2af
Author: Paul Phillips <paulp@improving.org>
Date:   Fri Oct 5 22:19:52 2012 -0700

    Responded to comment about how many isCoercibles there are.

    I make the case that there is only one.
    (cherry picked from commit 883f1ac88dd7cec5882d42d6b48d7f267d1f6e00)

1 files changed, 11 insertions, 0 deletions

diff --git a/test/files/pos/t6482.scala b/test/files/pos/t6482.scala
new file mode 100644
index 0000000000..24ea38e519
--- /dev/null
+++ b/test/files/pos/t6482.scala
@@ -0,0 +1,11 @@
+final class TraversableOnceOps[+A](val collection: TraversableOnce[A]) extends AnyVal {
+  def reduceLeftOption[B >: A](op: (B, A) => B): Option[B] =
+    if (collection.isEmpty) None else Some(collection.reduceLeft[B](op))
+}
+// error: type arguments [B] do not conform to method reduceLeft's type parameter bounds [B >: A]
+//            if (collection.isEmpty) None else Some(collection.reduceLeft[B](op))
+//                                                                        ^
+
+class Foo[+A <: AnyRef](val xs: List[A]) extends AnyVal {
+  def baz[B >: A](x: B): List[B] = x :: xs
+}