From 64a55ba2141b52f91ac6b290d406272e7500c8ad Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Mon, 21 Oct 2024 20:14:21 +0500
Subject: [PATCH 1/2] Implement fmt::format_to into std::vector<char>

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/base.h   | 27 +++++++++++++++++++++++++--
 test/compile-test.cc | 11 ++++++++---
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 4d846f44f0fe..bdb4f5e9c220 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -1973,15 +1973,38 @@ template <typename T = char> class counting_buffer : public buffer<T> {
 template <typename T>
 struct is_back_insert_iterator<basic_appender<T>> : std::true_type {};
 
+template <typename T, typename It, typename = void>
+struct has_container_append : std::false_type {};
+template <typename T, typename It>
+struct has_container_append<T, It,
+                            void_t<decltype(std::declval<T>().append(
+                                std::declval<It>(), std::declval<It>()))>>
+    : std::true_type {};
+
 // An optimized version of std::copy with the output value type (T).
-template <typename T, typename InputIt, typename OutputIt,
-          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value)>
+template <
+    typename T, typename InputIt, typename OutputIt,
+    FMT_ENABLE_IF(
+        is_back_insert_iterator<OutputIt>::value&& has_container_append<
+            decltype(get_container(std::declval<OutputIt>())), InputIt>::value)>
 FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
     -> OutputIt {
   get_container(out).append(begin, end);
   return out;
 }
 
+template <typename T, typename InputIt, typename OutputIt,
+          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value &&
+                        !has_container_append<
+                            decltype(get_container(std::declval<OutputIt>())),
+                            InputIt>::value)>
+FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
+    -> OutputIt {
+  auto& c = get_container(out);
+  c.insert(c.end(), begin, end);
+  return out;
+}
+
 template <typename T, typename InputIt, typename OutputIt,
           FMT_ENABLE_IF(!is_back_insert_iterator<OutputIt>::value)>
 FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
diff --git a/test/compile-test.cc b/test/compile-test.cc
index e3a5dca2bc7c..043588e49050 100644
--- a/test/compile-test.cc
+++ b/test/compile-test.cc
@@ -8,6 +8,7 @@
 #include "fmt/compile.h"
 
 #include <type_traits>
+#include <vector>
 
 #include "fmt/chrono.h"
 #include "fmt/ranges.h"
@@ -229,10 +230,14 @@ TEST(compile_test, unknown_format_fallback) {
   EXPECT_EQ(" 42 ",
             fmt::format(FMT_COMPILE("{name:^4}"), fmt::arg("name", 42)));
 
-  std::vector<char> v;
-  fmt::format_to(std::back_inserter(v), FMT_COMPILE("{name:^4}"),
+  std::vector<char> v1;
+  fmt::format_to(std::back_inserter(v1), FMT_COMPILE("{}"), 42);
+  EXPECT_EQ("42", fmt::string_view(v1.data(), v1.size()));
+
+  std::vector<char> v2;
+  fmt::format_to(std::back_inserter(v2), FMT_COMPILE("{name:^4}"),
                  fmt::arg("name", 42));
-  EXPECT_EQ(" 42 ", fmt::string_view(v.data(), v.size()));
+  EXPECT_EQ(" 42 ", fmt::string_view(v2.data(), v2.size()));
 
   char buffer[4];
   auto result = fmt::format_to_n(buffer, 4, FMT_COMPILE("{name:^5}"),

From ca497db44c69e92b652a551f16abbf65bf1a492e Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Thu, 24 Oct 2024 01:18:09 +0500
Subject: [PATCH 2/2] Move get_container to the trait

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/base.h | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index bdb4f5e9c220..169b08619c90 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -1973,20 +1973,20 @@ template <typename T = char> class counting_buffer : public buffer<T> {
 template <typename T>
 struct is_back_insert_iterator<basic_appender<T>> : std::true_type {};
 
-template <typename T, typename It, typename = void>
-struct has_container_append : std::false_type {};
-template <typename T, typename It>
-struct has_container_append<T, It,
-                            void_t<decltype(std::declval<T>().append(
-                                std::declval<It>(), std::declval<It>()))>>
-    : std::true_type {};
+template <typename OutputIt, typename InputIt, typename = void>
+struct has_back_insert_iterator_container_append : std::false_type {};
+template <typename OutputIt, typename InputIt>
+struct has_back_insert_iterator_container_append<
+    OutputIt, InputIt,
+    void_t<decltype(get_container(std::declval<OutputIt>())
+                        .append(std::declval<InputIt>(),
+                                std::declval<InputIt>()))>> : std::true_type {};
 
 // An optimized version of std::copy with the output value type (T).
-template <
-    typename T, typename InputIt, typename OutputIt,
-    FMT_ENABLE_IF(
-        is_back_insert_iterator<OutputIt>::value&& has_container_append<
-            decltype(get_container(std::declval<OutputIt>())), InputIt>::value)>
+template <typename T, typename InputIt, typename OutputIt,
+          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value&&
+                            has_back_insert_iterator_container_append<
+                                OutputIt, InputIt>::value)>
 FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
     -> OutputIt {
   get_container(out).append(begin, end);
@@ -1995,9 +1995,8 @@ FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
 
 template <typename T, typename InputIt, typename OutputIt,
           FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value &&
-                        !has_container_append<
-                            decltype(get_container(std::declval<OutputIt>())),
-                            InputIt>::value)>
+                        !has_back_insert_iterator_container_append<
+                            OutputIt, InputIt>::value)>
 FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
     -> OutputIt {
   auto& c = get_container(out);