How to make Apt prefer a more recent virtual package over a real one

[actionmystique]

Hello guys,

I've recently built a package which provides multiple virtual ones, more specifically snmp which provides libsnmp-base, libsnmp30 and so on...
It is installed on my system & provided on my online PPA.
When I try to install another package (php7.0-snmp) which depends on libsnmp30 & libsnmp-base, Synaptics offers me to uninstall snmp in order to install the required dependencies.
It seems that Apt always prefers real packages over virtual ones, despite the facts that:

the virtual ones have a more recent version
they are already installed


How can I install php7.0-snmp without having to uninstall snmp?

[actionmystique]

Aptitude offers more solutions, but none involves the preferred & most logical one: keep snmp because it already offers the most recent needed dependencies.

sudo aptitude install php7.0-snmpThe following NEW packages will be installed:
libsnmp30{a} php7.0-snmp
0 packages upgraded, 2 newly installed, 0 to remove and 1 not upgraded.
Need to get 832 kB of archives. After unpacking 3,382 kB will be used.
The following packages have unmet dependencies:
snmp : Conflicts: libsnmp30 but 5.7.3+dfsg-1ubuntu4 is to be installed.
The following actions will resolve these dependencies:

Remove the following packages:
1) snmp

Install the following packages:
2) libsnmp-base [5.7.3+dfsg-1ubuntu4 (xenial)]

Accept this solution? [Y/n/q/?] n
The following actions will resolve these dependencies:

Keep the following packages at their current version:
1) libsnmp30 [Not Installed]
2) php7.0-snmp [Not Installed]

Accept this solution? [Y/n/q/?] n
The following actions will resolve these dependencies:

Install the following packages:
1) libsnmp-base [5.7.3+dfsg-1ubuntu4 (xenial)]

Downgrade the following packages:
2) snmp [5.8~git20160427.8d07349f-14 (now) -> 5.7.3+dfsg-1ubuntu4 (xenial)]

Accept this solution? [Y/n/q/?] n

*** No more solutions available ***

[actionmystique]

The explanation lies into a very strange debian policy (https://www.debian.org/doc/debian-policy/ch-relationships.html#s-virtual) which does not make sense and should be changed
"...the version number of the concrete package which provides a particular virtual package will not be considered when considering a dependency on or conflict with the virtual package name.".
One workaround is to offer transitional packages for all the virtual ones.