From a7d7b894cdcb3e5a474338856966765e49ec956a Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Sun, 27 Apr 2025 08:51:18 -0700
Subject: [PATCH] Implement the s specifier for error_code

---
 include/fmt/std.h | 18 ++++++++++++++----
 test/std-test.cc  | 26 ++++++++++++--------------
 2 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/include/fmt/std.h b/include/fmt/std.h
index 1b0ea5af..62a45f42 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -422,6 +422,12 @@ template <> struct formatter<std::error_code> {
     char c = *it;
     if ((c >= '0' && c <= '9') || c == '{')
       it = detail::parse_width(it, end, specs_, width_ref_, ctx);
+    if (it == end) return it;
+
+    if (*it == 's') {
+      specs_.set_type(presentation_type::string);
+      ++it;
+    }
     return it;
   }
 
@@ -431,10 +437,14 @@ template <> struct formatter<std::error_code> {
     auto specs = specs_;
     detail::handle_dynamic_spec(specs.dynamic_width(), specs.width, width_ref_,
                                 ctx);
-    memory_buffer buf;
-    buf.append(string_view(ec.category().name()));
-    buf.push_back(':');
-    detail::write<char>(appender(buf), ec.value());
+    auto buf = memory_buffer();
+    if (specs_.type() == presentation_type::string) {
+      buf.append(ec.message());
+    } else {
+      buf.append(string_view(ec.category().name()));
+      buf.push_back(':');
+      detail::write<char>(appender(buf), ec.value());
+    }
     return detail::write<char>(ctx.out(), string_view(buf.data(), buf.size()),
                                specs);
   }
diff --git a/test/std-test.cc b/test/std-test.cc
index 6874f025..a5c2c3c9 100644
--- a/test/std-test.cc
+++ b/test/std-test.cc
@@ -12,7 +12,7 @@
 #include <string>
 #include <vector>
 
-#include "fmt/os.h"  // fmt::system_category
+#include "fmt/os.h"       // fmt::system_category
 #include "gtest-extra.h"  // StartsWith
 
 #ifdef __cpp_lib_filesystem
@@ -49,7 +49,7 @@ TEST(std_test, path) {
 }
 
 // Intentionally delayed include to test #4303
-#include "fmt/ranges.h"
+#  include "fmt/ranges.h"
 
 // Test ambiguity problem described in #2954.
 TEST(ranges_std_test, format_vector_path) {
@@ -276,18 +276,16 @@ TEST(std_test, variant) {
 
 TEST(std_test, error_code) {
   auto& generic = std::generic_category();
-  EXPECT_EQ("generic:42",
-            fmt::format(FMT_STRING("{0}"), std::error_code(42, generic)));
-  EXPECT_EQ("  generic:42",
-            fmt::format(FMT_STRING("{:>12}"), std::error_code(42, generic)));
-  EXPECT_EQ("generic:42  ",
-            fmt::format(FMT_STRING("{:12}"), std::error_code(42, generic)));
-  EXPECT_EQ("system:42",
-            fmt::format(FMT_STRING("{0}"),
-                        std::error_code(42, fmt::system_category())));
-  EXPECT_EQ("system:-42",
-            fmt::format(FMT_STRING("{0}"),
-                        std::error_code(-42, fmt::system_category())));
+  EXPECT_EQ(fmt::format("{}", std::error_code(42, generic)), "generic:42");
+  EXPECT_EQ(fmt::format("{:>12}", std::error_code(42, generic)),
+            "  generic:42");
+  EXPECT_EQ(fmt::format("{:12}", std::error_code(42, generic)), "generic:42  ");
+  EXPECT_EQ(fmt::format("{}", std::error_code(42, fmt::system_category())),
+            "system:42");
+  EXPECT_EQ(fmt::format("{}", std::error_code(-42, fmt::system_category())),
+            "system:-42");
+  auto ec = std::make_error_code(std::errc::value_too_large);
+  EXPECT_EQ(fmt::format("{:s}", ec), ec.message());
 }
 
 template <typename Catch> void exception_test() {