As reported earlier, Boost.Build's predefined macro testing support is still misdetecting in RC 1: Using Boost 1.71 RC 1 with VS2019: b2 libs/outcome/test Performing configuration checks - default address-model : 32-bit - default architecture : x86 - !BOOST_COMP_GNUC : no - BOOST_COMP_GNUC >= 6.0 : no - !BOOST_COMP_CLANG : no - BOOST_COMP_CLANG >= 4.0 : no - Boost.Config Feature Check: cxx14_variable_templates : yes - Boost.Config Feature Check: cxx14_constexpr : yes - !BOOST_COMP_GNUC : no - BOOST_COMP_GNUC >= 6.0 : no - !BOOST_COMP_CLANG : no - BOOST_COMP_CLANG >= 4.0 : no - Boost.Config Feature Check: cxx14_variable_templates : yes - Boost.Config Feature Check: cxx14_constexpr : yes ...found 1 target... Using Boost 1.71 RC 1 with GCC 7.3 on Linux: g++ --version g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0 Copyright (C) 2017 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. ./b2 libs/outcome/test Performing configuration checks - !BOOST_COMP_GNUC : no - BOOST_COMP_GNUC >= 6.0 : no - !BOOST_COMP_CLANG : no - BOOST_COMP_CLANG >= 4.0 : no - default address-model : 64-bit - default architecture : x86 - Boost.Config Feature Check: cxx14_variable_templates : yes - Boost.Config Feature Check: cxx14_constexpr : yes - !BOOST_COMP_GNUC : no - BOOST_COMP_GNUC >= 6.0 : no - !BOOST_COMP_CLANG : no - BOOST_COMP_CLANG >= 4.0 : no - Boost.Config Feature Check: cxx14_variable_templates : yes - Boost.Config Feature Check: cxx14_constexpr : yes ...found 1 target...