VC++ compiler upgrade 2010->2015 redefinition; 'constexpr' specifier mismatch

Haven't posted here before but ill give it a try, working on upgrading a VC++ project from the vs 2010 compiler to the vs 2015 compiler and I've run into this error, any help would be appreciated.

C2475 'std::less<_Kty>::operator ()': redefinition; 'constexpr' specifier mismatch

This is the code generating the error.

    bool std::less < sp < ref::db::string > >::operator()( const sp < ref::db::string >& first, const sp < ref::db::string >& second ) const {
    return first->cpp() < second->cpp();}