This opens the door to possibly being smarter about the existing Data.Monoid.First (and Last), encoding them as pattern synonyms for Maybe (Data.Semigroup.First a) (or Last) as well, but we'd likely need to give that a lot more thought / testing.
The potential downside is using a big scary extension in what will become a fairly mainstream module in 8.4, but I think we can phrase our way around
Yes First/Last is a good use for pattern synonyms the issue when it comes to the libraries part of Haskell-Prime. As for this proposal I am in no hurry to see it implemented, I figured pattern synonyms would take time to get adopted to base. What do you mean by "phrase our way" around the issue? (The pattern synonyms could be added to the documentation. . "If you are okay with PatternSynonyms..")