Ran into this warning while building on OSX:
../../src/amount.h:75:22: warning: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Wconstexpr-not-const] constexpr Amount operator-() { return Amount(-amount); } ^ const
It looks like the rest of the constexpr in this file are either friends or already have const applied.