From 193d6a4fd4ea7ced2259db32d6e27a5cee6c46b4 Mon Sep 17 00:00:00 2001
From: Max Kellermann <max.kellermann@gmail.com>
Date: Mon, 13 Jun 2022 21:19:17 +0200
Subject: [PATCH] system/Error: add LastErrorCategory()

---
 src/system/Error.hxx | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/system/Error.hxx b/src/system/Error.hxx
index c507656f6..e58b1bd94 100644
--- a/src/system/Error.hxx
+++ b/src/system/Error.hxx
@@ -50,10 +50,24 @@ FormatSystemError(std::error_code code, const char *fmt,
 #include <windef.h> // for HWND (needed by winbase.h)
 #include <winbase.h> // for FormatMessageA()
 
+/**
+ * Returns the error_category to be used to wrap WIN32 GetLastError()
+ * values.  The C++ standard does not define this well, and this value
+ * is mostly guessed.
+ *
+ * TODO: verify
+ */
+[[gnu::const]]
+static inline const std::error_category &
+LastErrorCategory() noexcept
+{
+	return std::system_category();
+}
+
 static inline std::system_error
 MakeLastError(DWORD code, const char *msg) noexcept
 {
-	return std::system_error(std::error_code(code, std::system_category()),
+	return std::system_error(std::error_code(code, LastErrorCategory()),
 				 msg);
 }
 
@@ -173,7 +187,7 @@ static inline bool
 IsFileNotFound(const std::system_error &e) noexcept
 {
 #ifdef _WIN32
-	return e.code().category() == std::system_category() &&
+	return e.code().category() == LastErrorCategory() &&
 		e.code().value() == ERROR_FILE_NOT_FOUND;
 #else
 	return IsErrno(e, ENOENT);
@@ -185,7 +199,7 @@ static inline bool
 IsPathNotFound(const std::system_error &e) noexcept
 {
 #ifdef _WIN32
-	return e.code().category() == std::system_category() &&
+	return e.code().category() == LastErrorCategory() &&
 		e.code().value() == ERROR_PATH_NOT_FOUND;
 #else
 	return IsErrno(e, ENOTDIR);
@@ -197,7 +211,7 @@ static inline bool
 IsAccessDenied(const std::system_error &e) noexcept
 {
 #ifdef _WIN32
-	return e.code().category() == std::system_category() &&
+	return e.code().category() == LastErrorCategory() &&
 		e.code().value() == ERROR_ACCESS_DENIED;
 #else
 	return IsErrno(e, EACCES);