From 864bdf9638f9895db3770e017207e94d25b816cb Mon Sep 17 00:00:00 2001
From: Dean Glazeski <dnglaze@gmail.com>
Date: Sun, 2 Mar 2025 08:47:03 -0700
Subject: [PATCH] Report error on duplicate named arg names (#4367)

---
 include/fmt/base.h  | 16 ++++++++++++++++
 test/format-test.cc |  2 ++
 2 files changed, 18 insertions(+)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index b886317d..11d3ccff 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -1064,13 +1064,28 @@ template <typename Char> struct named_arg_info {
   int id;
 };
 
+template <typename Char>
+FMT_CONSTEXPR void check_for_duplicate(named_arg_info<Char>* named_args,
+                                       int named_arg_index,
+                                       basic_string_view<Char> arg_name) {
+  if (named_arg_index <= 0) return;
+
+  for (auto i = 0; i < named_arg_index; ++i) {
+    if (basic_string_view<Char>(named_args[i].name) == arg_name) {
+      report_error("duplicate named args found");
+    }
+  }
+}
+
 template <typename Char, typename T, FMT_ENABLE_IF(!is_named_arg<T>::value)>
 void init_named_arg(named_arg_info<Char>*, int& arg_index, int&, const T&) {
   ++arg_index;
 }
+
 template <typename Char, typename T, FMT_ENABLE_IF(is_named_arg<T>::value)>
 void init_named_arg(named_arg_info<Char>* named_args, int& arg_index,
                     int& named_arg_index, const T& arg) {
+  check_for_duplicate<Char>(named_args, named_arg_index, arg.name);
   named_args[named_arg_index++] = {arg.name, arg_index++};
 }
 
@@ -1084,6 +1099,7 @@ template <typename T, typename Char,
           FMT_ENABLE_IF(is_static_named_arg<T>::value)>
 FMT_CONSTEXPR void init_static_named_arg(named_arg_info<Char>* named_args,
                                          int& arg_index, int& named_arg_index) {
+  check_for_duplicate<Char>(named_args, named_arg_index, T::name);
   named_args[named_arg_index++] = {T::name, arg_index++};
 }
 
diff --git a/test/format-test.cc b/test/format-test.cc
index c16f895b..e2c595c9 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -599,6 +599,8 @@ TEST(format_test, named_arg) {
   EXPECT_THROW_MSG((void)fmt::format(runtime("{a} {}"), fmt::arg("a", 2), 42),
                    format_error,
                    "cannot switch from manual to automatic argument indexing");
+  EXPECT_THROW_MSG((void)fmt::format("{a}", fmt::arg("a", 1),
+      fmt::arg("a", 10)), format_error, "duplicate named args found");
 }
 
 TEST(format_test, auto_arg_index) {