User can still be assigned to issues despite having no Assignable User permission

A user can still be assigned to an issue despite having no Assignable User permission in that project.
However, the criteria is that you must specify the Assignable User permission of the project's permission scheme to include the Reporter role.
Note that a user does not need to be the reporter, (in fact the user does not even need to be in any groups) to be assigned to issues in this project, as long as the Reporter role is added to the assignable user permission.

REPLICATION STEPS:

1. Install a fresh new JIRA 5.2.11 instance.
2. Create a new user, user_a. Remove all groups from this user.
3. Create a new project.
4. Modify the project's permission scheme to include the Reporter role into the Assignable User permission.
5. Now create a new issue. In the "Assignee" field, type in the username user_a in full. Click on the Create button.
6. Observe that the issue is created successfully, and the user is really assigned to the issue.

OTHER NOTES:

1. Typing the username of the user into the Assignee field does not auto-complete, however clicking on "Assign" or "create issue" button will still successfully assign the user.