We have Role of QA manager that was granted permission to Assign Issues. Developer role had been also set in Assignable Users section of the permission scheme.
Now if I set User for my QA manager with QA Manager Role and ask him to re-assign the issue to Developer - he doesn't see Developer users in the Asssignee list. The only options available are: his own username and Unassigned.
In compare we have PM role that was setup with same permissions as QA Manager role - and users set with role of PM can see all Developers names.