According to spirit document,
"All numeric parsers check for overflow conditions based on the type T the corresponding uint_parser<> has been instantiated with. If the parsed number overflows this type the parsing fails. Please be aware that the overflow check is not based on the type of the supplied attribute but solely depends on the template parameter T."
the valid input range of x3::uint_parser<T> should be 0 - std::numeric_limits<T>::max(). However, the current implementation takes values from std::numeric_limits<T>::max()+1 - std::numeric_limits<std::make_unsigned_t<T>>::max() also as valid input.