ClassCastException: gp cannot be cast to gq

EDIT: my Jira search is weak; this is a duplicate of MC-67961. Feel free to close.

If a server with compression already enabled tries to modify the compression threshold, the client drops to the Connection Lost screen with "Internal Exception: java.lang.ClassCastException: gp cannot be cast to gq".

This exception originates inside method "gr.a(int)", which appears to have a copy/paste error where it performs an "instanceof" check and cast on different objects.

The error was produced using a custom server but it is certainly a client bug.