Simplify ostream opt-in API

include/fmt/ostream.h

@@ -78,14 +78,12 @@ void format_value(buffer<Char>& buf, const T& value,
   output.exceptions(std::ios_base::failbit | std::ios_base::badbit);
   buf.try_resize(buf.size());
 }
+}  // namespace detail
 
 // Formats an object of type T that has an overloaded ostream operator<<.
-template <typename T, typename Char>
-struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
-    : private formatter<basic_string_view<Char>, Char> {
-  using formatter<basic_string_view<Char>, Char>::parse;
-
-  template <typename OutputIt>
+template <typename Char>
+struct basic_ostream_formatter : formatter<basic_string_view<Char>, Char> {
+  template <typename T, typename OutputIt>
   auto format(const T& value, basic_format_context<OutputIt, Char>& ctx) const
       -> OutputIt {
     auto buffer = basic_memory_buffer<Char>();
@@ -93,7 +91,17 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
     return formatter<basic_string_view<Char>, Char>::format(
         {buffer.data(), buffer.size()}, ctx);
   }
+};
 
+using ostream_formatter = basic_ostream_formatter<char>;
+
+namespace detail {
+
+// Formats an object of type T that has an overloaded ostream operator<<.
+template <typename T, typename Char>
+struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
+    : basic_ostream_formatter<Char> {
+  using basic_ostream_formatter<Char>::format;
   // DEPRECATED!
   template <typename OutputIt>
   auto format(const T& value, basic_printf_context<OutputIt, Char>& ctx) const
@@ -105,9 +113,6 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
 };
 }  // namespace detail
 
-template <typename T, typename Char = char>
-using ostream_formatter = detail::fallback_formatter<T, Char>;
-
 FMT_MODULE_EXPORT
 template <typename Char>
 void vprint(std::basic_ostream<Char>& os, basic_string_view<Char> format_str,