apt appears to be confused when installing a backport that version depends on other backports

This was seen in trusty where backports are pretty widely used.

We backported the whole lxc+lxd stack (golang, libseccomp, cgmanager, lxcfs, lxc, lxd) with some added version dependencies everywhere it made sense so that when pulling lxd, everything would be installed with the right version.

The assumption was that since lxd didn't exist in 14.04 and that as backports are enabled there by default, a simple "apt-get install lxd" would work.

That's not the case, it fails with:
root@blah:~# apt-get install lxd
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 lxd : Depends: lxc (>= 1.1.4-0ubuntu1.1~) but 1.0.7-0ubuntu0.9 is to be installed
       Recommends: lxd-client but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Showing more resolver details show:
 Broken lxd:amd64 Depends on lxc [ amd64 ] < none -> 1.0.7-0ubuntu0.9 | 1.1.4-0ubuntu1.1~ubuntu14.04.1 > ( admin ) (>= 1.1.4-0ubuntu1.1~)
 Considering lxc:amd64 0 as a solution to lxd:amd64 9999
 Re-Instated liblxc1:amd64
 Re-Instated python3-lxc:amd64
 Re-Instated lxc:amd64

I then tried installing with "apt-get install lxd/trusty-backports" which got me roughly the same thing:
root@blah:~# apt-get install lxd/trusty-backports
Reading package lists... Done
Building dependency tree
Reading state information... Done
Selected version '0.21-0ubuntu3~ubuntu14.04.1' (Ubuntu:14.04/trusty-backports [amd64]) for 'lxd'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 lxd : Depends: lxc (>= 1.1.4-0ubuntu1.1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

So in the end, I had to use "apt-get -t trusty-backports install lxd" which did succeed in pulling everything from backports.

This seems like an apt bug as apt is clever enough to go pick things from backports when they're available nowhere else, one would expect that it would do the same when picking the dependencies of such a backport when they are in the same pocket as the backport, instead of failing to resolve the dependencies.

It'd be great if people could use a straight apt-get install for non-existing packages or the /trusty-backports packages to pull specific packages from backports during install, instead of having to rely on a separate apt-get run with -t trusty-backports specified.

Also it looks like our existing backports documentation points users to the /trusty-backports syntax which will not work for any reasonably complex backport which involves more than one package.