prepare tests, fix incorrect handling of named args with simple {} replacement fields

include/fmt/compile.h

@@ -454,9 +454,14 @@ template <typename Char, typename T, int N> struct field {
 
   template <typename OutputIt, typename... Args>
   constexpr OutputIt format(OutputIt out, const Args&... args) const {
-    // This ensures that the argument type is convertile to `const T&`.
-    const T& arg = get<N>(args...);
-    return write<Char>(out, arg);
+    if constexpr (is_named_arg<typename std::remove_cv<T>::type>::value) {
+      decltype(T::value) arg = get<N>(args...).value;
+      return write<Char>(out, arg);
+    } else {
+      // This ensures that the argument type is convertile to `const T&`.
+      const T& arg = get<N>(args...);
+      return write<Char>(out, arg);
+    }
   }
 };
 
@@ -775,8 +780,15 @@ FMT_INLINE std::basic_string<typename S::char_type> format(const S&,
 #ifdef __cpp_if_constexpr
   if constexpr (std::is_same<typename S::char_type, char>::value) {
     constexpr basic_string_view<typename S::char_type> str = S();
-    if constexpr (str.size() == 2 && str[0] == '{' && str[1] == '}')
-      return fmt::to_string(detail::first(args...));
+    if constexpr (str.size() == 2 && str[0] == '{' && str[1] == '}') {
+      auto first = detail::first(args...);
+      if constexpr (detail::is_named_arg<typename std::remove_cv<
+                        decltype(first)>::type>::value) {
+        return fmt::to_string(first.value);
+      } else {
+        return fmt::to_string(first);
+      }
+    }
   }
 #endif
   constexpr auto compiled = detail::compile<Args...>(S());