From 4211d8653918282e2f7cad6ec3d70006ffbbce18 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=92=D0=BB=D0=B0=D0=B4=D0=B8=D1=81=D0=BB=D0=B0=D0=B2=20?=
 =?UTF-8?q?=D0=A9=D0=B0=D0=BF=D0=BE=D0=B2?= <phprus@gmail.com>
Date: Sat, 8 May 2021 16:51:51 +0500
Subject: [PATCH] Add a formatter specialization for std::error_code.

---
 include/fmt/os.h | 17 +++++++++++++++++
 test/os-test.cc  | 24 ++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/fmt/os.h b/include/fmt/os.h
index 482b1a81..cd40c031 100644
--- a/include/fmt/os.h
+++ b/include/fmt/os.h
@@ -129,6 +129,23 @@ class error_code {
   int get() const FMT_NOEXCEPT { return value_; }
 };
 
+template <typename Char> struct formatter<std::error_code, Char> {
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext>
+  FMT_CONSTEXPR auto format(const std::error_code& ec, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    out = detail::write<Char>(out, to_string_view(ec.category().name()));
+    out = detail::write<Char>(out, Char(':'));
+    out = detail::write<Char>(out, ec.value());
+    return out;
+  }
+};
+
 #ifdef _WIN32
 namespace detail {
 // A converter from UTF-16 to UTF-8.
diff --git a/test/os-test.cc b/test/os-test.cc
index b2c38af7..c7e45021 100644
--- a/test/os-test.cc
+++ b/test/os-test.cc
@@ -74,6 +74,30 @@ TEST(os_test, error_code) {
   EXPECT_EQ(error_code(42).get(), 42);
 }
 
+TEST(os_test, format_std_error_code) {
+  EXPECT_EQ("generic:42",
+            fmt::format(FMT_STRING("{0}"),
+                        std::error_code(42, std::generic_category())));
+  EXPECT_EQ("system:42",
+            fmt::format(FMT_STRING("{0}"),
+                        std::error_code(42, std::system_category())));
+  EXPECT_EQ("system:-42",
+            fmt::format(FMT_STRING("{0}"),
+                        std::error_code(-42, std::system_category())));
+}
+
+TEST(os_test, format_std_error_code_wide) {
+  EXPECT_EQ(L"generic:42",
+            fmt::format(FMT_STRING(L"{0}"),
+                        std::error_code(42, std::generic_category())));
+  EXPECT_EQ(L"system:42",
+            fmt::format(FMT_STRING(L"{0}"),
+                        std::error_code(42, std::system_category())));
+  EXPECT_EQ(L"system:-42",
+            fmt::format(FMT_STRING(L"{0}"),
+                        std::error_code(-42, std::system_category())));
+}
+
 TEST(os_test, format_windows_error) {
   LPWSTR message = 0;
   auto result = FormatMessageW(