Fix ParVector#padTo

This was throwing a UnsupportedOperationError for small operations.

The parallel collections test suite sets `-minSuccessfulTests 5` in
test/files/scalacheck/parallel-collections/pc.scala, which is far
lower thatn the default of 100, and means that we are less likely
to falsify properties.

This parameter seems to have been added in #2476, assuming I'm reading
it correctly. Not sure of the motiviation, perhaps just to make the
slowest part of the scalacheck test suite run faster?

I haven't changed the paramater now, but instead have included a one
element collection in generator.

I also found that when the test failed, Scalacheck would try to minimize
the example, but did so assuming that the elements of the tuple of
test data could be independentally shrunk. This breaks the invariant
that the two collections contain equal elements, and led to spurious
error reports. I have disabled shrinking in all tests tests affected
by this.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/library/scala/collection/parallel/immutable/package.scala b/src/library/scala/collection/parallel/immutable/package.scala
index 8fd84eaf4d..3cafdba5f7 100644
--- a/src/library/scala/collection/parallel/immutable/package.scala
+++ b/src/library/scala/collection/parallel/immutable/package.scala
@@ -20,7 +20,12 @@ package immutable {
     self =>
 
     def apply(idx: Int) = if (0 <= idx && idx < length) elem else throw new IndexOutOfBoundsException("" + idx)
-    override def seq = throw new UnsupportedOperationException
+    override def seq: collection.immutable.Seq[T] = new collection.AbstractSeq[T] with collection.immutable.Seq[T] {
+        override def length: Int = self.length
+        override def apply(idx: Int): T = self.apply(idx)
+        override def iterator: Iterator[T] = Iterator.continually(elem).take(length)
+        override def par: ParSeq[T] = self
+      }
     def update(idx: Int, elem: T) = throw new UnsupportedOperationException
 
     class ParIterator(var i: Int = 0, val until: Int = length, elem: T = self.elem) extends SeqSplitter[T] {