As noted here, the change in #855 was not actually source compatible, since overload resolution doesn't distinguish methods by whether they are async or not. Code in an AsyncParsableCommand extension that previously called ParsableCommand.parse or parseAsRoot now calls the new async versions, causing a compilation failure:

         let command = try MyCommand.parseAsRoot()
//                     |   `- note: call is 'async'
//                     `- error: expression is 'async' but is not marked with 'await'

The fix for clients is to add the await keyword; working on a source compatible fix for the issue.