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.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 2bdae4164a..7e4f50ecd7 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3987,7 +3987,11 @@ trait Typers extends Modes with Adaptations with Tags {
             ReturnWithoutTypeError(tree, enclMethod.owner)
           } else {
             context.enclMethod.returnsSeen = true
+            //TODO: also pass enclMethod.tree, so that adaptAnnotations can check whether return is in tail position
+            pushAnnotationContext(tree)
             val expr1: Tree = typed(expr, EXPRmode | BYVALmode, restpt.tpe)
+            popAnnotationContext()
+            
             // Warn about returning a value if no value can be returned.
             if (restpt.tpe.typeSymbol == UnitClass) {
               // The typing in expr1 says expr is Unit (it has already been coerced if