How to get the latest commit of a dependency

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If you want something from the latest commits that's unreleased you will have to compile it from source.

Clearly the easiest thing to do is to just add it to your workspace. I'm not sure why you don't want to do that. That is the standard thing to do.

As @gvdhoorn mentions you can add one level of complexity and add an underlay. Tutorial here

If you want to you can build a custom binary package based off of master using tools like bloom. But that's generally not recommended. And as you mention you're not familiar with this, and it will be a lot of custom work for something that may be hard to reproduce.

rosdep is not the right tool, it's a meta package manager. If someone else was packaging this in a PPA you could update your rosdep sources to support that. But that's not the case.

Stepping back though. If you want the latest and greatest commits, you should be ready to put them into your workspace. The underlay is an optimization to improve performance with multiple workspaces extending the same one.

And remember when you use an updated version of something from source like this. If there's any API or ABI changes you also need to check out all the downstream dependencies from source and recompile them too to make sure you don't have crashes due to binary incompatibilities.