Improve addressof()

Signed-off-by: Vladislav Shchapov <[email protected]>

include/fmt/core.h

@@ -104,6 +104,22 @@
 #  define FMT_CONSTEXPR
 #endif
 
+#if FMT_MSC_VERSION && _MSC_FULL_VER >= 190024215
+#  define FMT_HAS_BUILTIN_ADDRESSOF
+#elif FMT_GCC_VERSION >= 700
+#  define FMT_HAS_BUILTIN_ADDRESSOF
+#elif defined(__has_builtin)
+#  if __has_builtin(__builtin_addressof)
+#    define FMT_HAS_BUILTIN_ADDRESSOF
+#  endif
+#endif
+
+#if FMT_CPLUSPLUS >= 201703L && defined(FMT_HAS_BUILTIN_ADDRESSOF)
+#  define FMT_CONSTEXPR_BUILTIN_ADDRESSOF constexpr
+#else
+#  define FMT_CONSTEXPR_BUILTIN_ADDRESSOF
+#endif
+
 #if ((FMT_CPLUSPLUS >= 202002L) &&                            \
      (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE > 9)) || \
     (FMT_CPLUSPLUS >= 201709L && FMT_GCC_VERSION >= 1002)
@@ -1233,6 +1249,23 @@ template <typename Context> struct custom_value {
   void (*format)(void* arg, parse_context& parse_ctx, Context& ctx);
 };
 
+#if FMT_GCC_VERSION && FMT_GCC_VERSION < 500
+// A workaround for gcc 4.8 to make void_t work in a SFINAE context.
+template <typename...> struct void_t_impl {
+  using type = void;
+};
+template <typename... T> using void_t = typename void_t_impl<T...>::type;
+#else
+template <typename...> using void_t = void;
+#endif
+
+template <typename T, typename Enable = void>
+struct has_address_operator : std::false_type {};
+
+template <typename T>
+struct has_address_operator<T, void_t<decltype(&std::declval<T>())>>
+    : std::true_type {};
+
 // A formatting argument value.
 template <typename Context> class value {
  public:
@@ -1281,22 +1314,35 @@ template <typename Context> class value {
   FMT_INLINE value(const named_arg_info<char_type>* args, size_t size)
       : named_args{args, size} {}
 
-  template <typename T> FMT_CONSTEXPR FMT_INLINE value(T& val) {
+  // Workaround for types with overloaded operator&.
+  // std::addressof is non-constexpr before C++17.
+  template <typename T, FMT_ENABLE_IF(has_address_operator<T>::value)>
+  FMT_CONSTEXPR_BUILTIN_ADDRESSOF FMT_INLINE value(T& val) {
     using value_type = remove_const_t<T>;
-#if defined(_LIBCPP_VERSION)
-    // Workaround for formatter<std::vector<bool>::reference, Char>.
-    // libc++ std::vector<bool>::reference has an overloaded operator&.
-    // std::addressof is non-constexpr before C++17.
+#ifdef FMT_HAS_BUILTIN_ADDRESSOF
     custom.value = const_cast<value_type*>(__builtin_addressof(val));
 #else
-    custom.value = const_cast<value_type*>(&val);
+    custom.value = reinterpret_cast<value_type*>(
+        const_cast<char*>(&reinterpret_cast<const volatile char&>(val)));
 #endif
     // Get the formatter type through the context to allow different contexts
     // have different extension points, e.g. `formatter<T>` for `format` and
     // `printf_formatter<T>` for `printf`.
     custom.format = format_custom_arg<
         value_type, typename Context::template formatter_type<value_type>>;
   }
+
+  template <typename T, FMT_ENABLE_IF(!has_address_operator<T>::value)>
+  FMT_CONSTEXPR FMT_INLINE value(T& val) {
+    using value_type = remove_const_t<T>;
+    custom.value = const_cast<value_type*>(&val);
+    // Get the formatter type through the context to allow different contexts
+    // have different extension points, e.g. `formatter<T>` for `format` and
+    // `printf_formatter<T>` for `printf`.
+    custom.format = format_custom_arg<
+        value_type, typename Context::template formatter_type<value_type>>;
+  }
+
   value(unformattable);
   value(unformattable_char);
   value(unformattable_pointer);
@@ -1509,14 +1555,6 @@ FMT_CONSTEXPR auto copy_str(R&& rng, OutputIt out) -> OutputIt {
   return detail::copy_str<Char>(rng.begin(), rng.end(), out);
 }
 
-#if FMT_GCC_VERSION && FMT_GCC_VERSION < 500
-// A workaround for gcc 4.8 to make void_t work in a SFINAE context.
-template <typename...> struct void_t_impl { using type = void; };
-template <typename... T> using void_t = typename void_t_impl<T...>::type;
-#else
-template <typename...> using void_t = void;
-#endif
-
 template <typename It, typename T, typename Enable = void>
 struct is_output_iterator : std::false_type {};