[SR-5353] NSDecimalNumber cast to Int64 fails on some large numbers

[swift-ci]

Previous ID	SR-5353
Radar	None
Original Reporter	vlas (JIRA User)
Type	Bug

Attachment: Download

Environment

Apple Swift version 4.0 (swiftlang-900.0.45.6 clang-900.0.26)
Apple Swift version 4.0-dev (LLVM 1a69f372ee, Clang 17f727b492, Swift a8ba0772cd)

Additional Detail from JIRA

Votes	0
Component/s	Compiler, Standard Library
Labels	Bug, Foundation
Assignee	None
Priority	Medium

md5: 9c092cf213be7d5b64c0a4aaa961ffeb

Issue Description:

Attempting to conditionally cast some numbers larger than `90071992547409930` (`0x0C7FFFFFFFFFDFB2`) stored in `NSDecimalNumber` to `Int64` using `as?` fails (results in `nil`). Experimentally, all such numbers are multiples of 10, but not every multiple of 10 fails – only some of them, e.g.:

• 90071992547409930

• 90071992547409970

• 90071992547409990

• 90071992547410010

• 90071992547410030

• 90071992547410070

• 90071992547410090

• 90071992547410130

• 90071992547410150

• etc.

To exercise the issue, simply create a decimal number and cast it to Int64: `NSDecimalNumber(value: largeNumber) as? Int64`. Note that this issue does not occur if a normal `NSNumber` is used instead. This is a regression from Swift 3 where such numbers would be cast correctly.

An example where this behaviour would cause problems is decoding JSON using `JSONSerialization` on macOS 10.13 and iOS 11 where (some?) numbers are decoded internally into `NSDecimalNumber`. In cases where a cast to Int or Int64 during decoding would previously succeed, it would now fail.

Attached is a sample macOS command line project that demonstrates the issue.

[belkadan]

@phausler, is this a Swift issue, or would Objective-C Foundation have this behavior as well? (I suppose the "cast to Int64" is Swift-specific, but…)

[pmuller]

Expected behaviour. Downcasting is not the same as type conversion.