[Someone]

I think this would work: first, determine the range. For unsigned variants, we have whatever_max = (whatever)(-1), if it isn’t already defined.

That makes parsing the unsigned variant relatively simple. Undefined behavior would be end-of-game, but unsigned addition and multiplication will not invoke that (if you need he,p, look at your favorite multiple-precision library for ideas about detecting wraparound)

For parsing signed integers, derive the min and max values of the signed variant from the max value of the unsigned one ((whatever_max - 1) / 2 or something like that

Then parse your input to the unsigned variant first, then check whether it fits in the unsigned version.