@moiseev @natecook1000, @xwu you all likely have some thoughts here, too.

@swift-ci create

I question whether there are realistic (or any) scenarios for which the desired solution is a fixed-width integer type that (a) behaves like it's got two's complement behavior in all observable respects; and (b) doesn't actually use two's complement. While this is fairly common for extant BigInt implementations, I'm not familiar with any such fixed-width types, and I really don't see how it gains us anything to word it in such a way.

@xwu Right. It's only a matter of precision, and I am perhaps being overcautious.

The scenario in which it could reasonably happen is something like "I have an existing highly optimized 256-bit integer library in assembly that uses sign-magnitude, and I want to make it available in Swift as a FixedWidthInteger type."

Conformance to FixedWidthInteger would not be an enjoyable experience for such a signed type, I'd imagine, even if not forbidden. Down the road, hopefully, the standard library will be able to offer Int128 (which LLVM already supports?). Then, if the compiler can be evolved to a point to support DoubleWidth properly, we can get to a point where all useful conforming types are vended by the standard library itself.