The decimal-literal in a user-defined-integer-literal might be too large for an unsigned long long to represent (in implementations with extended integer types). In such cases, the original intent appears to have been to call a raw literal operator or a literal operator template; however, the existing wording of 5.13.9 [lex.ext] paragraph 3 always calls the unsigned long long literal operator if it exists, regardless of the value of the decimal-literal.