For clarity: I'm tagging this "blocked" since it depends on a runtime update for correctness. As fsb4000 says in the OP, we'll merge this for 17.4 when the redist is unlocked.

Note that this affects the behavior of future_category and iostream_category.

D:\GitHub\STL\out\build\x64>type meow.cpp

#include <future>
#include <iostream>
#include <system_error>
using namespace std;

int main() {
    cout << "future_category: " << error_code{0, future_category()}.message() << "\n";
    cout << "generic_category: " << error_code{0, generic_category()}.message() << "\n";
    cout << "iostream_category: " << error_code{0, iostream_category()}.message() << "\n";
    cout << "system_category: " << error_code{0, system_category()}.message() << "\n";
}

D:\GitHub\STL\out\build\x64>cl /EHsc /nologo /W4 /MTd meow.cpp
meow.cpp

Before #2560:

future_category: unknown error
generic_category: unknown error
iostream_category: unknown error
system_category: The operation completed successfully.

With #2560 (shipped in VS 2022 17.2):

future_category: unknown error
generic_category: success
iostream_category: unknown error
system_category: The operation completed successfully.

This PR:

future_category: success
generic_category: success
iostream_category: success
system_category: The operation completed successfully.

This seems reasonable to me, as future_errc and io_errc use nonzero error codes:

This seems reasonable to me, as future_errc and io_errc use nonzero error codes

The fact that error_code and error_condition have an operator bool that returns value() != 0 effectively means that meow{0} must represent success for every is_error_code_enum / is_error_condition_enum type meow. If anything, I'd call this an improvement.

Confirming that this is blocked - at this time, we aren't certain whether code is flowing into 17.4 Preview 1 (locked) or Preview 2 (unlocked). Moving to cryo-stasis for now.

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

Thanks for cleaning up this code and improving the behavior of the error categories! 🧹 🚀 😸