This looks nice at first sight (maybe as a future direction, since this does not look required for the pitch to provide value).
Yet beware: such a conformance extension would ONLY accept declarations that fulfill requirements. This is the hard version of the "group protocol conformances in extensions" styling, because even related methods would become forbidden in such an extension:
conformance extension SortedArray: Collection {
var first: ... // OK
var last: ... // OK
var median: ... // error: property 'median' does not fulfill any protocol requirement.
}
To me, this is the exact point were something nice that always help turns into an ill-designed tool that frequently annoys. I let the community consider if they prefer something good or something too perfect.
As you say, @sveinhal, we'd ignore methods that do not match the visibility of the protocol:
conformance extension SortedArray: Collection {
var first: ... // OK
var last: ... // OK
private func someHelper() { ... } // OK
}