Merge pull request scala#10903 from som-snytt/issue/13052-slice-overflow

Split the SliceIterator on drop overflow [ci: last-only]

Author: lrytz

src/library/scala/collection/Iterator.scala

@@ -1257,12 +1257,15 @@ object Iterator extends IterableFactory[Iterator] {
         else if (until <= lo) 0               // empty
         else if (unbounded) until - lo        // now finite
         else adjustedBound min (until - lo)   // keep lesser bound
+      val sum = dropping + lo
       if (rest == 0) empty
+      else if (sum < 0) {
+        dropping = Int.MaxValue
+        remaining = 0
+        this.concat(new SliceIterator(underlying, start = sum - Int.MaxValue, limit = rest))
+      }
       else {
-        dropping = {
-          val sum = dropping + lo
-          if (sum < 0) Int.MaxValue else sum
-        }
+        dropping = sum
         remaining = rest
         this
       }

test/junit/scala/collection/IteratorTest.scala

@@ -221,6 +221,15 @@ class IteratorTest {
     assertSameElements(List(3) ++ List(1, 2, 3).reverseIterator.drop(1), List(3, 2, 1))
   }
 
+  @Test def `drop does overflow t13052`: Unit = {
+    var counter = 0L
+    val it = Iterator.continually(counter.tap(_ => counter += 1))
+    val n = 1_000_000_000
+    val dropped = it.drop(n).drop(n).drop(n).drop(50)
+    assertEquals(3L * n + 50L, dropped.next())
+    assertEquals(3L * n + 100L, dropped.drop(49).next())
+  }
+
   @Test def dropIsChainable(): Unit = {
     assertSameElements(1 to 4, Iterator from 0 take 5 drop 1)
     assertSameElements(3 to 4, Iterator from 0 take 5 drop 3)
@@ -891,7 +900,7 @@ class IteratorTest {
   }
 
   @Test
-  def t11106(): Unit = {
+  def `t11106 still lazy after withFilter`: Unit = {
     var i = 0
     Iterator.continually(0)
       .map(_ => {i += 1; i})