On Thu, 27 Jun 2024 at 00:02, Niall Douglas <s_sourceforge_at_[hidden]> wrote:
> We have layout compatibility between `_Atomic` and its equivalent
> `std::atomic<>`

I don't think we actually do. Or, quoth a vendor, "No. It's qoi and
GCC fails at it in some cases".

> and in real world code in a mixed C and C++ codebase
> people rely on the above functions being interchangeable when operating
> on the same memory i.e. a header file might declare a struct member as
> `std::atomic` if `__cplusplus`, and `_Atomic` if not.

Right, but making those functions constexpr in C++ doesn't change the layout
of anything, and it doesn't change the runtime behavior of anything,
so the functions remain interoperable on platforms where they
are interoperable to begin with.