CPS: enable return expressions in CPS code if they are in tail position

Adds a stack of context trees to AnnotationChecker(s). Here, it is used to
enforce that adaptAnnotations will only adapt the annotation of a return
expression if the expected type is a CPS type.

The remove-tail-return transform is reasonably general, covering cases such as
try-catch-finally. Moreover, an error is thrown if, in a CPS method, a return
is encountered which is not in a tail position such that it will be removed
subsequently.

2 files changed, 22 insertions, 0 deletions

diff --git a/test/files/continuations-neg/ts-1681-nontail-return.check b/test/files/continuations-neg/ts-1681-nontail-return.check
new file mode 100644
index 0000000000..8fe15f154b
--- /dev/null
+++ b/test/files/continuations-neg/ts-1681-nontail-return.check
@@ -0,0 +1,4 @@
+ts-1681-nontail-return.scala:10: error: return expressions in CPS code must be in tail position
+      return v
+      ^
+one error found
diff --git a/test/files/continuations-neg/ts-1681-nontail-return.scala b/test/files/continuations-neg/ts-1681-nontail-return.scala
new file mode 100644
index 0000000000..af86ad304f
--- /dev/null
+++ b/test/files/continuations-neg/ts-1681-nontail-return.scala
@@ -0,0 +1,18 @@
+import scala.util.continuations._
+
+class ReturnRepro { 
+  def s1: Int @cpsParam[Any, Unit] = shift { k => k(5) } 
+  def caller = reset { println(p(3)) }
+
+  def p(i: Int): Int @cpsParam[Unit, Any] = { 
+    val v= s1 + 3 
+    if (v == 8)
+      return v
+    v + 1
+  } 
+}
+
+object Test extends App {
+  val repro = new ReturnRepro
+  repro.caller
+}