[test/Prototypes] Fix DoubleWidth think-os #70289
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Aligns the implementation with fixes already committed in the apple/swift-numerics repository
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stephentyrone
approved these changes
Dec 7, 2023
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Aligns the implementation with fixes already committed in the apple/swift-numerics repository:
Since it turns out there's more than one calling site for
_dividethat doesn't ensure the invariant asserted on line 598, we should actually deal with this in one place by replacing the assertion with a guard that handles the special case.Additionally, fix another think-o related to full-width multiplication where a carry isn't accounted for, causing unintended overflow with certain inputs.
This type now gives correct output verified for addition/subtraction/multiplication/nonzero-division-with-overflow for every pair of 16-bit unsigned integers using
DoubleWidth<DoubleWidth<UInt4>>(with a customUInt4implementation).Resolves apple/swift-numerics#272.