I'm guessing this might be solvable by updating to a newer version of boost (currently I'm running ver 1.71.0) but before updating, I wondered if there might be something obvious that 'm missing.... I'm building with Visual Studio 2019 and I get a whole bunch of errors simply by adding this line to an empty source file:- #include "boost/rational.hpp" It seems to be the same 6 errors but at various different lines - i.e. these errors, over & over again:- Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\ammintrin.h(146,19): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\ammintrin.h(146,8): error C2146: syntax error: missing ';' before identifier '_mm_macc_ps' Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\ammintrin.h(147,20): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\ammintrin.h(147,9): error C2146: syntax error: missing ';' before identifier '_mm_macc_pd' Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\ammintrin.h(148,19): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\ammintrin.h(148,1): error C2086: 'int __m128': redefinition I just wondered if this might ring a bell with someone? e.g. might there be some preprocesser #define that I should've defined? (or a #define that I shouldn't be defining...) Thanks for any suggestions, John