I will add some tests later. @roystgnr please let me know if this looks okay to you before I go too far.

Sorry I missed the comment here!

Definitely we want to add this feature.

I'd have made the range argument a pointer with a nullptr default, but out of force of habit rather than because it's a good idea, and I'm happy with std::optional here. Someone correct me if I'm wrong, but best practices now is to use std::optional for any object that's lightweight enough to not worry about copying, right? (and probably also for heavyweight objects, eventually, when C++26 adds optional<T&> support?) Our ranges are currently like 48 bytes, so the copies should be fine.

Dear @roystgnr,

I added one test case and modified it to support project_solution with elem_ranges as well.
Could you please review it and assist us in activating the CI?

Thanks!

Dear @roystgnr,

We tried to support libMesh project_vector and project_solution with target variables as well.
And, I modified the test to test this as well.
If you are available and have a chance, could you please review it and help us mark this PR as "ready for review"?

Thanks for your time!

It appears that https://civet.inl.gov/repo/8/ does not recognize this PR, so as @roystgnr suggested, we plan to close this PR and open a new PR (#4368).