> 2. What happens if package P (already installed on the user's system)
> starts recommending package Q (not installed on the user's system)? Will Q
> get auto-installed together with P's update, or not? I believe it's
> important to keep auto-installation enabled for *new* weak relationships.
New weak dependencies of package P are installed.
Installed P-1-1.noarch (no recommends)
Available P-1-2.noarch (recommends ddd) will install ddd on upgrade if possible.
> 3. Similarly to above (perhaps exactly the same case), what happens when
> package Q (not installed) starts supplementing package P (installed), will
> it get auto-installed or not?
No, Q will be not installed. With supplements it is difficult to known when it appears, because that information is not on RPMDB.
While it makes sense technically, this might be quite confusing for packagers. Up until now I think there were no real-world differences between forward (recommends) and backward (supplements) dependencies. This (and also the first answer) should get documented in the Change proposal and in the packaging guidelines [1]. Can you please add an action item to the proposal to adjust relevant Fedora docs?