Extending fmt::join to support C++20-only ranges. (#2549)

include/fmt/format.h

@@ -2748,7 +2748,12 @@ using arg_join FMT_DEPRECATED_ALIAS = join_view<It, Sentinel, Char>;
 template <typename It, typename Sentinel, typename Char>
 struct formatter<join_view<It, Sentinel, Char>, Char> {
  private:
-  using value_type = typename std::iterator_traits<It>::value_type;
+  using value_type =
+#ifdef __cpp_lib_ranges
+      std::iter_value_t<It>;
+#else
+      typename std::iterator_traits<It>::value_type;
+#endif
   using context = buffer_context<Char>;
   using mapper = detail::arg_mapper<context>;
 
@@ -2782,11 +2787,13 @@ struct formatter<join_view<It, Sentinel, Char>, Char> {
     auto it = value.begin;
     auto out = ctx.out();
     if (it != value.end) {
-      out = value_formatter_.format(map(*it++), ctx);
+      out = value_formatter_.format(map(*it), ctx);
+      ++it;
       while (it != value.end) {
         out = detail::copy_str<Char>(value.sep.begin(), value.sep.end(), out);
         ctx.advance_to(out);
-        out = value_formatter_.format(map(*it++), ctx);
+        out = value_formatter_.format(map(*it), ctx);
+        ++it;
       }
     }
     return out;