NM VPN does not allow IPv6-only connections?

[Joel Goguen <jgoguen jgoguen ca>]

When connecting to a VPN using NetworkManager (specifically tried using
OpenConnect but the bug looks to be in the NetworkManager core code) with IPv4
disabled, connecting fails. NetworkManager attempts to get an IPv4 configuration
anyway and fails if it isn't present.

On one VPN, with IPv4 disabled and no IPv4 DHCP in use, connecting failed
because no IPv4 config could be found. With the same VPN profile, IPv4 disabled,
but IPv4 addresses being handed out via DHCP, NetworkManager still looks for a
connection and configures IPv4 even though it's disabled.

On a VPN with no IPv4 available, IPv4 disabled in NM:

Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <info>  [1539381667.4587]
vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-65304bb12667,"VPN",0]: VPN
connection: (IP Config Get) reply received.
Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <info>  [1539381667.4601]
vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-
65304bb12667,"VPN",8:(vpn0)]: VPN connection: (IP4 Config Get) reply received
Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <warn>  [1539381667.4604]
vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-
65304bb12667,"VPN",8:(vpn0)]: invalid IP4 config received!
Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <warn>  [1539381667.4606]
vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-
65304bb12667,"VPN",8:(vpn0)]: VPN connection: did not receive valid IP config
information

On a VPN with IPv4 available, IPv4 disabled in NM, the connection succeeds and
an IPv4 address is assigned.

With a wireless network, disabling IPv4 works as expected.

Reported as 
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/62 but that
hasn't seen any activity in 3 months…

-- 
Joel Goguen