Clean core-test and fix linkage errors on older gcc

include/fmt/format.h

@@ -1154,54 +1154,6 @@ template <typename T = void>
 struct null {};
 }  // namespace internal
 
-struct monostate {};
-
-/**
-  \rst
-  Visits an argument dispatching to the appropriate visit method based on
-  the argument type. For example, if the argument type is ``double`` then
-  ``vis(value)`` will be called with the value of type ``double``.
-  \endrst
- */
-template <typename Visitor, typename Context>
-FMT_CONSTEXPR typename internal::result_of<Visitor(int)>::type
-    visit(Visitor &&vis, const basic_format_arg<Context> &arg) {
-  typedef typename Context::char_type char_type;
-  switch (arg.type_) {
-  case internal::none_type:
-    break;
-  case internal::named_arg_type:
-    FMT_ASSERT(false, "invalid argument type");
-    break;
-  case internal::int_type:
-    return vis(arg.value_.int_value);
-  case internal::uint_type:
-    return vis(arg.value_.uint_value);
-  case internal::long_long_type:
-    return vis(arg.value_.long_long_value);
-  case internal::ulong_long_type:
-    return vis(arg.value_.ulong_long_value);
-  case internal::bool_type:
-    return vis(arg.value_.int_value != 0);
-  case internal::char_type:
-    return vis(static_cast<char_type>(arg.value_.int_value));
-  case internal::double_type:
-    return vis(arg.value_.double_value);
-  case internal::long_double_type:
-    return vis(arg.value_.long_double_value);
-  case internal::cstring_type:
-    return vis(arg.value_.string.value);
-  case internal::string_type:
-    return vis(basic_string_view<char_type>(
-                 arg.value_.string.value, arg.value_.string.size));
-  case internal::pointer_type:
-    return vis(arg.value_.pointer);
-  case internal::custom_type:
-    return vis(typename basic_format_arg<Context>::handle(arg.value_.custom));
-  }
-  return vis(monostate());
-}
-
 enum alignment {
   ALIGN_DEFAULT, ALIGN_LEFT, ALIGN_RIGHT, ALIGN_CENTER, ALIGN_NUMERIC
 };