Add back lazylists

Author: liufengyun

tests/init/neg/lazylist1.scala

@@ -0,0 +1,20 @@
+class LazyList[A]
+
+object LazyList {
+  inline implicit def toDeferred[A](l: LazyList[A]): Deferred[A] =
+    new Deferred(l)
+
+  final class Deferred[A](l: => LazyList[A]) {
+    def #:: [B >: A](elem: => B): LazyList[B] = ???
+  }
+}
+
+import LazyList._
+
+final class Test {
+  lazy val a: LazyList[Int] = 5 #:: b
+  lazy val b: LazyList[Int] = 10 #:: a
+
+  val x: LazyList[Int] = 5 #:: y
+  val y: LazyList[Int] = 10 #:: x  // error
+}

tests/init/neg/lazylist2.scala

@@ -0,0 +1,43 @@
+import scala.language.implicitConversions
+
+trait LazyList[+A] {
+  def isEmpty: Boolean = true
+  def head: A
+  def tail: LazyList[A]
+}
+
+object LazyList {
+  inline implicit def toHelper[A](l: => LazyList[A]): Helper[A] = new Helper(l)
+  final class Helper[A](l: => LazyList[A]) {
+    def #:: [B >: A](elem: => B): LazyList[B] = new LazyList[B] {
+      override def isEmpty: Boolean = false
+      override def head: B = elem
+      override def tail: LazyList[B] = l
+    }
+  }
+}
+
+import LazyList._
+
+final class Test1 {
+  lazy val a: LazyList[Int] = 5 #:: b
+  val b: LazyList[Int] = 10 #:: a
+
+  a.head // ok
+  b.head // ok
+
+  val x: LazyList[Int] = 5 #:: y
+  val y: LazyList[Int] = 10 #:: x   // error
+}
+
+final class Test2 {
+  lazy val a: LazyList[Int] = 5 #:: b
+  lazy val b: LazyList[Int] = 10 #:: 30 #:: c
+  val c: LazyList[Int] = 20 #:: a
+}
+
+final class Test3 {
+  val a: LazyList[Int] = n #:: a
+  a.head
+  val n: Int = 20    // error
+}