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, 6 insertions, 0 deletions

diff --git a/test/files/neg/t6231.check b/test/files/neg/t6231.check
new file mode 100644
index 0000000000..b27961d393
--- /dev/null
+++ b/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