Regarding this thread, as well as Jens' July 2016 posts, about parentheses, tuples, etc.: Jens' patience and enthusiasm for this topic are remarkable, and his writing on the subject is excellent. Thank you, Jens. Also, kudos to Letan and Thomas for their efforts and sharp counterpoints. Great discussion, all around.
These issues are a big deal. No matter their outcome, they have real implications for the long-run development of the Swift language.
I would be glad to see (or, if need be, help to create) a top-down decision tree for how parentheses are parsed and type checked by the compiler. I suspect that the parser and type checker are missing subtle opportunities to infer the intent of parentheses from context. I also suspect that there are edge cases where inference is impossible. Airing that sort of document would show us the specific problems, inform the discussion, and also might lead to creative solutions from the community.
The compiler has been built and modified over a long period of time by a lot of people. My impression is that, out of practical necessity, many modifications have been made as local patches rather than systemic integrations. At this stage, any real change to how parentheses are handled probably involves a frightening amount of retooling within the compiler. The work and the attendant risks of error are daunting, but waiting has its own costs. Whatever happens, great respect is due to the core team; they do amazing work.
Please see the commentary on SR-8089. In particular, there may be some significant under-the-hood work on tuples in the near future. My supposition is that that work may clear the way for better discussions about above-the-hood aspects of the issues addressed by this thread.
Let's find a way to keep this thread near the top of the heap.