Separate optional and variant support from other STL containers

[funchal]

I'd like to propose that we:

• move std::optional and std::variant conversion support, which are currently in pybind11/stl.h, to a new file (say pybind11/utility.h since they are in the STL utility library in C++17).
• include pybind11/utility.h from pybind11/stl.h so that we maintain full backward compatibility.
• update the docs Overview page to mention the new header.

My motivation for this proposal is that the conversion for other containers (set/vector/map) in pybind11/stl.h comes with a major downside, documented in http://pybind11.readthedocs.io/en/stable/advanced/cast/stl.html#automatic-conversion, and I believe that optional/variant can be useful in their own right.

If you like the idea, I'm happy to prepare a pull request. Please let me know. #

[wjakob]

Hi @funchal ,

This sounds reasonable to me. Do you want to create a PR? (please also mention this in the documentation and the changelog file).

Best, Wenzel

[funchal]

Here you go: https://github.com/pybind/pybind11/pull/1210

[funchal]

Quick reminder, there is still interest in resolving this issue. stl.h has too much stuff in it some of it I want and some I don't. I currently have to workaround this would be great to get some solution in place.