More lazylist tests

Bring back LazyCons, since that version also works now under the new rules

Author: odersky

tests/pos-custom-args/captures/lazylists.scala

@@ -22,7 +22,8 @@ extension [A](xs: {*} LazyList[A])
       def head: B = f(xs.head)
       def tail: {this} LazyList[B] = xs.tail.map(f)  // OK
       def concat(other: {f} LazyList[A]): {this, f} LazyList[A] = ??? : ({xs, f} LazyList[A]) // OK
-    new Mapped
+    if xs.isEmpty then LazyNil
+    else new Mapped
 
 def test(cap1: Cap, cap2: Cap) =
   def f(x: String): String = if cap1 == cap1 then "" else "a"
@@ -35,4 +36,7 @@ def test(cap1: Cap, cap2: Cap) =
       def isEmpty = false
       def head = f("")
       def tail = LazyNil
-    new Initial
+    new Initial
+  val xsc: {cap1} LazyList[String] = xs
+  val ys = xs.map(g)
+  val ysc: {cap1, cap2} LazyList[String] = ys

tests/pos-custom-args/captures/lazylists1.scala

@@ -0,0 +1,35 @@
+class CC
+type Cap = {*} CC
+
+trait LazyList[+A]:
+  this: ({*} LazyList[A]) =>
+
+  def isEmpty: Boolean
+  def head: A
+  def tail: {this} LazyList[A]
+
+object LazyNil extends LazyList[Nothing]:
+  def isEmpty: Boolean = true
+  def head = ???
+  def tail = ???
+
+class LazyCons[+T](val x: T, val xs: {*} () => {*} LazyList[T]) extends LazyList[T]:
+  this: ({*} LazyList[T]) =>
+
+  def isEmpty = false
+  def head = x
+  def tail: {this} LazyList[T] = xs()
+
+extension [A](xs: {*} LazyList[A])
+  def map[B](f: {*} A => B): {xs, f} LazyList[B] =
+    if xs.isEmpty then LazyNil
+    else LazyCons(f(xs.head), () => xs.tail.map(f))
+
+def test(cap1: Cap, cap2: Cap) =
+  def f(x: String): String = if cap1 == cap1 then "" else "a"
+  def g(x: String): String = if cap2 == cap2 then "" else "a"
+
+  val xs = LazyCons("", () => if f("") == f("") then LazyNil else LazyNil)
+  val xsc: {cap1} LazyList[String] = xs
+  val ys = xs.map(g)
+  val ysc: {cap1, cap2} LazyList[String] = ys