Boost 1.91: warning C4702: unreachable code

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Building boost 1.91 on Windows fails with

E:\jenkins\workspace\gerrit_windows_wsl\workdir\UnpackedTarball\boost\boost\optional\detail\union_optional.hpp(600) : error C2220: the following warning is treated as an error
E:\jenkins\workspace\gerrit_windows_wsl\workdir\UnpackedTarball\boost\boost\optional\detail\union_optional.hpp(600) : warning C4702: unreachable code