There was some consensus at the time we discussed the acceptance of c++ to avoid c++ features like auto variables and lambda functions (if I am remembering correctly). This comment in PR 19713 refers to a triage meeting but the protocol is quite preliminary and not complete. this other comment refers to this meeting where it says "We should limit C++ explicitly, we can have clang-tidy", but the actual limitations were not set out. That PR also has a discussion about not using symbols from the c++ standard library to avoid the need to ship another dll/so.

I think we should document the use of templating, and additionally state which features we wish to avoid. By creating the document, we can finalize the discussion of the set of features that are acceptable/to be avoided.

There now is a short statement in build.rst, and the set of flags we use (NPY_CXX_FLAGS) can give an indication of what features we use and what features we avoid

Thanks, @mattip. That covers the basic question of "Which C++ standard can be used?"