This stems from the open PR #15719 on Collection Routing.
That PR is very big and adds many new features to Rails, so I'd like to follow a conservative approach and add one feature at the time, making sure it's fully tested and backwards-compatible.
The first feature is to change the match of a path like GET /posts/1,3,4:
- currently, Rails matches to
posts#showwith the params set toid: "1,3,4" - the idea is instead to match to
posts#indexwith the params set toids: ["1", "3", "4"].