scala/scala — pull request #1946 by retronym (2013-01-21 21:15:49+00:00)
Title: SI-6231 Report unsupported free var capture by a trait.

_Does not close the ticket, merely advances a crasher to an implementation restriction._

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

Review by @gkossakowski