Don't wrap named arg in cref and clarify docs

include/fmt/args.h

@@ -168,29 +168,21 @@ class dynamic_format_arg_store
   /**
     \rst
     Adds a reference to the argument into the dynamic store for later passing to
-    a formatting function. Supports named arguments wrapped in
+    a formatting function.
-    ``std::reference_wrapper`` via ``std::ref()``/``std::cref()``.
 
     **Example**::
 
       fmt::dynamic_format_arg_store<fmt::format_context> store;
-      char str[] = "1234567890";
+      char band[] = "Rolling Stones";
-      store.push_back(std::cref(str));
+      store.push_back(std::cref(band));
-      int a1_val{42};
+      band[9] = 'c'; // Changing str affects the output.
-      auto a1 = fmt::arg("a1_", a1_val);
+      std::string result = fmt::vformat("{}", store);
-      store.push_back(std::cref(a1));
+      // result == "Rolling Scones"
-
-      // Changing str affects the output but only for string and custom types.
-      str[0] = 'X';
-
-      std::string result = fmt::vformat("{} and {a1_}");
-      assert(result == "X234567890 and 42");
     \endrst
   */
   template <typename T> void push_back(std::reference_wrapper<T> arg) {
     static_assert(
-        detail::is_named_arg<typename std::remove_cv<T>::type>::value ||
+        need_copy<T>::value,
-            need_copy<T>::value,
         "objects of built-in types and string views are always copied");
     emplace_arg(arg.get());
   }
@@ -198,7 +190,7 @@ class dynamic_format_arg_store
   /**
     Adds named argument into the dynamic store for later passing to a formatting
     function. ``std::reference_wrapper`` is supported to avoid copying of the
-    argument.
+    argument. The name is always stored by reference.
   */
   template <typename T>
   void push_back(const detail::named_arg<char_type, T>& arg) {

include/fmt/core.h

@@ -1619,8 +1619,9 @@ inline auto make_args_checked(const S& format_str,
 
 /**
   \rst
-  Returns a named argument to be used in a formatting function. It should only
+  Returns a named argument to be used in a formatting function.
-  be used in a call to a formatting function.
+  It should only be used in a call to a formatting function or
+  `dynamic_format_arg_store::push_back`.
 
   **Example**::
 

test/core-test.cc

@@ -469,24 +469,10 @@ TEST(FormatDynArgsTest, NamedStrings) {
 
 TEST(FormatDynArgsTest, NamedArgByRef) {
   fmt::dynamic_format_arg_store<fmt::format_context> store;
-
+  char band[] = "Rolling Stones";
-  // Note: fmt::arg() constructs an object which holds a reference
+  store.push_back(fmt::arg("band", std::cref(band)));
-  // to its value. It's not an aggregate, so it doesn't extend the
+  band[9] = 'c'; // Changing str affects the output.
-  // reference lifetime. As a result, it's a very bad idea passing temporary
+  EXPECT_EQ(fmt::vformat("{band}", store), "Rolling Scones");
-  // as a named argument value. Only GCC with optimization level >0
-  // complains about this.
-  //
-  // A real life usecase is when you have both name and value alive
-  // guarantee their lifetime and thus don't want them to be copied into
-  // storages.
-  int a1_val{42};
-  auto a1 = fmt::arg("a1_", a1_val);
-  store.push_back("abc");
-  store.push_back(1.5f);
-  store.push_back(std::cref(a1));
-
-  std::string result = fmt::vformat("{a1_} and {} and {} and {}", store);
-  EXPECT_EQ("42 and abc and 1.5 and 42", result);
 }
 
 TEST(FormatDynArgsTest, NamedCustomFormat) {