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

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] =

src/compiler/scala/tools/nsc/transform/Mixin.scala

@@ -1215,9 +1215,24 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
           // refer to fields in some implementation class via an abstract
           // getter in the interface.
           val iface  = toInterface(sym.owner.tpe).typeSymbol
-          val getter = sym getter iface orElse abort("No getter for " + sym + " in " + iface)
+          val ifaceGetter = sym getter iface
+
+          def si6231Restriction() {
+            // See SI-6231 comments in LamdaLift for ideas on how to lift the restriction.
+            val msg = sm"""Implementation restriction: local ${iface.fullLocationString} is unable to automatically capture the
+                |free variable ${sym} on behalf of ${currentClass}. You can manually assign it to a val inside the trait,
+                |and refer that that val in ${currentClass}. For more details, see SI-6231."""
+            reporter.error(tree.pos, msg)
+          }
 
-          typedPos(tree.pos)((qual DOT getter)())
+          if (ifaceGetter == NoSymbol) {
+            if (sym.isParamAccessor) {
+              si6231Restriction()
+              EmptyTree
+            }
+            else abort("No getter for " + sym + " in " + iface)
+          }
+          else typedPos(tree.pos)((qual DOT ifaceGetter)())
 
         case Assign(Apply(lhs @ Select(qual, _), List()), rhs) =>
           // assign to fields in some implementation class via an abstract

test/files/neg/t6231.check

@@ -0,0 +1,6 @@
+t6231.scala:4: error: Implementation restriction: local trait Bug$X$1 is unable to automatically capture the
+free variable value ev$1 on behalf of anonymous class anonfun$qux$1. You can manually assign it to a val inside the trait,
+and refer that that val in anonymous class anonfun$qux$1. For more details, see SI-6231.
+      def qux = { () => ev }
+                        ^
+one error found

test/files/neg/t6231.scala

@@ -0,0 +1,15 @@
+object Bug {
+  def bar(ev: Any) = {
+    trait X {
+      def qux = { () => ev }
+    }
+    new X {}.qux()
+
+    // workaround
+    trait Y {
+      val ev2 = ev // manually capture `ev` so that `ev2` is added to the trait interface.
+      def qux = { () => ev2 }
+    }
+  }
+}
+