AC

• Visiting Special:SimilarEditors without the similareditors right shows an error
• Visiting Special:SimilarEditors shows the "Find similar users" form
• Submitting the form with an invalid value shows an error. Examples of invalid values are as follows:
  • "" (the empty string)
  • A username that doesn't exist
  • An IP address that isn't a valid IPv{4,6} address
• Submitting the form with a valid value submits a request to the similarusers service
• The response from the service is displayed as a standard paged (T304525) table, e.g. see system messages and blocked users

Design Spec

See also the parent task: T291509: [Epic] Similar Editors tool.