Make LazyList.cons.apply lazier

Port of scala/scala#9095.

Co-authored-by: NthPortal <nthportal@gmail.com>

Author: ijuma

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

@@ -1264,7 +1264,7 @@ object LazyList extends SeqFactory[LazyList] {
      *  @param hd   The first element of the result lazy list
      *  @param tl   The remaining elements of the result lazy list
      */
-    def apply[A](hd: => A, tl: => LazyList[A]): LazyList[A] = newLL(sCons(hd, tl))
+    def apply[A](hd: => A, tl: => LazyList[A]): LazyList[A] = newLL(sCons(hd, newLL(tl.state)))
 
     /** Maps a lazy list to its head and tail */
     def unapply[A](xs: LazyList[A]): Option[(A, LazyList[A])] = #::.unapply(xs)

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

@@ -715,6 +715,20 @@ class LazyListLazinessTest {
   //   assertLazyAllSkipping(op, 4)
   // }
 
+  private def genericCons_unapply_properlyLazy(unapply: LazyList[Int] => Option[(Int, LazyList[Int])]): Unit = {
+    assertLazyAllSkipping(unapply, 1)
+  }
+
+  @Test
+  def cons_unapply_properlyLazy(): Unit = {
+    genericCons_unapply_properlyLazy(LazyList.cons.unapply)
+  }
+
+  @Test
+  def `#::_unapply_properlyLazy`(): Unit = {
+    genericCons_unapply_properlyLazy(LazyList.#::.unapply)
+  }
+
   /* factory laziness tests */
 
   @Test
@@ -776,14 +790,12 @@ class LazyListLazinessTest {
     assertRepeatedlyLazy(factory)
   }
 
-  @Test
-  def `#:: properlyLazy`(): Unit = {
+  private def genericCons_properlyLazy(cons: (=> Int, => LazyList[Int]) => LazyList[Int]): Unit = {
     val headInitFactory = lazyListFactory { init =>
       def gen(index: Int): LazyList[Int] = {
         def elem(): Int = { init.evaluate(index); index }
         if (index >= LazinessChecker.count) LazyList.empty
-        // else elem() #:: gen(index + 1)
-        else gen(index + 1).#::(elem())
+        else cons(elem(), gen(index + 1))
       }
 
       gen(0)
@@ -795,7 +807,7 @@ class LazyListLazinessTest {
         if (index >= LazinessChecker.count) LazyList.empty
         else {
           init.evaluate(index)
-          index #:: gen(index + 1)
+          cons(index, gen(index + 1))
         }
       }
 
@@ -804,6 +816,16 @@ class LazyListLazinessTest {
     assertRepeatedlyLazy(tailInitFactory)
   }
 
+  @Test
+  def cons_properlyLazy(): Unit = {
+    genericCons_properlyLazy(LazyList.cons(_, _))
+  }
+
+  @Test
+  def `#::_properlyLazy`(): Unit = {
+    genericCons_properlyLazy(_ #:: _)
+  }
+
   @Test
   def `#::: properlyLazy`(): Unit = {
     val headInitFactory = lazyListFactory { init =>