boostorg/fusion — pull request #26 by ldionne (2014-10-27 14:17:44+00:00)
Title: Improve support for constexpr in fusion::nil and its iterators.

`fusion::nil` is a pretty trivial type, and it would be nice to make it constexpr-constructible. The same goes for its iterators. The patch was tested with Apple's clang 6.0.