Currently CC experiment allows types likeList[{*} Proc], with my favorite aliastype Proc = Unit => Unit. It seems to me such types should be rejected as ill-formed. The basic argument is that according to the formalism, all elements of such a list should be in an unopenable box. Since we don't have boxes in the surface syntax, the most reasonable thing to do seems to be simply forbidding such types.

Even an immutable collection likeList could be used to leak scoped capabilities if we can use{*} Proc as its elements. Similarly, a mutable collection likeListBuffer[{*} Proc] could be easily used to leak a scoped capability and use it outside of its scope.

However, forbidding such types rules out the most natural way to migrate existing code to capture checking, i.e. simply putting{*} as the c.s. of every type. If we are concerned about this, perhaps we should introduce an annotation like@unsafeCap that could be used to allow such types, as in:List[@unsafeCap {*} Proc]?

/cc@odersky