Background: My organization uses Boost internally for one of its software products, which is itself a C++ library. Users of our library also often use Boost. As such, we must compile our Boost in a different namespace so that the Boost symbols loaded when our library loads do not interfere with whatever different-version Boost symbols our users might load. Historically, another member of the team has been responsible for building new versions of Boost, but now it's me. I am doing this the same way our code/documentation has previously done it, but it is no longer working, and the errors I'm receiving are not familiar to the team member who was previously responsible for this. The last time we built a new Boost version was 1.80.0. I am now doing it with 1.87.0. The example: Further below, I include a lot of information and build commands, but that might not be useful to you. To help, I have created a minimal reproduction program for reproducing the error. You can clone and use it here: https://github.com/beamerblvd/unable-find-target-with-boost-bcp The good: With minimal effort and using our existing tools and configurations, I was easily able to bootstrap `b2` and then run `b2` to build stock Boost with the normal `boost` namespace. All of that worked perfectly and without error. The bad: When I went to do the same thing but with our custom namespace (via bcp), things fell apart. Our existing tooling copies the untouched 1.87.0 source tree into {dir}/_src. {dir}/user-config.jam is configured as follows: ``` constant BOOST_VERSION : mybranchname-1.87.0.a ; constant MY_PLATFORM : macos15-clang16-arm-d ; constant MY_LABEL : MYextralLabel ; constant BOOST_USER_CONFIG : ; using clang : 16 : clang++ : <cxxflags>-fvisibility=hidden <cxxflags>-fvisibility-inlines-hidden ; ``` Within {dir}/_src, I call `bootstrap.sh` and then build `bcp` with: bc -sBOOST_ROOT={dir}/_src \ --boost-build=tools/build/src \ cxxstd=20 \ -d \ 2 \ -j \ 2 \ -q \ --prefix={dir}/bcp \ --build-dir={dir}/bcp \ --stagedir={dir}/bcp/stage \ --user-config={dir}/user-config.jam \ --layout=system \ toolset=clang-darwin \ threading=multi \ link=shared \ architecture=arm \ instruction-set=nocona \ address-model=64 \ --without-python \ --disable-icu \ variant=release tools/bcp That takes a few minutes to complete and does so without error. Once `bcp` is built, I run it with: mkdir {dir}/_dst && \ {dir}/bcp/bin/bcp --boost={dir}/_src --namespace=boost_MYextralLabel --namespace-alias Jamroot boost boost-build.jam boostcpp.jam bootstrap.bat bootstrap.sh build status tools [insert_name_of_every_dir_in_libs] {dir}/_dst That takes a few minutes to complete and does so without error. The contents of {dir}/_dst look as I would expect them to look, with diffs against {dir}/_src reflecting the change in namespace. Once complete, I cd into {dir}/_dst, re-bootstrap, and run `b2` to compile the modified Boost: b2 -sBOOST_ROOT={dir}/_dst \ --boost-build=tools/build/src \ cxxstd=20 \ -d \ 2 \ -j \ 2 \ -q \ --prefix={dir} \ --build-dir={dir}/shobj \ --stagedir={dir}/stage \ --user-config={dir}/user-config.jam \ --layout=system \ toolset=clang-darwin \ threading=multi \ link=shared \ architecture=arm \ instruction-set=nocona \ address-model=64 \ --without-python \ --disable-icu \ variant=debug This fails quickly, here: ... - native syslog supported : yes [2] - pthread supports robust mutexes : no [2] - Boost.Regex is header-only : yes [2] error: Unable to find file or target named error: '/boost/align//boost_align' error: referred to from project at error: '{dir}/_dst/libs/atomic/build' make: *** [build] Error 1 I have tried dozens of different things over the past two days and can't remember most of them, but they didn't change the outcome, so I don't supposed they're relevant. The one thing that *did* change the outcome is that, after running `bcp` and before re-bootstrapping in {dir}/_dst, I tried deleting the {dir}/_dst/boost directory. This still failed, but it got slightly further and failed with a different (albeit extremely similar) error: ... - std_wstreambuf : no [2] - std_wstreambuf : no [3] - BOOST_COMP_GNUC >= 4.3.0 : no [2] - BOOST_COMP_GNUC >= 4.3.0 : no [4] error: Unable to find file or target named error: 'libs/thread/build//boost_MYextralLabel_thread' error: referred to from project at error: '.' make: *** [build] Error 1 I feel like I've exhausted my ability to discern what the problem is. Other than updating a handful of compiler flags (like changing cxxstd=17 to cxxstd=20), I'm doing this the same way we have (allegedly) always done it, but it's not working anymore and I don't know why. I would appreciate any guidance you can offer. Thank you, Nick