SI-7474 Record extra errors in Throwable#suppressedExceptions

... in parallel collection operations.

Followup to bcbe38d, which did away with the the approach to
use a composite exception when more than one error happened.

src/library/scala/collection/parallel/Tasks.scala

@@ -66,13 +66,10 @@ trait Task[R, +Tp] {
   }
 
   private[parallel] def mergeThrowables(that: Task[_, _]) {
-    // TODO: As soon as we target Java >= 7, use Throwable#addSuppressed
-    // to pass additional Throwables to the caller, e. g.
-    // if (this.throwable != null && that.throwable != null)
-    //   this.throwable.addSuppressed(that.throwable)
-    // For now, we just use whatever Throwable comes across “first”.
-    if (this.throwable == null && that.throwable != null)
-      this.throwable = that.throwable
+     if (this.throwable != null && that.throwable != null)
+       this.throwable.addSuppressed(that.throwable)
+     else if (this.throwable == null && that.throwable != null)
+       this.throwable = that.throwable
   }
 
   // override in concrete task implementations to signal abort to other tasks

test/files/run/t5375.check

@@ -1 +0,0 @@
-Runtime exception

test/files/run/t5375.scala

@@ -1,8 +1,16 @@
 object Test extends App {
   val foos = (1 to 1000).toSeq
-  try
-    foos.par.map(i => if (i % 37 == 0) sys.error("i div 37") else i)
-  catch {
-    case ex: RuntimeException => println("Runtime exception")
+  try {
+    foos.par.map(i => if (i % 37 == 0) throw new MultipleOf37Exception(i) else i)
+    assert(false)
+  } catch {
+    case ex: MultipleOf37Exception =>
+      assert(ex.getSuppressed.size > 0)
+      assert(ex.getSuppressed.forall(_.isInstanceOf[MultipleOf37Exception]))
+      assert(ex.i == 37)
+      assert(ex.getSuppressed.map(_.asInstanceOf[MultipleOf37Exception].i).toList == List(74, 148, 259, 518))
+    case _: Throwable =>
+      assert(false)
   }
+  class MultipleOf37Exception(val i: Int) extends RuntimeException
 }