empty instantiations of self types

[CeylonMigrationBot]

[@gavinking] We could add some reasoning around types like Comparable<Anything> which are empty due to the self type constraint. Perhaps just say that Nothing covers Comparable<Anything>, since I'm not sure if saying Comparable<Anything> is exactly Nothing would be decidable.

These case is distinguishable because:

• the type parameter is a self type and is not covariant, and
• the type argument does not inherit the type (and is not marked out).

[Migrated from ceylon/ceylon-spec#1383]

[CeylonMigrationBot]

[@RossTate] Is this practical? If I recall, the analysis you ran a while ago showed that no one uses self types in ways that would be relevant to this optimization.

[CeylonMigrationBot]

[@gavinking] Not sure if it's useful or not. But after looking at the code, there is no really obvious place to incorporate this reasoning without complicating the algorithms. So: not for 1.2.