SI-6231 Report unsupported free var capture by a trait.

If a class nested in a trait captures a free variable from
the enclosing scope of the trait, the transformation to
add that variable to the `init` method of the trait
implementation class happens *after* the abstract trait
interface has been extracted. This would lead to a crash
when trying to find the corresponding interface method.

This commit detects this situation and reports an
implementation restriction. The enclosed test case
shows a workaround.

To lift this restriction, LambdaLifter should add the getters
and make sure they end up in the trait interface. Looks like
Martin tried this once:

    // LambdaLift.scala
    //
    // Disabled attempt to to add getters to freeParams
    // this does not work yet. Problem is that local symbols need local names
    // and references to local symbols need to be transformed into
    // method calls to setters.
    // def paramGetter(param: Symbol): Tree = {
    //   val getter = param.newGetter setFlag TRANS_FLAG resetFlag PARAMACCESSOR // mark because we have to add them to interface
    //   sym.info.decls.enter(getter)
    //   val rhs = Select(gen.mkAttributedThis(sym), param) setType param.tpe
    //   DefDef(getter, rhs) setPos tree.pos setType NoType
    // }
    // val newDefs = if (sym.isTrait) freeParams ::: (ps map paramGetter) else freeParams

1 files changed, 2 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/LambdaLift.scala b/src/compiler/scala/tools/nsc/transform/LambdaLift.scala
index 4a23e65ad2..cd2bffcfde 100644
--- a/src/compiler/scala/tools/nsc/transform/LambdaLift.scala
+++ b/src/compiler/scala/tools/nsc/transform/LambdaLift.scala
@@ -352,6 +352,7 @@ abstract class LambdaLift extends InfoTransform {
 
             copyDefDef(tree)(vparamss = List(vparams ++ freeParams))
           case ClassDef(_, _, _, _) =>
+            // SI-6231
             // Disabled attempt to to add getters to freeParams
             // this does not work yet. Problem is that local symbols need local names
             // and references to local symbols need to be transformed into
@@ -369,7 +370,7 @@ abstract class LambdaLift extends InfoTransform {
         tree
     }
 
-/*  Something like this will be necessary to eliminate the implementation
+/*  SI-6231: Something like this will be necessary to eliminate the implementation
  *  restiction from paramGetter above:
  *  We need to pass getters to the interface of an implementation class.
     private def fixTraitGetters(lifted: List[Tree]): List[Tree] =