Stable hashCodes for case objects.

Somehow case objects fell through the cracks and have had default
hashCode implementations, leading to new hashCodes on each jvm run. Now
we use the productPrefix, and inline it right into the bytecode.

  scala> None.## == "None".##
  res0: Boolean = true

Closes #4752, no review.

1 files changed, 10 insertions, 0 deletions

diff --git a/test/files/run/bug4752.scala b/test/files/run/bug4752.scala
new file mode 100644
index 0000000000..3d5c166a7a
--- /dev/null
+++ b/test/files/run/bug4752.scala
@@ -0,0 +1,10 @@
+object Test {
+  object Bippy {
+    case object Dingus
+  }
+
+  def main(args: Array[String]): Unit = {
+    assert(None.## == "None".##, None)
+    assert(Test.Bippy.Dingus.## == "Dingus".##, Test.Bippy.Dingus)
+  }
+}