The story starts with https://travis-ci.org/vinniefalco/beast/jobs/375049193 which gives the error dyld: Symbol not found: __ZN5boost6system6detail19system_category_ncxEv Referenced from: /Users/travis/build/vinniefalco/boost-root/bin.v2/libs/beast/test/beast/core/clang-darwin-4.2.1/debug/threadapi-pthread/threading-multi/fat-tests Expected in: /usr/local/lib/libboost_system.dylib boost::system::detail::system_category_ncx is a new function (in develop), and careful reading of the above reveals that the compiler is trying to link to a preinstalled libboost_system.dylib, in /usr/local/lib. Looking at the log, Boost.System is not even being built. This is odd; the Beast root Jamfile contains <library>/boost/system//boost_system (at https://github.com/boostorg/beast/blob/develop/Jamfile#L78) but even if it didn't, it also links to Filesystem, which _is_ being built, and Filesystem also depends on System in its build/Jamfile: https://github.com/boostorg/filesystem/blob/develop/build/Jamfile.v2#L29 So Boost.System should be built, but it isn't. Its Jamfile hasn't been touched in 10 years: https://github.com/boostorg/system/blob/develop/build/Jamfile.v2 In addition, there's not even a `mklink-or-dir boost/system` in the log, even though the link must be created because the new headers are being used (this could be a separate issue). The Linux/g++ job, https://travis-ci.org/vinniefalco/beast/jobs/375049187, also doesn't seem to build Boost.System, but it works for some reason. When I try `b2 libs/beast/test` on Windows, with msvc-14.1, Boost.System is being built, in the very beginning: ...patience... ...patience... ...patience... ...found 6992 targets... ...updating 1090 targets... link.hardlink ..\..\boost\utility\string_view.hpp Hardlink created for ..\..\boost\utility\string_view.hpp <<===>> ..\utility\include\boost\utility\string_view.hpp compile-c-c++ ..\..\bin.v2\libs\beast\test\beast\core\type_traits.test\msvc-14.1\debug\threadapi-win32\threading-multi\type_traits.obj type_traits.cpp compile-c-c++ ..\..\bin.v2\libs\beast\test\beast\core\type_traits.test\msvc-14.1\debug\threadapi-win32\threading-multi\main.obj main.cpp compile-c-c++ ..\..\bin.v2\libs\system\build\msvc-14.1\debug\threading-multi\error_code.obj error_code.cpp msvc.link.dll ..\..\bin.v2\libs\system\build\msvc-14.1\debug\threading-multi\boost_system-vc141-mt-gd-x32-1_66.dll (The 1.66 version instead of 1.68 was caused by a typo in the superproject which is now fixed.) When I try the same on Ubuntu Trusty, the result is the same as on Travis; no `mklink-or-dir boost/system`, no building of Boost.System, yet everything compiles and works, and I don't have a preinstalled Boost on my VM. It makes no sense at all. Does anyone have any ideas what might be happening here?