Simplify the adaptation of types of return expressions

Add `adaptTypeOfReturn` hook to `AnnotationCheckers`.
Move adaptation of types of return expressions from `addAnnotations`
to `typedReturn` via `adaptTypeOfReturn` hook.

This resolves an inconsistency where previously types could have
a plus marker without additional CPS annotations. This also adds
additional test cases.

4 files changed, 53 insertions, 11 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 79c1e660f0..8ba2d9e0fd 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -4096,7 +4096,8 @@ trait Typers extends Modes with Adaptations with Tags {
               if (typed(expr).tpe.typeSymbol != UnitClass)
                 unit.warning(tree.pos, "enclosing method " + name + " has result type Unit: return value discarded")
             }
-            treeCopy.Return(tree, checkDead(expr1)) setSymbol enclMethod.owner setType NothingClass.tpe
+            treeCopy.Return(tree, checkDead(expr1)).setSymbol(enclMethod.owner)
+                                                   .setType(adaptTypeOfReturn(expr1, restpt.tpe, NothingClass.tpe))
           }
         }
       }
diff --git a/src/continuations/plugin/scala/tools/selectivecps/CPSAnnotationChecker.scala b/src/continuations/plugin/scala/tools/selectivecps/CPSAnnotationChecker.scala
index 122fe32ed3..eb96f87e0e 100644
--- a/src/continuations/plugin/scala/tools/selectivecps/CPSAnnotationChecker.scala
+++ b/src/continuations/plugin/scala/tools/selectivecps/CPSAnnotationChecker.scala
@@ -220,16 +220,32 @@ abstract class CPSAnnotationChecker extends CPSUtils with Modes {
         res
       } else if (retMode && !hasPlusMarker(tree.tpe) && annotsTree.isEmpty && annotsExpected.nonEmpty) {
         // add a marker annotation that will make tree.tpe behave as pt, subtyping wise
-        // tree will look like having no annotation
+        // tree will look like having any possible annotation
         
-        // note that we are only adding a plus marker if the method's result type is a CPS type
-        // (annotsExpected.nonEmpty == cpsParamAnnotation(pt).nonEmpty)
+        // note 1: we are only adding a plus marker if the method's result type is a cps type
+        //         (annotsExpected.nonEmpty == cpsParamAnnotation(pt).nonEmpty)
+        // note 2: we are not adding the expected cps annotations, since they will be added
+        //         by adaptTypeOfReturn (see below).
         val res = tree modifyType (_ withAnnotations List(newPlusMarker()))
         vprintln("adapted annotations (return) of " + tree + " to " + res.tpe)
         res
       } else tree
     }
 
+    // only adapt type if this return will
+    // (a) be removed (in tail position and method's result type (pt) is cps type), or
+    // (b) cause an error
+    override def adaptTypeOfReturn(tree: Tree, pt: Type, default: => Type): Type = {
+      // only adapt if method's result type (pt) is cps type
+      val annots = cpsParamAnnotation(pt)
+      if (annots.nonEmpty) {
+        // return type of `tree` without plus marker, but only if it doesn't have other cps annots
+        if (hasPlusMarker(tree.tpe) && !hasCpsParamTypes(tree.tpe))
+          tree.setType(removeAttribs(tree.tpe, MarkerCPSAdaptPlus))
+        tree.tpe
+      } else default
+    }
+
     def updateAttributesFromChildren(tpe: Type, childAnnots: List[AnnotationInfo], byName: List[Tree]): Type = {
       tpe match {
         // Would need to push annots into each alternative of overloaded type
@@ -483,13 +499,6 @@ abstract class CPSAnnotationChecker extends CPSUtils with Modes {
           }
           tpe
 
-        case ret @ Return(expr) =>
-          // only change type if this return will (a) be removed (in tail position) or (b) cause
-          // an error (not in tail position)
-          if (expr.tpe != null && hasPlusMarker(expr.tpe))
-            ret setType expr.tpe
-          ret.tpe
-
         case _ =>
           tpe
       }
diff --git a/src/continuations/plugin/scala/tools/selectivecps/SelectiveANFTransform.scala b/src/continuations/plugin/scala/tools/selectivecps/SelectiveANFTransform.scala
index 1783264e5c..ba87cadfeb 100644
--- a/src/continuations/plugin/scala/tools/selectivecps/SelectiveANFTransform.scala
+++ b/src/continuations/plugin/scala/tools/selectivecps/SelectiveANFTransform.scala
@@ -43,6 +43,12 @@ abstract class SelectiveANFTransform extends PluginComponent with Transform with
         case If(cond, r1 @ Return(thenExpr), r2 @ Return(elseExpr)) =>
           treeCopy.If(tree, cond, transform(thenExpr), transform(elseExpr))
 
+        case If(cond, r1 @ Return(thenExpr), elseExpr) =>
+          treeCopy.If(tree, cond, transform(thenExpr), transform(elseExpr))
+
+        case If(cond, thenExpr, r2 @ Return(elseExpr)) =>
+          treeCopy.If(tree, cond, transform(thenExpr), transform(elseExpr))
+
         case If(cond, thenExpr, elseExpr) =>
           treeCopy.If(tree, cond, transform(thenExpr), transform(elseExpr))
 
diff --git a/src/reflect/scala/reflect/internal/AnnotationCheckers.scala b/src/reflect/scala/reflect/internal/AnnotationCheckers.scala
index 9f102c5712..05f80c8a0c 100644
--- a/src/reflect/scala/reflect/internal/AnnotationCheckers.scala
+++ b/src/reflect/scala/reflect/internal/AnnotationCheckers.scala
@@ -47,6 +47,13 @@ trait AnnotationCheckers {
      *  before. If the implementing class cannot do the adaptiong, it
      *  should return the tree unchanged.*/
     def adaptAnnotations(tree: Tree, mode: Int, pt: Type): Tree = tree
+
+    /** Adapt the type of a return expression. The decision of an annotation checker
+     *  whether the type should be adapted is based on the type of the expression
+     *  which is returned, as well as the result type of the method (pt).
+     *  By default, this method simply returns the passed `default` type.
+     */
+    def adaptTypeOfReturn(tree: Tree, pt: Type, default: => Type): Type = default
   }
 
   // Syncnote: Annotation checkers inaccessible to reflection, so no sync in var necessary.
@@ -118,4 +125,23 @@ trait AnnotationCheckers {
     annotationCheckers.foldLeft(tree)((tree, checker) =>
       checker.adaptAnnotations(tree, mode, pt))
   }
+
+  /** Let a registered annotation checker adapt the type of a return expression.
+   *  Annotation checkers that cannot do the adaptation should simply return
+   *  the `default` argument.
+   *  
+   *  Note that the result is undefined if more than one annotation checker
+   *  returns an adapted type which is not a subtype of `default`.
+   */
+  def adaptTypeOfReturn(tree: Tree, pt: Type, default: => Type): Type = {
+    val adaptedTypes = annotationCheckers flatMap { checker =>
+      val adapted = checker.adaptTypeOfReturn(tree, pt, default)
+      if (!(adapted <:< default)) List(adapted)
+      else List()
+    }
+    adaptedTypes match {
+      case fst :: _ => fst
+      case List() => default
+    }
+  }
 }