SI-9331 Fix canEqual for case classes with HK type params

Time for the courage of our convictions: follow the advice of my
TODO comment from SI-8244 / f62e280825 and fix `classExistentialType`
once and for all.

This is the change in the generated `canEquals` method in the test
case; we no longer get a kind conformance error.

```
--- sandbox/old.log	2015-05-27 14:31:27.000000000 +1000
+++ sandbox/new.log	2015-05-27 14:31:29.000000000 +1000
@@ -15,7 +15,7 @@
       case _ => throw new IndexOutOfBoundsException(x$1.toString())
     };
     override <synthetic> def productIterator: Iterator[Any] = runtime.this.ScalaRunTime.typedProductIterator[Any](Stuff.this);
-    <synthetic> def canEqual(x$1: Any): Boolean = x$1.$isInstanceOf[Stuff[Proxy[PP]]]();
+    <synthetic> def canEqual(x$1: Any): Boolean = x$1.$isInstanceOf[Stuff[_ <: [PP]Proxy[PP]]]();
     override <synthetic> def hashCode(): Int = ScalaRunTime.this._hashCode(Stuff.this);
     override <synthetic> def toString(): String = ScalaRunTime.this._toString(Stuff.this);
     override <synthetic> def equals(x$1: Any): Boolean = x$1 match {
@@ -38,9 +38,3 @@
   }
 }
```

I also heeded my own advice to pass in a prefix to this method.

1 files changed, 6 insertions, 0 deletions

diff --git a/test/files/pos/t9331.scala b/test/files/pos/t9331.scala
new file mode 100644
index 0000000000..00a667886f
--- /dev/null
+++ b/test/files/pos/t9331.scala
@@ -0,0 +1,6 @@
+import scala.language.higherKinds
+
+trait Proxy[+T]
+case class Stuff[+P[PP] <: Proxy[PP]]() {
+  // canEqual was incorrectly synthetized and started reporting a kind error.
+}