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The `std::error_code`/`std::error_category` functionality is designed to support multiple error domains. On Unix, both system calls and libc functions return the same error codes, and thus, libc++ today treats `generic_category()` and `system_category()` as being equivalent. However, on Windows, libc functions return `errno.h` error codes in the `errno` global, but system calls return the very different `winerror.h` error codes via `GetLastError()`. As such, there is a need to map the winerror.h error codes into generic errno codes. In libc++, however, the system_error facility does not implement this mapping; instead the mapping is hidden inside libc++, used directly by the std::filesystem implementation. That has a few problems: 1. For std::filesystem APIs, the concrete windows error number is lost, before users can see it. The intent of the distinction between std::error_code and std::error_condition is that the error_code return has the original (potentially more detailed) error code. 2. User-written code which calls Windows system APIs requires this same mapping, so it also can also return error_code objects that other (cross-platform) code can understand. After this commit, an `error_code` with `generic_category()` is used to report an error from `errno`, and, on Windows only, an `error_code` with `system_category()` is used to report an error from `GetLastError()`. On Unix, system_category remains identity-mapped to generic_category, but is never used by libc++ itself. The windows error code mapping is moved into system_error, so that conversion of an `error_code` to `error_condition` correctly translates the `system_category()` code into a `generic_category()` code, when appropriate. This allows code like: `error_code(GetLastError(), system_category()) == errc::invalid_argument` to work as expected -- as it does with MSVC STL. (Continued from old phabricator review [D151493](https://reviews.llvm.org/D151493))