We've seen a lot of examples with clarity problems in this thread, but not all of them are related to trailing closures or argument labels, and IMO if we're going to make a change to the language, it's really important to be clear about which problems it solves. As I have said before, the core problem with filter is that we don't know the polarity of the predicate, and seeing where: does nothing whatsoever to help with that. The only cure for making these calls clearer at the call site is to get that information into the name, e.g.
a.selecting { x in x % 2 == 0 }