Implement fmt::format_to into std::vector<char>

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/base.h

@@ -1971,12 +1971,26 @@ 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 It,
+          FMT_ENABLE_IF(has_container_append<T, It>::value)>
+FMT_CONSTEXPR20 void copy_container_append(T& container, It begin, It end) {
+  container.append(begin, end);
+}
+
 template <typename T, typename InputIt, typename OutputIt,
           FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value)>
 FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
     -> OutputIt {
-  get_container(out).append(begin, end);
+  copy_container_append(get_container(out), begin, end);
   return out;
 }