Increase laziness of #:: for LazyList

Port of scala/scala#8985.

Co-authored-by: NthPortal <[email protected]>

Author: ijuma

compat/src/main/scala-2.11_2.12/scala/collection/compat/immutable/LazyList.scala

@@ -1277,7 +1277,7 @@ object LazyList extends SeqFactory[LazyList] {
     /** Construct a LazyList consisting of a given first element followed by elements
      *  from another LazyList.
      */
-    def #::[B >: A](elem: => B): LazyList[B] = newLL(sCons(elem, l()))
+    def #::[B >: A](elem: => B): LazyList[B] = newLL(sCons(elem, newLL(l().state)))
 
     /** Construct a LazyList consisting of the concatenation of the given LazyList and
      *  another LazyList.

compat/src/test/scala/test/scala/collection/LazyListLazinessTest.scala

@@ -217,7 +217,7 @@ class LazyListLazinessTest {
   }
 
   @Test
-  def lazyAppendedAll_appendedAll_properlyLazy(): Unit = {
+  def lazyAppendedAll_properlyLazy(): Unit = {
     genericAppendedColl_properlyLazy(_ lazyAppendedAll _)
   }
 
@@ -778,7 +778,7 @@ class LazyListLazinessTest {
 
   @Test
   def `#:: properlyLazy`(): Unit = {
-    val factory = lazyListFactory { init =>
+    val headInitFactory = lazyListFactory { init =>
       def gen(index: Int): LazyList[Int] = {
         def elem(): Int = { init.evaluate(index); index }
         if (index >= LazinessChecker.count) LazyList.empty
@@ -788,12 +788,25 @@ class LazyListLazinessTest {
 
       gen(0)
     }
-    assertRepeatedlyLazy(factory)
+    assertRepeatedlyLazy(headInitFactory)
+
+    val tailInitFactory = lazyListFactory { init =>
+      def gen(index: Int): LazyList[Int] = {
+        if (index >= LazinessChecker.count) LazyList.empty
+        else {
+          init.evaluate(index)
+          index #:: gen(index + 1)
+        }
+      }
+
+      LazyList.empty lazyAppendedAll gen(0) // prevent initial state evaluation
+    }
+    assertRepeatedlyLazy(tailInitFactory)
   }
 
   @Test
   def `#::: properlyLazy`(): Unit = {
-    val factory = lazyListFactory { init =>
+    val headInitFactory = lazyListFactory { init =>
       def gen(index: Int): LazyList[Int] = {
         def elem(): LazyList[Int] = LazyList.fill(1) { init.evaluate(index); index }
         if (index >= LazinessChecker.count) LazyList.empty
@@ -802,7 +815,20 @@ class LazyListLazinessTest {
 
       gen(0)
     }
-    assertRepeatedlyLazy(factory)
+    assertRepeatedlyLazy(headInitFactory)
+
+    val tailInitFactory = lazyListFactory { init =>
+      def gen(index: Int): LazyList[Int] = {
+        if (index >= LazinessChecker.count) LazyList.empty
+        else {
+          init.evaluate(index)
+          LazyList.fill(1)(index) #::: gen(index + 1)
+        }
+      }
+
+      LazyList.empty lazyAppendedAll gen(0) // prevent initial state evaluation
+    }
+    assertRepeatedlyLazy(tailInitFactory)
   }
 
   @Test

compat/src/test/scala/test/scala/collection/LazyListTest.scala

@@ -161,7 +161,7 @@ class LazyListTest {
     cyc.tail.tail.head
     assertEquals("LazyList(1, 2, 3, <not computed>)", cyc.toString)
     cyc.tail.tail.tail.head
-    assertEquals("LazyList(1, 2, 3, 4, <cycle>)", cyc.toString)
+    assertEquals("LazyList(1, 2, 3, 4, <not computed>)", cyc.toString)
     cyc.tail.tail.tail.tail.head
     assertEquals("LazyList(1, 2, 3, 4, <cycle>)", cyc.toString)
   }