Changes a GenServer callback return into another valid OTP return tuple. It runs only in modules that use or declare the GenServer behaviour.

The mutator covers return values from handle_call/3, handle_cast/2, handle_info/2, and handle_continue/2:

handle_call/3
  {:reply, reply, new_state}                     ->  {:noreply, new_state}
  {:reply, reply, new_state, action}             ->  {:noreply, new_state, action}
  {:noreply, new_state}                          ->  {:stop, :normal, new_state}
  {:noreply, new_state, action}                  ->  {:stop, :normal, new_state}
  {:stop, reason, new_state}                     ->  {:noreply, new_state}
  {:stop, reason, reply, new_state}              ->  {:reply, reply, new_state}

handle_cast/2, handle_info/2, handle_continue/2
  {:noreply, new_state}                          ->  {:stop, :normal, new_state}
  {:noreply, new_state, action}                  ->  {:stop, :normal, new_state}
  {:stop, reason, new_state}                     ->  {:noreply, new_state}

Here action is a timeout, :hibernate, or {:continue, term()}. Each mutation preserves the original state, reply, and action where the new tuple accepts them.

Other tuple shapes do not match. This excludes init/1 returns, :ignore, two-element {:stop, reason} tuples, and returns from callbacks such as terminate/2 and code_change/3. Actions and stop reasons are not mutated.