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, 18 insertions, 0 deletions

diff --git a/test/files/neg/t963.scala b/test/files/neg/t963.scala
new file mode 100644
index 0000000000..0cc2034299
--- /dev/null
+++ b/test/files/neg/t963.scala
@@ -0,0 +1,18 @@
+import scala.util.Random
+
+// Only y1 (val/val) should actually compile.
+object Test {
+  val r = new Random()
+
+  val y1 : { val x : java.lang.Integer  } = new { val x = new java.lang.Integer(r.nextInt) }
+  val w1 : y1.x.type = y1.x
+
+  val y2 : { val x : java.lang.Integer  } = new { def x = new java.lang.Integer(r.nextInt) }
+  val w2 : y2.x.type = y2.x
+
+  val y3 : { def x : java.lang.Integer  } = new { val x = new java.lang.Integer(r.nextInt) }
+  val w3 : y3.x.type = y3.x
+
+  val y4 : { def x : java.lang.Integer  } = new { def x = new java.lang.Integer(r.nextInt) }
+  val w4 : y4.x.type = y4.x
+}