I'm sorry, I take it back. Things were fine until I included <boost/hana.hpp> I had been testing with
<boost/hana/core/when.hpp>
and the likes of:
template<class T>
constexpr T pi = T(3.1415926535897932385);  // variable template
which would not work until I patched.

And to quote:
In this release, we've updated the C++ compiler and standard library with enhanced support for C++11 and C++14 features, as well as preliminary support for certain features expected to be in the C++17 standard. The most notable compiler changes include complete support for generalized constexpr.

Maybe next time....

Best, Dan.