[vitus]

Sure, then set one to 2^62 and the other to -2^62 (namely: 0b1100..00). It's overflow as far as unsigned arithmetic is concerned, but not in the case of signed arithmetic.

That said, when you're dealing with 256-bit integers, you're almost assuredly not working with signed arithmetic.

[immibis]

...so? They don't care about top limb overflow, at all. That's the point.