conversions. We limit the relationship between the result of the
conversion and the actual expected type to a subtyping relationship,
which effectively limits us to applying a single user-defined
conversion function (rather than a chain of such conversions).
Later, we can consider extending this to a chain of conversion
functions, but that will require us to cope with cyclic conversions
and minimizing conversion sequences (e.g., if A -> B -> C and A -> C
are both possible, we should prefer A -> C).
Swift SVN r2730
