Fix StackOverflowError in Page.latest

shared/src/main/scala/scala/util/parsing/input/PagedSeq.scala

@@ -243,7 +243,12 @@ private class Page[T: ClassTag](val num: Int) {
   /** The last page as currently present in the sequence; This can change as more
    *  elements get appended to the sequence.  */
   final def latest: Page[T] = {
-    if (later.next != null) later = later.next.latest
+    var oldLater = later
+    while (later.next != null) later = later.next
+    while (oldLater.next != null) {
+      oldLater = oldLater.next
+      oldLater.later = later
+    }
     later
   }
 

shared/src/test/scala/scala/util/parsing/input/gh64.scala

@@ -0,0 +1,16 @@
+package scala.util.parsing.input
+
+import org.junit.Assert._
+import org.junit.Test
+
+class gh64 {
+
+  @Test
+  def test: Unit = {
+    val len = 4096 * 20000
+    val i = Iterator.fill(len)(true)  // use `true` to make this test more lightweight
+    val pagedSeq = PagedSeq.fromIterator(i)
+    pagedSeq.slice(len - 1)  // load the whole pagedSeq without caching `latest` element
+    assertEquals(len, pagedSeq.length)  // should not throw StackOverflowError
+  }
+}