Tests fail with LTO due to ODR violations

Reproduce the LTO build for libs/thread/test/test_10964 and inspect the diagnostics against boost/thread/futures/launch.hpp and boost/thread/future.hpp. Use the attached build.log to compare the conflicting translation units and identify the relevant configuration difference. Done means test_10964 links successfully with the listed -Werror flags and the ODR/type-mismatch diagnostics are gone.

I tried to build with the following *FLAGS to optimize the build: -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing

Link-Time Optimization is a massively global compiler optimization pass which is pretty handy for producing faster executables. It also has the interesting property that because the compiler does whole-program analysis using bytecode, it can save type information and perform error checks that it normally doesn’t have enough insight for. In particular, checking for ODR issues and checking function type signature mismatches.

Note that all the -Werror=* flags are used to help detect cases where the compiler tries to optimize by assuming UB cannot exist in the source code -- if it does exist, ordinarily the code would be miscompiled, and this says to make the miscompilation a fatal error.

I got this error:

gcc.link ../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden/test_10964_lib

    "x86_64-pc-linux-gnu-g++"    -o "../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden/test_10964_lib" -Wl,--start-group "../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden/test_10964.o" "../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden/winrt_init.o" "../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden/tss_null.o" "../../../bin.v2/libs/thread/build/gcc-14.3/gentoorelease/x86_64/link-static/pch-off/threading-multi/visibility-hidden/libboost_thread.a" "../../../bin.v2/libs/exception/build/gcc-14.3/gentoorelease/x86_64/link-static/pch-off/threading-multi/visibility-hidden/libboost_exception.a" "../../../bin.v2/libs/atomic/build/gcc-14.3/gentoorelease/x86_64/link-static/pch-off/threading-multi/visibility-hidden/libboost_atomic.a" "../../../bin.v2/libs/chrono/build/gcc-14.3/gentoorelease/x86_64/link-static/pch-off/threading-multi/visibility-hidden/libboost_chrono.a" "../../../bin.v2/libs/container/build/gcc-14.3/gentoorelease/x86_64/link-static/pch-off/threading-multi/visibility-hidden/libboost_container.a" "../../../bin.v2/libs/date_time/build/gcc-14.3/gentoorelease/x86_64/link-static/pch-off/threading-multi/visibility-hidden/libboost_date_time.a"  -Wl,-Bstatic  -Wl,-Bdynamic -lrt -Wl,--end-group -fPIC -m64 -pthread -fvisibility=hidden -fvisibility-inlines-hidden -Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing -Wl,--defsym=__gentoo_check_ldflags__=0 

../../../boost/thread/futures/launch.hpp:17:3: error: type ‘boost::launch’ violates the C++ One Definition Rule [-Werror=odr]
   17 |   BOOST_SCOPED_ENUM_DECLARE_BEGIN(launch)
      |   ^
../../../boost/thread/futures/launch.hpp:17:3: note: an enum with different value name is defined in another translation unit
   17 |   BOOST_SCOPED_ENUM_DECLARE_BEGIN(launch)
      |   ^
../../../boost/thread/futures/launch.hpp:23:7: note: name ‘executor’ differs from name ‘inherit’ defined in another translation unit
   23 |       executor = 4,
      |       ^
../../../boost/thread/futures/launch.hpp:25:7: note: mismatching definition
   25 |       inherit = 8,
      |       ^
../../../boost/thread/future.hpp:838:16: error: virtual table of type ‘struct shared_state’ violates one definition rule [-Werror=odr]
  838 |         struct shared_state<void>:
      |                ^
../../../boost/thread/future.hpp:838:16: note: the conflicting type defined in another translation unit
  838 |         struct shared_state<void>:
      |                ^
../../../boost/thread/future.hpp:327:26: note: virtual method ‘set_continuation_ptr’
  327 |             virtual void set_continuation_ptr(continuation_ptr_type continuation, boost::unique_lock<boost::mutex>& lock)
      |                          ^
../../../boost/thread/future.hpp:362:26: note: ought to match virtual method ‘run_if_is_deferred’ but does not
  362 |             virtual bool run_if_is_deferred()
      |                          ^
../../../boost/thread/future.hpp:151:16: error: virtual table of type ‘struct shared_state_base’ violates one definition rule [-Werror=odr]
  151 |         struct shared_state_base : enable_shared_from_this<shared_state_base>
      |                ^
../../../boost/thread/future.hpp:151:16: note: the conflicting type defined in another translation unit
  151 |         struct shared_state_base : enable_shared_from_this<shared_state_base>
      |                ^
../../../boost/thread/future.hpp:327:26: note: virtual method ‘set_continuation_ptr’
  327 |             virtual void set_continuation_ptr(continuation_ptr_type continuation, boost::unique_lock<boost::mutex>& lock)
      |                          ^
../../../boost/thread/future.hpp:362:26: note: ought to match virtual method ‘run_if_is_deferred’ but does not
  362 |             virtual bool run_if_is_deferred()
      |                          ^
lto1: some warnings being treated as errors
lto-wrapper: fatal error: /usr/bin/x86_64-pc-linux-gnu-g++ returned 1 exit status
compilation terminated.
/usr/lib/gcc/x86_64-pc-linux-gnu/14/../../../../x86_64-pc-linux-gnu/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status
...skipped <p../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden>test_10964_lib.run for lack of <p../../../bin.v2/libs/thread/test/test_10964_lib.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden>test_10964_lib...
gcc.link ../../../bin.v2/libs/thread/test/test_10964.test/gcc-14.3/gentoorelease/x86_64/pch-off/threading-multi/visibility-hidden/test_10964

Originally reported downstream: https://bugs.gentoo.org/956660
Full build log: build.log