Alire has the ability to express dependencies that can be fulfilled by one of several projects (like OPAM does with the pipe operator). This supports at least two use cases:

• We need one of two libraries, no matter which one (e.g. some crypto, some sockets...).
• We do not want to bring in heavy dependencies if we can do with lighter ones.
  • In turn this can simplify finding solutions, since less libs are depended upon.

The first case I think is pretty simple. It appears, e.g., when there are no other conditions (like a lib is only available in windows and another on linux) on which to select one of several possible candidates.

The second case is a bit more tricky. In the old index it was tested with gnatcoll. It came in three flavors:

• gnatcoll, which depends in turn on gpr and maybe other libs
• gnatcoll.slim, which has no dependencies (and loses anything that depended on that)
• gnatcoll.xstrings, which is only the XString type from gnatcoll.

To make proper use of this, the old index had two features:

• "renaming" field, that was used by gnatcoll.slim and gnatcoll.xstrings to express that, at the time of resolution, they are equivalent to gnatcoll (i.e., once one of the three is included in a solution, the "gnatcoll" dependency is fulfilled.
• "or" operator in dependencies, that was used to express an ordered list of dependencies, only one of which was needed (in that preferred order): `gnatcoll.xstrings="1.0" or gnatcoll.slim="1.0" or gnatcoll="1.0".

Now, suppose there are projects B and C. Project B, in turn, depends on full gnatcoll. Project C, in turn, depends only on gnatcoll.xstrings. With the previous arrangement, a third project A that depends on B and C, would get a solution that can only be fulfilled by plain "gnatcoll". OTOH, a project D that depends only on C, would get the lighter "gnatcoll.xstrings" dependency:

A --> B --> gnatcoll
  \-> C -/

D --> C --> gnatcoll.xstrings

To make thinks even simpler for clients, the old project xstrings encapsulated the three dependencies above, so a client interested on xstrings did not need to specify the triple OR dependency:
C --> xstrings --> gnatcoll.xstrings

I'd like to recover this functionality (it's already there) if we can think of a nice toml syntax. Perhaps something using [[depends-on.one-of]] arrays?

Since this one is less trivial, I'm pinging both of you for opinions/experiences ;-) @pmderodat @Fabien-Chouteau

P.s.: apt uses "provides" and "|" for roughly the above two ideas (I'm sure there are some differences).