OS-X CMake build fails with: "invalid operands to binary expression"

I have a project I am trying to build. There appears to be an incompatibility with my c++ lib:

In file included from /Applications/http://ift.tt/1j5fWaK:
In file included from /Applications/http://ift.tt/1is3Coe:
/Applications/http://ift.tt/1OfM2oF: error: invalid operands to binary expression
      ('const RadiusResultSet::Item' and 'const RadiusResultSet::Item')
    _LIBCPP_INLINE_VISIBILITY bool operator()(const _T1& __x, const _T1& __y) const {return __x < __y;}

Can anyone suggest what might be going wrong with my build? I am using cmake, with xcode tools (Yosemite).