From 167ec541407c7207b5a527a537cb057c85afb756 Mon Sep 17 00:00:00 2001
From: Dean Glazeski <dnglaze@gmail.com>
Date: Sat, 22 Feb 2025 09:39:29 -0700
Subject: [PATCH] Report error on duplicate named arg names

#4282
---
 include/fmt/base.h                     | 10 ++++++++++
 test/compile-error-test/CMakeLists.txt |  8 ++++++++
 test/format-test.cc                    |  2 ++
 3 files changed, 20 insertions(+)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index a37858a843c5..7be57f09ce2a 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -1071,6 +1071,11 @@ void init_named_arg(named_arg_info<Char>*, int& arg_index, int&, const T&) {
 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) {
+  for (auto i = 0; i < named_arg_index; ++i) {
+    if (basic_string_view<Char>(named_args[i].name) == basic_string_view<Char>(arg.name)) {
+      report_error("duplicate named args found");
+    }
+  }
   named_args[named_arg_index++] = {arg.name, arg_index++};
 }
 
@@ -1084,6 +1089,11 @@ 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) {
+  for (auto i = 0; i < named_arg_index; ++i) {
+    if (basic_string_view<Char>(named_args[i].name) == basic_string_view<Char>(T::name)) {
+      report_error("duplicate named args found");
+    }
+  }
   named_args[named_arg_index++] = {T::name, arg_index++};
 }
 
diff --git a/test/compile-error-test/CMakeLists.txt b/test/compile-error-test/CMakeLists.txt
index a86996e21f95..b2586a76af27 100644
--- a/test/compile-error-test/CMakeLists.txt
+++ b/test/compile-error-test/CMakeLists.txt
@@ -209,6 +209,14 @@ if (CMAKE_CXX_STANDARD GREATER_EQUAL 20)
       #error
     #endif
   " ERROR)
+  expect_compile(format-string-duplicate-name-error "
+    #if defined(FMT_HAS_CONSTEVAL) && FMT_USE_NONTYPE_TEMPLATE_ARGS
+      using namespace fmt::literals;
+      fmt::print(\"{bar}\", \"bar\"_a=42, \"bar\"_a=43);
+    #else
+      #error
+    #endif
+  " ERROR)
 endif ()
 
 # Run all tests
diff --git a/test/format-test.cc b/test/format-test.cc
index c16f895bed29..ce5db19ec39f 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -582,6 +582,8 @@ TEST(format_test, named_arg) {
   EXPECT_EQ("1/a/A", fmt::format("{_1}/{a_}/{A_}", fmt::arg("a_", 'a'),
                                  fmt::arg("A_", "A"), fmt::arg("_1", 1)));
   EXPECT_EQ(fmt::format("{0:{width}}", -42, fmt::arg("width", 4)), " -42");
+  EXPECT_THROW_MSG(fmt::format("{enum}", fmt::arg("enum", 1), fmt::arg("enum", 10)),
+      format_error, "duplicate named args found");
   EXPECT_EQ("st",
             fmt::format("{0:.{precision}}", "str", fmt::arg("precision", 2)));
   EXPECT_EQ(fmt::format("{} {two}", 1, fmt::arg("two", 2)), "1 2");