Summary

If a user doesn't exist and you use a rest call like /rest/api/2/search?jql=assignee was admin, then Jira will return 400 Bad Request error rather than a warning that the user doesn't exist.

Steps to Reproduce

1. Create a TestUser
2. Create an issue, and assign it to TestUser
3. Assign the issue to another user, or unassign it
4. Delete the TestUser account
5. Execute the following REST call (since it's a GET, you can browse to it directly):

   $jira-url/rest/api/2/search?jql=assignee%20was%20testuser
   

Expected Results

The REST API should return a 200 OK response, with the warning message that the user does not exist. This is the behaviour when the query is assignee = testuser for example.

Actual Results

The REST API returns a 400 Bad Request response, with the warning message that the user does not exists.

Workaround

Performing a POST with the JQL as JSON data should return the results correctly. For example:

curl -X "POST" "http://localhost:47121/j7121/rest/api/2/search" \
     -u 'username:password' \
     -d $'{"jql": "assignee was testuser"}'

Notes

We notice the same issue in the UI too, where '=' is a warning and 'was' is an error