There is a lot of confusion in the WordPress ecosystem when it comes to the issues around bundling dependencies. Peter Suhm has published a detailed post about this exact issue: A Warning About Using Composer With WordPress, where I have left the following (partial) comment:
So imagine both of us loading library x in our plugins, both requiring a version that’s incompatible with each other. No matter how well tested your plugin is, if I get a chance to load my autoloader before yours, I can load a version of the library that is not compatible with yours and your plugin will break (at least, the end user will see it like that and the horror of finding the culprit begins)…
This obviously is not a Composer issue, but related to the way WordPress loads its plugins. It’s also not a well known issue, since Composer isn’t used a lot in the WordPress ecosystem (yet!), especially not in distributed plugins. But as Composer becomes more used, more collisions like this will happen. Without a proper dependency management system in place, this is really hard to solve.
My WordPress Composer Installer proof of concept is as close to solving this issue as I’ve ever been. If there is anyone more experienced with the plugins installer in WordPress and maybe knows their way around Composer as well, I could use some help. 🙂