donc/phobos — comment on commit c0b25d0903d13eec0e5016ec9a3b187b48630b57
File: std/internal/math/biguintcore.d line 711
By donc:

This is just an assert, it's not recoverable. It's just a convenience thing, it catches some extreme cases which would otherwise result in out-of-memory errors, making them fail much more quickly, and be more debuggable. Unfortunately it's nowhere near precise enough to work except in cases where you've done something really absurd (like raising a million digit number to the power of 2^^63). But I found that absurd cases happen quite often, so it's useful in practice.