Enabling TLS certificate validation in the unit tests (CDRIVER-3579) revealed that IPv6 hostname matching on macOS is buggy. Specifically, the logic does not consider the IPv6 addresses 0:0:0:0:0:0:0:1 and [::1] to be equivalent. As a result, a certificate trust failure is encountered in that situation. Once that is fixed, the workaround code in test-mongoc-client.c should be removed. It can be found by searching for the reference to this ticket.