...
    unittest::Barrier barrier(2);
    ConnectionThread connectThread(tf.tla().listenerPort(), [&](ConnectionThread& conn) {
        ON_BLOCK_EXIT([&] { barrier.countDownAndWait(); }); // This line is causing the build failure.

        // Linger timeout = 0 causes a RST packet on close.
        struct linger sl = {1, 0};
        if (setsockopt(conn.socket().rawFD(),
                       SOL_SOCKET,
                       SO_LINGER,
                       reinterpret_cast<SetsockoptPtr>(&sl),
                       sizeof(sl)) != 0) {
            auto err = make_error_code(std::errc{errno});
            LOGV2_ERROR(6060300, "setsockopt", "error"_attr = err.message());
        }
    });
...