Fix for one of the oldest open soundness bugs.

Closes SI-963, since it was one of my random 30 it won the prize.
The trick after adding the stability check (which has been sitting
there commented out for 3+ years) was that implicit search depended
on the wrongness, because memberWildcardType would create scopes
with members of the form

  ?{ val name: tp }

And since a def shouldn't match that, fixing it broke everything
until I flipped it around: memberWildcardType should be seeking

  ?{ def name: tp }

It could also search for a mutable value: the relevant quality
is that it not be stable so it doesn't have a tighter type than
the members it hopes to match.

1 files changed, 2 insertions, 2 deletions

diff --git a/test/files/buildmanager/t2556_3/t2556_3.check b/test/files/buildmanager/t2556_3/t2556_3.check
index 37808d2b31..34f90f7f9b 100644
--- a/test/files/buildmanager/t2556_3/t2556_3.check
+++ b/test/files/buildmanager/t2556_3/t2556_3.check
@@ -9,10 +9,10 @@ invalidate B.scala because it references invalid (no longer inherited) definitio
 compiling Set(B.scala, C.scala)
 B.scala:3: error: type mismatch;
  found   : C
- required: ?{val x: ?}
+ required: ?{def x: ?}
 Note that implicit conversions are not applicable because they are ambiguous:
  both method any2Ensuring in object Predef of type [A](x: A)Ensuring[A]
  and method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A]
- are possible conversion functions from C to ?{val x: ?}
+ are possible conversion functions from C to ?{def x: ?}
     println( (new C).x )
               ^