User defined literal arguments are not constexpr?

Question

I\'m testing out user defined literals. I want to make _fac return the factorial of the number.

Having it call a constexpr function works,

Answer 1

I may be wrong, but I think constexpr functions can also be called with non-constant arguments (in which case they don't give a constant expression and are evaluated at runtime). Which wouldn't work well with non-type template arguments.