Cleanup base API

include/fmt/base.h

@@ -657,7 +657,6 @@ using buffered_context =
     conditional_t<std::is_same<Char, char>::value, context,
                   generic_context<basic_appender<Char>, Char>>;
 
-template <typename Context> struct arg_mapper;
 FMT_EXPORT template <typename Context> class basic_format_arg;
 FMT_EXPORT template <typename Context> class basic_format_args;
 FMT_EXPORT template <typename Context> class dynamic_format_arg_store;
@@ -694,6 +693,11 @@ struct has_to_string_view<
     T, void_t<decltype(detail::to_string_view(std::declval<T>()))>>
     : std::true_type {};
 
+/// String's character (code unit) type. detail:: is intentional to prevent ADL.
+template <typename S,
+          typename V = decltype(detail::to_string_view(std::declval<S>()))>
+using char_t = typename V::value_type;
+
 template <typename Char>
 using unsigned_char = typename conditional_t<std::is_integral<Char>::value,
                                              std::make_unsigned<Char>,
@@ -825,11 +829,6 @@ FMT_DEPRECATED FMT_NORETURN inline void throw_format_error(
   report_error(message);
 }
 
-/// String's character (code unit) type.
-template <typename S,
-          typename V = decltype(detail::to_string_view(std::declval<S>()))>
-using char_t = typename V::value_type;
-
 enum class presentation_type : unsigned char {
   // Common specifiers:
   none = 0,
@@ -2098,10 +2097,6 @@ template <typename T> struct format_as_result {
 };
 template <typename T> using format_as_t = typename format_as_result<T>::type;
 
-template <typename T>
-struct has_format_as
-    : bool_constant<!std::is_same<format_as_t<T>, void>::value> {};
-
 #define FMT_MAP_API static FMT_CONSTEXPR FMT_ALWAYS_INLINE
 
 // Maps formatting arguments to reduce the set of types we need to work with.