• What is your evaluation of the proposal?
Well written, concise and to the point. +1 on support for default values as well.
• Is the problem being addressed significant enough to warrant a change to Swift?
Absolutely, not to mention is part of our Generic Manifesto https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generic-subscripts and was raised before by Radek Pietruszewski on https://bugs.swift.org/browse/SR-115
• Does this proposal fit well with the feel and direction of Swift?
Yes, this fits very well with how generic code is written right now.
• How much effort did you put into your review? A glance, a quick reading, or an in-depth study?