Hi, as mentioned in another thread, there are compilation problems with the reworked none_t on MSVC when using precompiled headers. To reproduce: precompiled header stdafx.h: #pragma once #include s.cpp: #include "stdafx.h" struct s { void f(boost::optional<int> const& = boost::none) {} }; On MSVC12, this results in the error message: s.cpp(3): error C3083: ' ?? A0x06d21c56': the symbol to the left of a '::' must be a type