Function format

test/format-test.cc:2116–2119

template <>
struct formatter<adl_test::fmt::detail::foo> : formatter<std::string> {
  auto format(adl_test::fmt::detail::foo, format_context& ctx) const
      -> decltype(ctx.out()) {
    return formatter<std::string>::format("foo", ctx);
  }
};
FMT_END_NAMESPACE