I believe a restriction for `constexpr`/`consteval` is that you can do memory allocations but they must all be freed at the end of the evaluation. Arbitrary precision means arbitrary memory usage means allocation that's the runtime's responsibility. Maybe there's a trick that gets around that (define a type at compile time that's just big enough for this specific number and return that?), but I don't see it as possible without something weird.

You can allocate as much as you want, but the allocation cannot survive to runtime time. So the final result would have to be copied on a large enough static buffer.