Building with the new(?) LLVM-based g++ on Mac, I'm getting the following
/Users/cdyer/eigen-dev/Eigen/src/Core/arch/CUDA/Half.h:285:9: error: use of undeclared identifier '_cvtss_sh'
h.x = _cvtss_sh(ff, 0);
^
/Users/cdyer/eigen-dev/Eigen/src/Core/arch/CUDA/Half.h:338:10: error: use of undeclared identifier '_cvtsh_ss'
return _cvtsh_ss(h.x);
Poking around, it looks like there was a recent change to LLVM to add these intrinsics this change hasn't made it into the standard Xcode release (which most mac developers use, I think).
Adding
#undef EIGEN_HAS_FP16_C
will fix this, but I'm not quite sure what the appropriate ifdef guard is. Fixing this could be quite useful for your Mac users.
(Apologies for not submitting to Bugzilla, even after reseting my password I'm unable to log in properly).