Comment 13 for bug 1633479

I can also confirm trusty and precise as shown above.
They behave differently, in that instead of exiting failure, dhclient actually tries again and ends up getting an address. So in precise you would see:

| # dhclient -6 -1 -v $dev
| Internet Systems Consortium DHCP Client 4.1-ESV-R4
| Copyright 2004-2011 Internet Systems Consortium.
| All rights reserved.
| For info, please visit https://www.isc.org/software/dhcp/
|
| Bound to *:546
| Listening on Socket/eth0
| Sending on Socket/eth0
| PRC: Confirming active lease (INIT-REBOOT).
|
| XMT: Forming Confirm, 0 ms elapsed.
| XMT: X-- IA_NA 3e:28:e6:37
| XMT: | X-- Confirm Address fd42:198e:d2a5:1d10:1e4c:ac80:7f2e:f59e
| XMT: V IA_NA appended.
| XMT: Confirm on eth0, interval 920ms.
| send_packet6: Cannot assign requested address
| dhc6: sendpacket6() sent -1 of 84 bytes
| [***** short pause here *****]
| XMT: Forming Confirm, 920 ms elapsed.
| XMT: X-- IA_NA 3e:28:e6:37
| XMT: | X-- Confirm Address fd42:198e:d2a5:1d10:1e4c:ac80:7f2e:f59e
| XMT: V IA_NA appended.
| XMT: Confirm on eth0, interval 1910ms.
| send_packet6: Cannot assign requested address
| dhc6: sendpacket6() sent -1 of 84 bytes
| [***** short pause here *****]
| XMT: Forming Confirm, 2830 ms elapsed.
| XMT: X-- IA_NA 3e:28:e6:37
| XMT: | X-- Confirm Address fd42:198e:d2a5:1d10:1e4c:ac80:7f2e:f59e
| XMT: V IA_NA appended.
| XMT: Confirm on eth0, interval 3710ms.
| RCV: Reply message on eth0 from fe80::3896:5cff:fe61:baf0.
| RCV: X-- Server ID: 00:01:00:01:1f:c5:e1:fb:b8:ae:ed:75:5f:9a
| message status code Success: "all addresses still on link"
| PRC: Bound to lease 00:01:00:01:1f:c5:e1:fb:b8:ae:ed:75:5f:9a.

After the fix you see:
| # dhclient -6 -1 -v $dev
| Internet Systems Consortium DHCP Client 4.1-ESV-R4
| Copyright 2004-2011 Internet Systems Consortium.
| All rights reserved.
| For info, please visit https://www.isc.org/software/dhcp/
|
| Bound to *:546
| Listening on Socket/eth0
| Sending on Socket/eth0
| PRC: Confirming active lease (INIT-REBOOT).
|
| XMT: Forming Confirm, 0 ms elapsed.
| XMT: X-- IA_NA 3e:28:e6:37
| XMT: | X-- Confirm Address fd42:198e:d2a5:1d10:1e4c:ac80:7f2e:f59e
| XMT: V IA_NA appended.
| XMT: Confirm on eth0, interval 920ms.
| send_packet6: Cannot assign requested address
| dhc6: sendpacket6() sent -1 of 84 bytes

So I'm marking this fixed on precise and trusty also as we fix the error messages like:
  send_packet6: Cannot assign requested address
  dhc6: sendpacket6() sent -1 of 84 bytes