TYPO3 Core

Possible solution would be to use the "suggests" array in the emconf to announce such optional "dependencies".
In your case you would add realurl to the suggests constraints.

In case a suggested extension is depending on the suggesting one, the dependencies take precedence for the loading order.

+1 for Markus idea.

Well it was tmaroschik's idea while we discussed this. I just posted it. ;-)

Ok this is more and more a real problem. rtehtmlarea needs to be loaded after setup extension in order to add the rte settings to the user settings. This is not a dependency and not really a suggestion. We could misuse "suggests" for this, but it does not really seem right to me. :-(

I'm working on a first draft.

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30006

Can be tested with #58694.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30005

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30006

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30005

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30005

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30005

Is it not strange to use the "suggest" option for this? Don't we need new options like "before" and/or "after"?

If you unsinstall both setup and rtehtmlarea, and then install rtehtmlarea, without first installing setup, the extension manager does not even care about suggesting to install setup first...

suggest does not mean that EM should "force" you to install an extension.
I understand it more as a listing of what the author thinks is a good addition to his extension.

Markus Klein wrote:

suggest does not mean that EM should "force" you to install an extension.
I understand it more as a listing of what the author thinks is a good addition to his extension.

My understanding is that the extension manager should "suggest" to install it, and I think it used to do so.

Ok, but with a checkbox in front to have it optional.

Still the question remains, can we introduce a before/after condition.

In fact, the extension being installed could be a pre-requisite to some of its "suggested" extensions.

Stanislas: Have a look at the unit tests please.

This is the reason why dependencies take precedence over suggestions.
So if a "suggested" extension depends on the ext being installed (directly or indirectly), the suggestion is not taken into account for loading order calculation.

Can we say in this regard that "suggest" would be a soft dependency? In case the suggested extension is installed, the "suggest" becomes a "require" for the Package Manager which will compute the correct order of extensions accordingly. This is how I understand it...

Yes that's how this should be.