SI-5314 - CPS transform of return statement fails

Enable return expressions in CPS code if they are in tail position. Note that tail returns are
only removed in methods that do not call `shift` or `reset` (otherwise, an error is reported).

Addresses the issues pointed out in a previous pull request:
https://github.com/scala/scala/pull/720

- Addresses all issues mentioned here:
  https://github.com/scala/scala/pull/720#issuecomment-6429705

- Move transformation methods to SelectiveANFTransform.scala:
  https://github.com/scala/scala/pull/720#commitcomment-1477497

- Do not keep a list of tail returns.

Tests:
- continuations-neg/t5314-missing-result-type.scala
- continuations-neg/t5314-type-error.scala
- continuations-neg/t5314-npe.scala
- continuations-neg/t5314-return-reset.scala
- continuations-run/t5314.scala
- continuations-run/t5314-2.scala
- continuations-run/t5314-3.scala

2 files changed, 7 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Modes.scala b/src/compiler/scala/tools/nsc/typechecker/Modes.scala
index e9ea99faab..d942d080cb 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Modes.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Modes.scala
@@ -86,6 +86,10 @@ trait Modes {
    */
   final val TYPEPATmode   = 0x10000
 
+  /** RETmode is set when we are typing a return expression.
+   */
+  final val RETmode       = 0x20000
+
   final private val StickyModes   = EXPRmode | PATTERNmode | TYPEmode | ALTmode
 
   final def onlyStickyModes(mode: Int) =
@@ -133,4 +137,4 @@ trait Modes {
   def modeString(mode: Int): String =
     if (mode == 0) "NOmode"
     else (modeNameMap filterKeys (bit => inAllModes(mode, bit))).values mkString " "
-}
\ No newline at end of file
+}
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index a6d7424837..21bc890a20 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -4087,7 +4087,8 @@ trait Typers extends Modes with Adaptations with Tags {
             ReturnWithoutTypeError(tree, enclMethod.owner)
           } else {
             context.enclMethod.returnsSeen = true
-            val expr1: Tree = typed(expr, EXPRmode | BYVALmode, restpt.tpe)
+            val expr1: Tree = typed(expr, EXPRmode | BYVALmode | RETmode, restpt.tpe)
+            
             // 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