Thank you for the two suggestions, @thomasjpfan.

In fact item 13 is a move constructor which must take a r-value reference (see the && for item 13) in order to be used.

We get a compile error because we aren't using r-value here, but self.neigh_{distance,indices} which are l-value.
I guess it makes sense not the be able to cast unique_ptr to shared_ptr because this would just break the uniqueness of the pointer. This was my prior understanding before this PR, and it seems that the problem we are meeting here likely illustrates it.

Is this right or am I missing something?

I will just revert for now to have the CI pass on the original PR, still I am interested in pursuing the discussions here, @thomasjpfan.

Yes you are correct.

My mental model was incorrect about the interaction between shared + unique pointers. Everything needs to be shared here. For the casting to work, we would need to move it.

That was an opportunity for me to learn as well. :)