NSRoundBankers is not working as documented by Apple

Question

So, according to Apple's documentation about NSRoundBankers:

Round to the closest possible return value; when halfway between two possibilities, return the possibility whose last digit is even.

While this is true for positive numbers, I am not getting the expected behaviour on negative numbers. Here is the piece of code I executed on the device and on the simulator, both printing the exact same results:

NSDecimalNumber *increment = [NSDecimalNumber decimalNumberWithMantissa:5 exponent:-2 isNegative:NO];
NSDecimalNumber *number = [NSDecimalNumber decimalNumberWithMantissa:10 exponent:-1 isNegative:YES];
NSDecimalNumberHandler *handler = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers scale:1 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];
while ([number compare:@1] == NSOrderedAscending)
{
    NSLog(@";%@;%@", number, [number decimalNumberByRoundingAccordingToBehavior:handler]);
    number = [number decimalNumberByAdding:increment];
}

On negative numbers, it's not returning the one whose last digit is even, it basically rounds down.

For example, for -0.85 i should be getting -0.8, but I am getting -0.9

Am I doing something wrong?

Left table shows the ACTUAL behaviour, in red marked the wrong rounded values.

Right table shows the EXPECTED behaviour, in green the correct rounded values.

Answer 1

Stumbled upon this today and it's probably a bug in Foundation. If the source code of swift-corelibs-foundation has anything to do with the actual Foundation code, I might have found the culprit: https://github.com/apple/swift-corelibs-foundation/blob/70f8af962ff182c78a81673e75fe725b5b1b7827/Foundation/Decimal.swift#L1970.

I believe 1 should be subtracted from remainder on the line 1970, not added to it. Rationale: if remainder is 5, what does adding 1 to it actually change? The case falls trough (line 1972) and the check on line 1974 succeeds either way, be it 5 or 6. If it were to become 4 for after subtracting, it will prevent adding 1 to self (line 1979) and keep the number even.

UPD the issue has been reported to Apple: FB7565793.

UPD 2 the issue has been fixed by Apple in the first betas of iOS 14 / macOS 11.0.