From aff640c32f9ada045e4f8c289119fb104e5c4290 Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Mon, 29 Jul 2024 21:18:20 +0500
Subject: [PATCH] Make fmt::appender implement std::output_iterator concept
 (#4093)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/format.h | 1 +
 test/base-test.cc    | 8 +-------
 test/format-test.cc  | 7 +++++++
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 1ad0d7ac8b3a..708c9051334e 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -110,6 +110,7 @@ namespace std {
 template <> struct iterator_traits<fmt::appender> {
   using iterator_category = output_iterator_tag;
   using value_type = char;
+  using difference_type = ptrdiff_t;
 };
 }  // namespace std
 
diff --git a/test/base-test.cc b/test/base-test.cc
index e03c5d9180bf..4afd99dab39e 100644
--- a/test/base-test.cc
+++ b/test/base-test.cc
@@ -93,7 +93,7 @@ TEST(string_view_test, compare) {
 }
 
 TEST(base_test, is_locking) {
-  EXPECT_FALSE(fmt::detail::is_locking<const char (&)[3]>());
+  EXPECT_FALSE(fmt::detail::is_locking<const char(&)[3]>());
 }
 
 TEST(base_test, is_output_iterator) {
@@ -116,12 +116,6 @@ TEST(base_test, is_back_insert_iterator) {
                std::front_insert_iterator<std::string>>::value);
 }
 
-TEST(base_test, buffer_appender) {
-#ifdef __cpp_lib_ranges
-  EXPECT_TRUE((std::output_iterator<fmt::appender, char>));
-#endif
-}
-
 #if !FMT_GCC_VERSION || FMT_GCC_VERSION >= 470
 TEST(buffer_test, noncopyable) {
   EXPECT_FALSE(std::is_copy_constructible<buffer<char>>::value);
diff --git a/test/format-test.cc b/test/format-test.cc
index 86c709816346..62d082a76c0c 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -26,6 +26,9 @@
 #include <string>              // std::string
 #include <thread>              // std::thread
 #include <type_traits>         // std::is_default_constructible
+#if FMT_CPLUSPLUS > 201703L && FMT_HAS_INCLUDE(<version>)
+#  include <version>
+#endif
 
 #include "gtest-extra.h"
 #include "mock-allocator.h"
@@ -42,6 +45,10 @@ using fmt::detail::uint128_fallback;
 using testing::Return;
 using testing::StrictMock;
 
+#ifdef __cpp_lib_concepts
+static_assert(std::output_iterator<fmt::appender, char>);
+#endif
+
 enum { buffer_size = 256 };
 
 TEST(uint128_test, ctor) {