Thank you @tkremenek for bringing this to review even if the outcome wasn't entirely conclusive. In order to avoid situations like this in future could I suggest with appropriate temerity that we introduce a "pre-review" state into the Swift Evolution pipeline. There are 34 open PRs on the swift-evolution repo that could come to review at any time. Would it be possible to schedule proposals of interest to the core team formally by the act of merging the PR (automatically notifying the proposal author and allocating the SE-NNNN number) at least two weeks ahead of time. This would create a sort of "pre-review lobby" which could be on a dashboard somewhere where authors would be motivated to fine tune their proposal, ensure the implementation is up to date and receive final comments from the community before the proposal text is finalised.
All that said, I was able to reproduce a toolchain for the original implementation last night and can give definitive if rather belated answers to some of the questions that came up during the review. I'll not post the toolchain as it is from a time before ABI stability and only builds/works with Xcode 10.1.
This was marked as an error by the implementation. Internally, parsing of collection literals read ahead an expression and a token (skipping any conditionals) to see if the token was ':' or ',' to decide the dictionary/array-ness of the literal and goes down two distinct code paths once that has been determined.
In the original implementation which you can see from my reply to @rintaro's post it was decided to code it to require the trailing ',' inside conditionals. This wasn't strictly necessary and is probably a decision I'd revisit as it creates the need to add commas inside a conditional even if it is defines the last element of the collection.