SI-9542 Fix regression in value classes with enclosing refs

In SI-9473 / e2653736, I changed `mkAttributedRef` to avoid prefixing
references to statically owned symbols with a `ThisType`.

Turns out there was one place that depended on the old behaviour.

```
object Outer {
  trait T
  class C(val value: Any) extends AnyVal {
	def foo(t: T) = expr
  }
}
```

Is translated to:

```
object Outer {
  trait T
  class C(val value: Any) extends AnyVal {
	def foo(t: T) = Outer.this.C.foo$extension(C.this, t)
  }
  object C {
    def foo$extension($this: C, t: T) = expr
  }
}
```

After the change, the forwarder was instead:

```
	def foo(t: T) = Outer.C.foo$extension(C.this, t)
```

Note: this change is not actually necessary after the following
commit that makes subtyping unify the different module class
reference, that alone is enough to make the enclosed test
pass.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala b/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala
index d83cf68b38..9d02228ab5 100644
--- a/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala
+++ b/src/compiler/scala/tools/nsc/transform/ExtensionMethods.scala
@@ -244,7 +244,10 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
 
           // These three lines are assembling Foo.bar$extension[T1, T2, ...]($this)
           // which leaves the actual argument application for extensionCall.
-          val sel        = Select(gen.mkAttributedRef(companion), extensionMeth)
+          // SI-9542 We form the selection here from the thisType of the companion's owner. This is motivated
+          //         by the test case, and is a valid way to construct the reference because we know that this
+          //         method is also enclosed by that owner.
+          val sel        = Select(gen.mkAttributedRef(companion.owner.thisType, companion), extensionMeth)
           val targs      = origTpeParams map (_.tpeHK)
           val callPrefix = gen.mkMethodCall(sel, targs, This(origThis) :: Nil)