fix: convert std::error_code messages to UTF-8 on Windows

When formatting with {:s}, system error messages may use the active
code page instead of UTF-8. Convert from ACP via UTF-16 before writing.

Fixes #4436.

include/fmt/os.h

@@ -117,6 +117,8 @@ FMT_API const std::error_category& system_category() noexcept;
 namespace detail {
 FMT_API void format_windows_error(buffer<char>& out, int error_code,
                                   const char* message) noexcept;
+FMT_API bool append_system_error_message_as_utf8(buffer<char>& buf,
+                                                 string_view message) noexcept;
 }
 
 FMT_API std::system_error vwindows_error(int error_code, string_view fmt,

include/fmt/std.h

@@ -546,6 +546,13 @@ struct formatter<Variant, Char,
 
 #endif  // FMT_CPP_LIB_VARIANT
 
+#ifdef _WIN32
+namespace detail {
+FMT_API bool append_system_error_message_as_utf8(buffer<char>& buf,
+                                                 string_view message) noexcept;
+}
+#endif
+
 template <> struct formatter<std::error_code> {
  private:
   format_specs specs_;
@@ -584,7 +591,12 @@ template <> struct formatter<std::error_code> {
                                 ctx);
     auto buf = memory_buffer();
     if (specs_.type() == presentation_type::string) {
+#ifdef _WIN32
+      if (!detail::append_system_error_message_as_utf8(buf, ec.message()))
+        buf.append(ec.message());
+#else
       buf.append(ec.message());
+#endif
     } else {
       buf.append(string_view(ec.category().name()));
       buf.push_back(':');

src/os.cc

@@ -165,6 +165,29 @@ void report_windows_error(int error_code, const char* message) noexcept {
   do_report_error(detail::format_windows_error, error_code, message);
 }
 
+namespace detail {
+bool append_system_error_message_as_utf8(buffer<char>& buf,
+                                         string_view message) noexcept {
+  FMT_TRY {
+    if (message.empty()) return true;
+    const int size = static_cast<int>(message.size());
+    const int wide_size =
+        MultiByteToWideChar(CP_ACP, 0, message.data(), size, nullptr, 0);
+    if (wide_size == 0) return false;
+    std::wstring wide(static_cast<size_t>(wide_size), L'\0');
+    if (MultiByteToWideChar(CP_ACP, 0, message.data(), size, wide.data(),
+                            wide_size) == 0)
+      return false;
+    auto utf8_message = to_utf8<wchar_t>();
+    if (!utf8_message.convert(wide)) return false;
+    buf.append(string_view(utf8_message.c_str(), utf8_message.size()));
+    return true;
+  }
+  FMT_CATCH(...) {}
+  return false;
+}
+}  // namespace detail
+
 #endif  // _WIN32
 
 buffered_file::~buffered_file() noexcept {