I've just created LWG 4295 proposing this change, and am implementing it
via this patch.
libstdc++-v3/ChangeLog:
* include/experimental/memory (swap, make_observer_ptr): Add
constexpr.
(operator==, operator!=, operator<, operator>, operator<=)
(operator>=): Likewise.
* testsuite/experimental/memory/observer_ptr/make_observer.cc:
Checks for constant evaluation.
* testsuite/experimental/memory/observer_ptr/relops/relops.cc:
Likewise.
* testsuite/experimental/memory/observer_ptr/swap/swap.cc:
Likewise.
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>