Conform std::iterator_traits<fmt::appender> to [iterator.traits]/1 (f…

…mtlib#4185)

* Conform `std::iterator_traits<fmt::appender>` to [iterator.traits]/1

> In addition, the types
> ```c++
> iterator_traits<I>::pointer
> iterator_traits<I>::reference
> ```
> shall be defined as the iterator’s pointer and reference types; that is, for an iterator object `a` of class type, the same type as `decltype(a.operator->())` and `decltype(*a)`, respectively. The type `iterator_traits<I>::pointer` shall be void for an iterator of class type `I` that does not support `operator->`. Additionally, in the case of an output iterator, the types
> ```c++
> iterator_traits<I>::value_type
> iterator_traits<I>::difference_type
> iterator_traits<I>::reference
> ```
> may be defined as `void`.

* Remove unnecessary member types from basic_appender

This reverts commit 1accf6c.

* Address clang-format issue

include/fmt/base.h

@@ -2382,11 +2382,6 @@ template <typename T> class basic_appender {
   detail::buffer<T>* container;
 
  public:
-  using iterator_category = int;
-  using value_type = T;
-  using pointer = T*;
-  using reference = T&;
-  using difference_type = decltype(pointer() - pointer());
   using container_type = detail::buffer<T>;
 
   FMT_CONSTEXPR basic_appender(detail::buffer<T>& buf) : container(&buf) {}

include/fmt/format.h

@@ -119,11 +119,13 @@
 #endif
 
 namespace std {
-template <> struct iterator_traits<fmt::appender> {
+template <class T> struct iterator_traits<fmt::basic_appender<T>> {
   using iterator_category = output_iterator_tag;
-  using value_type = char;
-  using reference = char&;
-  using difference_type = fmt::appender::difference_type;
+  using value_type = T;
+  using difference_type =
+      decltype(static_cast<int*>(nullptr) - static_cast<int*>(nullptr));
+  using pointer = void;
+  using reference = void;
 };
 }  // namespace std