From c936e2e44ec753fb9dfe6c57e29e189bf5d3717d Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Sun, 27 Apr 2025 09:06:17 -0700
Subject: [PATCH] Implement debug format for error_code

---
 include/fmt/std.h | 20 ++++++++++++++------
 test/std-test.cc  |  2 ++
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/include/fmt/std.h b/include/fmt/std.h
index 62a45f42..f43dc74d 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -410,6 +410,7 @@ template <> struct formatter<std::error_code> {
  private:
   format_specs specs_;
   detail::arg_ref<char> width_ref_;
+  bool debug_ = false;
 
  public:
   FMT_CONSTEXPR auto parse(parse_context<>& ctx) -> const char* {
@@ -417,14 +418,16 @@ template <> struct formatter<std::error_code> {
     if (it == end) return it;
 
     it = detail::parse_align(it, end, specs_);
-    if (it == end) return it;
 
     char c = *it;
-    if ((c >= '0' && c <= '9') || c == '{')
+    if (it != end && ((c >= '0' && c <= '9') || c == '{'))
       it = detail::parse_width(it, end, specs_, width_ref_, ctx);
-    if (it == end) return it;
 
-    if (*it == 's') {
+    if (it != end && *it == '?') {
+      debug_ = true;
+      ++it;
+    }
+    if (it != end && *it == 's') {
       specs_.set_type(presentation_type::string);
       ++it;
     }
@@ -445,8 +448,13 @@ template <> struct formatter<std::error_code> {
       buf.push_back(':');
       detail::write<char>(appender(buf), ec.value());
     }
-    return detail::write<char>(ctx.out(), string_view(buf.data(), buf.size()),
-                               specs);
+    auto quoted = memory_buffer();
+    auto str = string_view(buf.data(), buf.size());
+    if (debug_) {
+      detail::write_escaped_string<char>(std::back_inserter(quoted), str);
+      str = string_view(quoted.data(), quoted.size());
+    }
+    return detail::write<char>(ctx.out(), str, specs);
   }
 };
 
diff --git a/test/std-test.cc b/test/std-test.cc
index a5c2c3c9..8e5fa06c 100644
--- a/test/std-test.cc
+++ b/test/std-test.cc
@@ -286,6 +286,8 @@ TEST(std_test, error_code) {
             "system:-42");
   auto ec = std::make_error_code(std::errc::value_too_large);
   EXPECT_EQ(fmt::format("{:s}", ec), ec.message());
+  EXPECT_EQ(fmt::format("{:?}", std::error_code(42, generic)),
+            "\"generic:42\"");
 }
 
 template <typename Catch> void exception_test() {