DE-078 17.3.2 [version.syn] Feature-test macro for noexcept `std::exchange`

[wg21bot]

noexcept std::exchange (P2401R0) is missing a feature macro Use-case: backporting library components to older IS versions. C++20 Example:

namespace c20 {
#ifdef __cpp_lib_constexpr_algorithms
using std::exchange;
#else
~~~ reimplement std::exchange in C++20 form ~~~
#endif
}

use c20::exchange(), replace with std::exchange() when project depends on C++20

Increase value of existing feature test macro __cpp_lib_exchange_function to match the YYYYMM of the application of P2401R0 to the IS.

[JeffGarland]

LWG disccused 2022-10-26. Doesn't seem like a design issue here -- so this be an LWG issue for resolution.

[JeffGarland]

poll: accept the resolution in DE-078 to bump the feature test macro for P2401

https://wiki.edg.com/bin/view/Wg21kona2022/LWG20221107-EA

F	A	N
4	3	1

no consensus to change

[jensmaurer]

Rejected. There is no consensus for a change.