Duplicate conflicting forward declaration of basic_string_ref

[white@…]

In Boost 1.61 Beta 1, a new error means to much forward declaration of basic_string_ref.

boost/log/utility/formatting_ostream.hpp:39:76: error: redefinition of default argument for 'class traits'

template<typename charT, typename traits = std::char_traits<charT> > class basic_string_ref;

Its also declared in boost/utility/string_ref_fwd.hpp and boost/utility/string_ref.hpp

Removing the fwd declaration in formatting_ostream.hpp:39 and all is working.

Boost 1.61.0 beta gcc 5.3.0 (x86_64-win32-seh-rev0, Built by MinGW-W64 project)

[Andrey Semashev]

Should be fixed in ​https://github.com/boostorg/log/commit/768ac379dbd1da098b9e36c50f641bd3af338e30. Thanks.