1013fe3c0cused to implement LWG2770, butcb0d4df974made LWG2770 unimplemented again because of CWG2386. This patch re-implements LWG2770, while keeping the libc++-specific implementation strategy (which is controversial as noted in LWG4040). Drive-by: - Make the test coverage for the controversial part noted in LWG4040 libc++-only. - Add the previously missed entry for LWG2770 to the documentation.