Even more careful handling of tailcalls.

See i321 doc for description of problem and decision taken.

1 files changed, 21 insertions, 5 deletions

diff --git a/tests/pos/tailcall/i321.scala b/tests/pos/tailcall/i321.scala
index 0d32dd625..33b3075de 100644
--- a/tests/pos/tailcall/i321.scala
+++ b/tests/pos/tailcall/i321.scala
@@ -1,10 +1,26 @@
+import scala.annotation.tailrec
+/**
+ * Illustrates that abstracting over type arguments without triggering Ycheck failure is tricky
+ *
+ * go1.loop refers to type parameter of i321, and captures value f
+ * if goo1.loop will abstract over T it will need to cast f or will trigger a Ycheck failure.
+ * One could decide to not abstract over type parameters in tail calls, but this leads us to go2 example
+ *
+ * In go2 we should abstract over i321.T, as we need to change it in recursive call.
+ *
+ * For now decision is such - we will abstract for top-level methods, but will not for inner ones.
+ */
+
 class i321[T >: Null <: AnyRef] {
 
-  def mapconserve(f: T => Int): Int = {
-    def loop(pending: T): Int = {
-        val head1 = f(pending)
-        loop(pending)
-      }
+  def go1(f: T => Int): Int = {
+    @tailrec def loop(pending: T): Int = {
+      val head1 = f(pending)
+      loop(pending)
+    }
     loop(null)
   }
+
+  final def go2[U >: Null <: AnyRef](t: i321[U]): Int = t.go2(this)
+
 }
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