Some operators against empty collection should throw UnsupportedOperationException

core/src/main/scala/scala/collection/parallel/ParIterableLike.scala

@@ -358,6 +358,8 @@ extends IterableOnce[T @uncheckedVariance]
    *  if this $coll is empty.
    */
   def reduce[U >: T](op: (U, U) => U): U = {
+    if (isEmpty) throw new UnsupportedOperationException("empty.reduce")
+
     tasksupport.executeAndWaitResult(new Reduce(op, splitter)).get
   }
 
@@ -463,10 +465,14 @@ extends IterableOnce[T @uncheckedVariance]
   }
 
   def min[U >: T](implicit ord: Ordering[U]): T = {
+    if (isEmpty) throw new UnsupportedOperationException("empty.min")
+
     tasksupport.executeAndWaitResult(new Min(ord, splitter)).get.asInstanceOf[T]
   }
 
   def max[U >: T](implicit ord: Ordering[U]): T = {
+    if (isEmpty) throw new UnsupportedOperationException("empty.max")
+
     tasksupport.executeAndWaitResult(new Max(ord, splitter)).get.asInstanceOf[T]
   }
 

junit/src/test/scala/scala/collection/parallel/mutable/ParArrayTest.scala

@@ -85,6 +85,11 @@ class ParArrayTest extends scala.collection.concurrent.ctries_old.Spec {
     assert( ParArray(1,2,3,4,5).reduce(_+_) == 15 )
   }
 
+  @Test
+  def `empty reduce`: Unit = {
+    evaluating { ParArray.empty[Int].reduce(_+_) }.shouldProduce[UnsupportedOperationException]()
+  }
+
   @Test
   def `simple count`: Unit = {
     assert( ParArray[Int]().count(_ > 7) == 0 )
@@ -127,4 +132,25 @@ class ParArrayTest extends scala.collection.concurrent.ctries_old.Spec {
   def `simple map test`: Unit = {
     assert(ParArray(1,2,3,4,5).map( (_:Int) * 10 ) == ParArray(10,20,30,40,50))
   }
+
+  @Test
+  def `empty min`: Unit = {
+    evaluating { ParArray.empty[Int].min }.shouldProduce[UnsupportedOperationException]()
+  }
+
+  @Test
+  def `empty max`: Unit = {
+    evaluating { ParArray.empty[Int].max }.shouldProduce[UnsupportedOperationException]()
+  }
+
+  @Test
+  def `empty minBy`: Unit = {
+    evaluating { ParArray.empty[String].minBy(_.length) }.shouldProduce[UnsupportedOperationException]()
+  }
+
+  @Test
+  def `emtpy maxBy`: Unit = {
+    evaluating { ParArray.empty[String].maxBy(_.length) }.shouldProduce[UnsupportedOperationException]()
+  }
+
 }