diff --git a/include/format b/include/format
index 633a731c..2427e79d 100644
--- a/include/format
+++ b/include/format
@@ -132,15 +132,17 @@ namespace std {
     enum indexing { unknown, manual, automatic }; // exposition only
     indexing indexing_;                           // exposition only
     size_t next_arg_id_;                          // exposition only
+    size_t num_args_;                             // exposition only
 
   public:
-    explicit constexpr basic_format_parse_context(basic_string_view<charT> fmt) noexcept;
+    explicit constexpr basic_format_parse_context(basic_string_view<charT> fmt,
+                                                  size_t num_args = 0) noexcept;
     basic_format_parse_context(const basic_format_parse_context&) = delete;
     basic_format_parse_context& operator=(const basic_format_parse_context&) = delete;
 
     constexpr const_iterator begin() const noexcept;
     constexpr const_iterator end() const noexcept;
-    constexpr void advance_to(iterator it);
+    constexpr void advance_to(const_iterator it);
 
     constexpr size_t next_arg_id();
     constexpr void check_arg_id(size_t id);
@@ -154,8 +156,10 @@ namespace std {
 
 namespace std {
 template<class charT>
-/* explicit */ constexpr basic_format_parse_context<charT>::basic_format_parse_context(basic_string_view<charT> fmt) noexcept
-: begin_(fmt.begin()), end_(fmt.end()), indexing_(unknown), next_arg_id_(0) {}
+/* explicit */ constexpr basic_format_parse_context<charT>::
+                   basic_format_parse_context(basic_string_view<charT> fmt,
+                                              size_t num_args) noexcept
+: begin_(fmt.begin()), end_(fmt.end()), indexing_(unknown), next_arg_id_(0), num_args_(num_args) {}
 
 template<class charT>
 constexpr typename basic_format_parse_context<charT>::const_iterator basic_format_parse_context<charT>::begin() const noexcept { return begin_; }
@@ -176,7 +180,10 @@ constexpr size_t basic_format_parse_context<charT>::next_arg_id() {
 }
 
 template<class charT>
-constexpr void basic_format_parse_context<charT>::check_arg_id(size_t) {
+constexpr void basic_format_parse_context<charT>::check_arg_id(size_t id) {
+  // clang doesn't support __builtin_is_constant_evaluated yet
+  //if (!(!__builtin_is_constant_evaluated() || id < num_args_))
+  //  throw format_error(invalid index is out of range");
   if (indexing_ == automatic)
     throw format_error("automatic to manual indexing");
   if (indexing_ == unknown)
@@ -186,17 +193,15 @@ constexpr void basic_format_parse_context<charT>::check_arg_id(size_t) {
 
 // http://fmtlib.net/Text%20Formatting.html#format.context
 namespace std {
-  template<class O, class charT> FMT_REQUIRES(OutputIterator<O, const charT&>)
+  template<class Out, class charT>
   class basic_format_context {
     basic_format_args<basic_format_context> args_; // exposition only
-    O out_;                                        // exposition only
+    Out out_;                                      // exposition only
 
   public:
-    using iterator = O;
+    using iterator = Out;
     using char_type = charT;
-
-    template<class T>
-      using formatter_type = formatter<T>;
+    template<class T> using formatter_type = formatter<T, charT>;
 
     basic_format_arg<basic_format_context> arg(size_t id) const;
 
@@ -205,7 +210,7 @@ namespace std {
 
     // Implementation details:
     using format_arg = basic_format_arg<basic_format_context>;
-    basic_format_context(O out, basic_format_args<basic_format_context> args, fmt::internal::locale_ref)
+    basic_format_context(Out out, basic_format_args<basic_format_context> args, fmt::internal::locale_ref)
     : args_(args), out_(out) {}
     fmt::internal::error_handler error_handler() const { return {}; }
     basic_format_arg<basic_format_context> arg(fmt::basic_string_view<charT>) const {
@@ -256,52 +261,55 @@ namespace std {
     class handle;
 
   private:
-    using char_type = typename Context::char_type;                     // exposition only
+    using char_type = typename Context::char_type;                       // exposition only
 
-  public: // public to workaround a bug in clang
     variant<monostate, bool, char_type,
             int, unsigned int, long long int, unsigned long long int,
             double, long double,
             const char_type*, basic_string_view<char_type>,
-            const void*, handle> value;                                // exposition only
+            const void*, handle> value;                                  // exposition only
 
-  private:
-    template<
-      FMT_CONCEPT(Integral) I,
+    template<typename T,
       typename = enable_if_t<
-        std::is_same_v<I, bool> ||
-        std::is_same_v<I, char_type> ||
-        (std::is_same_v<I, char> && std::is_same_v<char_type, wchar_t>) ||
-        detail::is_standard_integer_v<I> ||
-        detail::is_standard_unsigned_integer_v<I> ||
-        is_default_constructible_v<typename Context::template formatter_type<I>>
-        >>
-    explicit basic_format_arg(const I& n) noexcept;      // exposition only
-    explicit basic_format_arg(float n) noexcept;                       // exposition only
-    explicit basic_format_arg(double n) noexcept;                      // exposition only
-    explicit basic_format_arg(long double n) noexcept;                 // exposition only
-    explicit basic_format_arg(const char_type* s) noexcept;            // exposition only
-    explicit basic_format_arg(nullptr_t) noexcept;                     // exposition only
+        std::is_same_v<T, bool> ||
+        std::is_same_v<T, char_type> ||
+        (std::is_same_v<T, char> && std::is_same_v<char_type, wchar_t>) ||
+        detail::is_standard_integer_v<T> ||
+        detail::is_standard_unsigned_integer_v<T> ||
+        is_default_constructible_v<typename Context::template formatter_type<T>>
+    >> explicit basic_format_arg(const T& v) noexcept; // exposition only
+    explicit basic_format_arg(float n) noexcept;                         // exposition only
+    explicit basic_format_arg(double n) noexcept;                        // exposition only
+    explicit basic_format_arg(long double n) noexcept;                   // exposition only
+    explicit basic_format_arg(const char_type* s);                       // exposition only
 
     template<class traits>
       explicit basic_format_arg(
-        basic_string_view<char_type, traits> s) noexcept;              // exposition only
+        basic_string_view<char_type, traits> s) noexcept;                // exposition only
 
     template<class traits, class Allocator>
       explicit basic_format_arg(
-        const basic_string<char_type, traits, Allocator>& s) noexcept; // exposition only
+        const basic_string<char_type, traits, Allocator>& s) noexcept;   // exposition only
 
-    template<class T, typename = std::enable_if_t<std::is_same_v<T, void>>>
-      explicit basic_format_arg(const T* p) noexcept;                  // exposition only
+    explicit basic_format_arg(nullptr_t) noexcept;                       // exposition only
 
-    //template<class Visitor>
-    //  friend auto std::visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);
+    template<class T, typename = enable_if_t<is_void_v<T>>>
+      explicit basic_format_arg(const T* p) noexcept;                    // exposition only
+
+    // Fails due to a bug in clang
+    //template<class Visitor, class Ctx>
+    //  friend auto visit_format_arg(Visitor&& vis,
+    //                                    basic_format_arg<Ctx> arg);           // exposition only
+
+    friend auto get_value(basic_format_arg arg) {
+      return arg.value;
+    }
+
+    template <typename T, typename Char> friend struct detail::formatter;
 
     template<class Ctx, class... Args>
       friend format_arg_store<Ctx, Args...>
-        make_format_args(const Args&... args);                         // exposition only
-
-    template <typename T, typename Char> friend struct detail::formatter;
+        make_format_args(const Args&... args);                           // exposition only
 
   public:
     basic_format_arg() noexcept;
@@ -315,22 +323,21 @@ template<class Context>
 basic_format_arg<Context>::basic_format_arg() noexcept {}
 
 template<class Context>
-template<FMT_CONCEPT(Integral) I, typename>
-/* explicit */ basic_format_arg<Context>::basic_format_arg(const I& n) noexcept {
-  if constexpr (std::is_same_v<I, bool> || std::is_same_v<I, char_type>)
-    value = n;
-  else if constexpr (std::is_same_v<I, char> && std::is_same_v<char_type, wchar_t>)
-    value = static_cast<wchar_t>(n);
-  else if constexpr (detail::is_standard_integer_v<I> && sizeof(I) <= sizeof(int))
-    value = static_cast<int>(n);
-  else if constexpr (detail::is_standard_unsigned_integer_v<I> && sizeof(I) <= sizeof(unsigned))
-    value = static_cast<unsigned>(n);
-  else if constexpr (detail::is_standard_integer_v<I>)
-    value = static_cast<long long int>(n);
-  else if constexpr (detail::is_standard_unsigned_integer_v<I>)
-    value = static_cast<unsigned long long int>(n);
-  else if constexpr (is_default_constructible_v<typename Context::template formatter_type<I>>)
-    value = handle(n);
+template<class T, typename> /* explicit */ basic_format_arg<Context>::basic_format_arg(const T& v) noexcept {
+  if constexpr (std::is_same_v<T, bool> || std::is_same_v<T, char_type>)
+    value = v;
+  else if constexpr (std::is_same_v<T, char> && std::is_same_v<char_type, wchar_t>)
+    value = static_cast<wchar_t>(v);
+  else if constexpr (detail::is_standard_integer_v<T> && sizeof(T) <= sizeof(int))
+    value = static_cast<int>(v);
+  else if constexpr (detail::is_standard_unsigned_integer_v<T> && sizeof(T) <= sizeof(unsigned))
+    value = static_cast<unsigned>(v);
+  else if constexpr (detail::is_standard_integer_v<T>)
+    value = static_cast<long long int>(v);
+  else if constexpr (detail::is_standard_unsigned_integer_v<T>)
+    value = static_cast<unsigned long long int>(v);
+  else if constexpr (is_default_constructible_v<typename Context::template formatter_type<T>>)
+    value = handle(v);
 }
 
 template<class Context>
@@ -346,7 +353,7 @@ template<class Context>
   : value(n) {}
 
 template<class Context>
-/* explicit */ basic_format_arg<Context>::basic_format_arg(const typename basic_format_arg<Context>::char_type* s) noexcept
+/* explicit */ basic_format_arg<Context>::basic_format_arg(const typename basic_format_arg<Context>::char_type* s)
   : value(s) {
   assert(s != nullptr);
 }
@@ -386,7 +393,7 @@ namespace std {
 
     template<class T> explicit handle(const T& val) noexcept; // exposition only
 
-    friend class basic_format_arg<Context>;
+    friend class basic_format_arg<Context>;                   // exposition only
 
   public:
     void format(basic_format_parse_context<char_type>&, Context& ctx) const;
@@ -397,10 +404,10 @@ namespace std {
 template<class Context>
 template<class T> /* explicit */ basic_format_arg<Context>::handle::handle(const T& val) noexcept
   : ptr_(&val), format_([](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) {
-  typename Context::template formatter_type<T> f;
-  parse_ctx.advance_to(f.parse(parse_ctx));
-  format_ctx.advance_to(f.format(*static_cast<const T*>(ptr), format_ctx));
-}) {}
+    typename Context::template formatter_type<T> f;
+    parse_ctx.advance_to(f.parse(parse_ctx));
+    format_ctx.advance_to(f.format(*static_cast<const T*>(ptr), format_ctx));
+  }) {}
 
 template<class Context>
 void basic_format_arg<Context>::handle::format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const {
@@ -410,7 +417,7 @@ void basic_format_arg<Context>::handle::format(basic_format_parse_context<char_t
 // http://fmtlib.net/Text%20Formatting.html#format.visit
 template<class Visitor, class Context>
   auto visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg) {
-    return visit(vis, arg.value);
+    return visit(vis, get_value(arg));
   }
 }
 
@@ -465,7 +472,7 @@ template<class Context /*= format_context*/, class... Args>
 // http://fmtlib.net/Text%20Formatting.html#format.make_wargs
 template<class... Args>
   format_arg_store<wformat_context, Args...> make_wformat_args(const Args&... args) {
-    return {basic_format_arg<wformat_context>(args)...};
+    return make_format_args<wformat_context>(args...);
   }
 }
 
@@ -543,7 +550,7 @@ inline fmt::internal::type get_type(basic_format_arg<Context> arg) {
       return fmt::internal::string_type;
     if (std::is_same_v<T, const void*>)
       return fmt::internal::pointer_type;
-    assert(arg.value.index() == 12);
+    assert(get_value(arg).index() == 12);
     return fmt::internal::custom_type;
   }, arg);
 }