Simplify format string checks

include/fmt/prepare.h

@@ -187,7 +187,6 @@ class prepared_format {
  public:
   typedef FMT_CHAR(Format) char_type;
   typedef format_part<char_type> format_part_t;
-  typedef internal::checked_args<Format, Args...> checked_args;
 
   prepared_format(Format f)
       : format_(std::move(f)), parts_provider_(to_string_view(format_)) {}
@@ -218,7 +217,7 @@ class prepared_format {
   std::basic_string<char_type> format(const Args&... args) const {
     basic_memory_buffer<char_type> buffer;
     typedef back_insert_range<internal::basic_buffer<char_type>> range;
-    this->vformat_to(range(buffer), *checked_args(format_, args...));
+    this->vformat_to(range(buffer), make_args_checked(format_, args...));
     return to_string(buffer);
   }
 
@@ -229,7 +228,7 @@ class prepared_format {
             const Args&... args) const {
     internal::container_buffer<Container> buffer(internal::get_container(out));
     typedef back_insert_range<internal::basic_buffer<char_type>> range;
-    this->vformat_to(range(buffer), *checked_args(format_, args...));
+    this->vformat_to(range(buffer), make_args_checked(format_, args...));
     return out;
   }
 
@@ -245,16 +244,18 @@ class prepared_format {
   inline typename buffer_context<char_type>::type::iterator format_to(
       basic_memory_buffer<char_type, SIZE>& buf, const Args&... args) const {
     typedef back_insert_range<internal::basic_buffer<char_type>> range;
-    return this->vformat_to(range(buf), *checked_args(format_, args...));
+    return this->vformat_to(range(buf), make_args_checked(format_, args...));
   }
 
  private:
-  template <typename Range, typename Context>
-  typename Context::iterator vformat_to(Range out,
-                                        basic_format_args<Context> args) const {
+  typedef typename buffer_context<char_type>::type context;
+
+  template <typename Range>
+  typename context::iterator vformat_to(Range out,
+                                        basic_format_args<context> args) const {
     const auto format_view = internal::to_string_view(format_);
     basic_parse_context<char_type> parse_ctx(format_view);
-    Context ctx(out.begin(), args);
+    context ctx(out.begin(), args);
 
     const auto& parts = parts_provider_.parts();
     for (auto part_it = parts.begin(); part_it != parts.end(); ++part_it) {