fix #15489, don't check conversions during mixed signedness arithmetic

[JeffBezanson]

also skip checks for integer bitwise ops

[StefanKarpinski]

Wouldn't these definitions work for Integer as well? Would collapse all of these definitions.

[StefanKarpinski]

The basic observation here is that (a + b) % n == (a % n) + (b % n) % n for any a, b, n, so whatever the modulus of the target integer type, we can mod the original arguments into that, then do overflowing integer operations in that modulus and get the same result as if we'd done the operations in arbitrary precision and then reduced to the modulus of the target type. None of that depends on the signedness of the arguments, so we can do this completely universally for all kinds of integers – as long as they implement some kind of modular arithmetic.

[JeffBezanson]

That's right. But I was worried about integer types that might implement convert and promote_rule, but not %. Maybe we should add a fallback % that calls convert?

[StefanKarpinski]

The argument works for - and * for all n and for bitwise ops when n is a power of 2.

[StefanKarpinski]

Beautiful! 👏

[JeffBezanson]

Failure on 64-bit AV might be a variation of #23371?

[StefanKarpinski]

I don't think these failures are related, but I really wish we could get a clean CI run here.

[StefanKarpinski]

Ah, this feels very nice. I'm very pleased with how this turned out.