Comment by Ryan Govostes (JIRA)

A solution to the problem is to make the enum conform to the Comparable protocol, which requires a bit of boilerplate per this answer. I wonder what other implementation of the < operator you would ever use on an enum, though.

Enums aren't Comparable by default because you are normally allowed to reorder cases. Because cases can also have custom raw values, it's not clear whether you want to order by case order or by raw value order. (This is more likely to come up with String raw values.)

The diagnostics could definitely be better. cc @xedin

@swift-ci create

Compiler now produces a much better diagnostic which actually tells you why `>` can't be used:

error: protocol 'Comparable' requires that 'GameState' conform to 'Comparable'
if state > .BookOpened && state < .PageTurnedTwice {
         ^
note: where 'Self' = 'GameState'
public protocol Comparable: Equatable {
                ^

But I think we could go even further and actually point to the `>` in the diagnostic...

This is fixed in Swift 5.3. The compiler now reports:

error: referencing operator function '>' on 'Comparable' requires that 'GameState' conform to 'Comparable'
if state > .BookOpened && state < .PageTurnedTwice {
         ^
note: where 'Self' = 'GameState'
public protocol Comparable : Equatable {
                ^

Could you please verify using the latest Xcode 12 beta? Thank you!