"error: No best alternative for {dir}/libs/mpi/build/boost_mpi", but the build succeeds

From: Nicholas Williams (nicholas_at_[hidden])
Date: 2025-01-16 14:32:10

I'm building the full stock, unmodified Boost 1.87.0. I have previously
built Boost 1.80.0 without issue. After bootstrapping, when running `b2` to
actually build Boost, I get the following "errors":

...
- has stat::st_birthtimespec : yes [5]
- has fdopendir(O_NOFOLLOW) : yes [5]
- has dirent::d_type : yes [5]
- has POSIX *at APIs : no [5]
- has fallocate : no [5]
- cxx11_static_assert : yes [5]
error: No best alternative for {dir}/libs/mpi/build/boost_mpi with
<abi>aapcs <address-model>64 <architecture>arm <asynch-exceptions>off
<binary-format>mach-o <boost.beast.allow-deprecated>on
<boost.beast.separate-compilation>on <boost.cobalt.executor>any_io_executor
<boost.cobalt.pmr>std <context-impl>fcontext <coverage>off
<cxxstd-dialect>iso <cxxstd>20 <debug-symbols>off <exception-handling>on
<extern-c-nothrow>off <inlining>full <instruction-set>nocona
<known-warnings>hide <link>shared <optimization>speed <os>MACOSX <pch>on
<preserve-test-targets>on <profiling>off <python-debugging>off <python>3.9
<rtti>on <runtime-debugging>off <runtime-link>shared <stdlib>native
<strip>off <target-os>darwin <testing.execute>on <threadapi>pthread
<threading>multi <toolset-clang:platform>darwin <toolset-clang:version>14
<toolset>clang <variant>release <vectorize>off <visibility>hidden
<warnings-as-errors>off <warnings>on <x-deduced-platform>arm_64
matched: (empty)
matched: (empty)
error: No best alternative for {dir}/libs/mpi/build/boost_mpi with
<abi>aapcs <address-model>64 <architecture>arm <asynch-exceptions>off
<binary-format>mach-o <boost.beast.allow-deprecated>on
<boost.beast.separate-compilation>on <boost.cobalt.executor>any_io_executor
<boost.cobalt.pmr>std <coverage>off <cxxstd-dialect>iso <cxxstd>20
<debug-symbols>off <exception-handling>on <extern-c-nothrow>off
<inlining>full <instruction-set>nocona <known-warnings>hide <link>shared
<optimization>speed <os>MACOSX <pch>on <preserve-test-targets>on
<profiling>off <python-debugging>off <python>3.9 <rtti>on
<runtime-debugging>off <runtime-link>shared <stdlib>native <strip>off
<target-os>darwin <testing.execute>on <threadapi>pthread <threading>multi
<toolset-clang:platform>darwin <toolset-clang:version>14 <toolset>clang
<variant>release <vectorize>off <visibility>hidden <warnings-as-errors>off
<warnings>on <x-deduced-platform>arm_64
matched: (empty)
matched: (empty)
- gcc visibility : yes [2]
- cxx14_decltype_auto : yes [2]
- cxx14_generic_lambdas : yes [2]
...

I used scare quotes around "errors," because these errors do not appear to
actually break anything. The build completes fully and successfully, and
all the libraries are built as expected. It appears to be more of a warning
than an error, but I don't love warnings either, so I'd like to solve it.
The only relevant result I could find Googling about this was [this other
mailing list message](
https://lists.boost.org/Archives/boost/2024/11/258275.php), which was a
reply to "[release] Boost 1.87.0 Beta 2 Release Candidate is available,"
and to which nobody ever replied. Doesn't really clear anything up for me.

FWIW, I get these "errors" on RHEL 8 GCC 8, 9 GCC 11, and 9 GCC 14; Ubuntu
20.04 GCC 9, 22.04 GCC 11, and 24.04 GCC 13; macOS 12 Clang 14 (Intel and
ARM), 14 Clang 16 (Intel and ARM), and 15 Clang 16 (Intel and ARM); Windows
10 MSVC 2022 and 11 MSVR 2022; and Windows Server 2022 MSVC 2022 and 2025
MSVC 2022.

Any insights or guidance would be appreciated.

Thanks,

Nick