⟨12rnlahaye⟩ Bizarrely, it seems to be defined for 10.13 on my computer:
#   define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS                             \
        __attribute__((availability(macosx,strict,introduced=10.13)))           \
        __attribute__((availability(ios,strict,introduced=11.0)))               \
        __attribute__((availability(tvos,strict,introduced=11.0)))              \
        __attribute__((availability(watchos,strict,introduced=4.0)))
#   define _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS                              \
        _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS