SI-6415, overly eager evaluation in Stream.

The lengthCompare method in LinearSeqOptimized was looking one
step further than it needed to in order to give the correct
result, which was creating some unwanted side effects related to
Streams.

4 files changed, 57 insertions, 13 deletions

diff --git a/src/library/scala/collection/LinearSeqOptimized.scala b/src/library/scala/collection/LinearSeqOptimized.scala
index 0f0a405a85..48b0b5469e 100755
--- a/src/library/scala/collection/LinearSeqOptimized.scala
+++ b/src/library/scala/collection/LinearSeqOptimized.scala
@@ -247,14 +247,20 @@ trait LinearSeqOptimized[+A, +Repr <: LinearSeqOptimized[A, Repr]] extends Linea
   }
 
   override /*SeqLike*/
-  def lengthCompare(len: Int): Int =  {
-    var i = 0
-    var these = self
-    while (!these.isEmpty && i <= len) {
-      i += 1
-      these = these.tail
+  def lengthCompare(len: Int): Int = {
+    // TODO: Remove this method when incrementing a major revision
+    // This method is the same as in SeqLike, no need to redefine it
+    if (len < 0) 1
+    else {
+      var i = 0
+      val it = iterator
+      while (it.hasNext) {
+        if (i == len) return if (it.hasNext) 1 else 0
+        it.next()
+        i += 1
+      }
+      i - len
     }
-    i - len
   }
 
   override /*SeqLike*/
diff --git a/src/library/scala/collection/SeqLike.scala b/src/library/scala/collection/SeqLike.scala
index f65e2ef9cd..1be0dba29f 100644
--- a/src/library/scala/collection/SeqLike.scala
+++ b/src/library/scala/collection/SeqLike.scala
@@ -84,13 +84,17 @@ trait SeqLike[+A, +Repr] extends Any with IterableLike[A, Repr] with GenSeqLike[
    *  if computing `length` is cheap.
    */
   def lengthCompare(len: Int): Int = {
-    var i = 0
-    val it = iterator
-    while (it.hasNext && i <= len) {
-      it.next()
-      i += 1
+    if (len < 0) 1
+    else {
+      var i = 0
+      val it = iterator
+      while (it.hasNext) {
+        if (i == len) return if (it.hasNext) 1 else 0
+        it.next()
+        i += 1
+      }
+      i - len
     }
-    i - len
   }
 
   override /*IterableLike*/ def isEmpty: Boolean = lengthCompare(0) == 0
diff --git a/test/files/run/streams.check b/test/files/run/streams.check
index 7f894052d9..db6d2eebab 100644
--- a/test/files/run/streams.check
+++ b/test/files/run/streams.check
@@ -1,5 +1,8 @@
 Stream()
 Stream()
+true
+true
+true
 
 Array(1)
 Stream(1, ?)
@@ -8,12 +11,21 @@ Stream()
 Stream()
 Stream(1)
 Stream()
+true
+true
+true
+true
 
 Array(1, 2)
 Stream(2)
 Stream()
 Stream(1, 2)
 Stream()
+true
+true
+true
+true
+true
 
 999
 512
@@ -23,3 +35,5 @@ Stream(100001, ?)
 true
 true
 705082704
+
+true
diff --git a/test/files/run/streams.scala b/test/files/run/streams.scala
index 51b4e5d76c..03b2622edd 100644
--- a/test/files/run/streams.scala
+++ b/test/files/run/streams.scala
@@ -2,6 +2,9 @@ object Test extends App {
   val s0: Stream[Int] = Stream.empty
   println(s0.take(1))
   println(s0.takeWhile(_ > 0))
+  println(s0.lengthCompare(-5) > 0)
+  println(s0.lengthCompare(0) == 0)
+  println(s0.lengthCompare(5) < 0)
   println
 
   val s1 = Stream.cons(1, Stream.empty)
@@ -12,6 +15,10 @@ object Test extends App {
   println(s1.drop(2))
   println(s1.drop(-1))
   println(s1.dropWhile(_ > 0))
+  println(s1.lengthCompare(-5) > 0)
+  println(s1.lengthCompare(0) > 0)
+  println(s1.lengthCompare(1) == 0)
+  println(s1.lengthCompare(5) < 0)
   println
 
   val s2 = s1.append(Stream.cons(2, Stream.empty))
@@ -20,6 +27,11 @@ object Test extends App {
   println(s2.drop(2))
   println(s2.drop(-1))
   println(s2.dropWhile(_ > 0))
+  println(s2.lengthCompare(-5) > 0)
+  println(s2.lengthCompare(0) > 0)
+  println(s2.lengthCompare(1) > 0)
+  println(s2.lengthCompare(2) == 0)
+  println(s2.lengthCompare(5) < 0)
   println
 
   val s3 = Stream.range(1, 1000) //100000 (ticket #153: Stackoverflow)
@@ -43,4 +55,12 @@ object Test extends App {
   println(Stream.from(1).take(size).foldLeft(0)(_ + _))
   val arr = new Array[Int](size)
   Stream.from(1).take(size).copyToArray(arr, 0)
+
+  println
+
+  // ticket #6415
+  lazy val x = { println("evaluated"); 1 }
+  val s4 = 0 #:: x #:: Stream.empty
+
+  println(s4.isDefinedAt(0))
 }