What I meant by flagging the renaming of the generic `flatMap` to the more
specific-to-this-case `filterMap` is that all of the current uses of
`flatMap` share conceptual details: they all process whatever is in the
box, then perform one level of flattening.
This shared concept is hugely
I accept the motivation for this change, although I would mention that:
- in 2.5 years on a ~200k lines of Swift project
- we've seen a plenty of instances of `flatMap` used where `map` would have
been sufficient, but
- we've never burned time on tracking down the sort of compiler issue
described
