Thankyou both for the insightful comments. I do expect that a future update
of atomic operations in a constexpr context is sensible; and should also be
compatible with a future constexpr thread. I also think that as that is
currently only implicit in the linked constexpr shared_ptr implementation,
it would be clearer to use the libstdc++ interface to the atomic functions.
I'll update `__gnu_cxx::__atomic_add_dispatch` and
`__gnu_cxx::__exchange_and_add_dispatch` in the repo's <ext/atomicity.h>.

--
Paul