CWG2970 [intro.races] Races in practice with volatile sig_atomic_t bit-fields

[hubert-reinterpretcast]

Full name of submitter (unless configured in github; will be published with the issue): Hubert Tong

Reference (section label): intro.races

Link to reflector thread (if any): N/A

Issue description:
https://wg21.link/intro.races#22 says:

Two accesses to the same object of type volatile std::sig_atomic_t do not result in a data race if both occur in the same thread, even if one or more occurs in a signal handler.

However: Bit-fields are objects (https://wg21.link/basic.types.general#4), and volatile std::sig_atomic_t bit-field accesses are subject to tearing in practice and, therefore, should formally receive the data-race treatment.

Suggested resolution:

Two accesses to the same non-bit-field object of type volatile std::sig_atomic_t do not result in a data race if both occur in the same thread, even if one or more occurs in a signal handler.

[jensmaurer]

CWG2970