research_invention_state() trying to return -1 on error
research_invention_state() return type is enum tech_state, but it still has:
fc_assert_ret_val(NULL != valid_advance_by_number(tech), -1);
research_invention_state() return type is enum tech_state, but it still has:
fc_assert_ret_val(NULL != valid_advance_by_number(tech), -1);