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[SR-12091] AnyHashable changes hashValue of floating-point numbers #54527

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NevinBR opened this issue Jan 27, 2020 · 5 comments
Closed

[SR-12091] AnyHashable changes hashValue of floating-point numbers #54527

NevinBR opened this issue Jan 27, 2020 · 5 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella

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@NevinBR
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NevinBR commented Jan 27, 2020

Previous ID SR-12091
Radar rdar://problem/58999149
Original Reporter @NevinBR
Type Bug
Status Closed
Resolution Invalid
Environment

• Swift 5.1.3 (swiftlang-1100.0.282.1 clang-1100.0.33.15)
• MacOS 10.14.6 (18G103)

I observe the same results in both:
• Xcode 11.3.1 playground for MacOS
• Swift REPL in Terminal

Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug
Assignee None
Priority Medium

md5: f2c77ce20074961f74e964f514ed3697

Issue Description:

My understanding is that AnyHashable should preserve the hashValue of what it wraps. However, this does not hold for floating-point values which represent an integer in the range -0x1p63..<0x1p64:

let x = 1.0
let y = AnyHashable(x)
x.hashValue == y.hashValue  // false
@xwu
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xwu commented Jan 27, 2020

As commented on the Swift forums thread, this is in the intended behavior and is implemented [here|

swift/stdlib/public/core/FloatingPointTypes.swift.gyb

Line 1189 in 3ec5e76

// Float and Double are bridged with NSNumber, so we have to follow
]. As explained in the comments there, it is required for NSNumber bridging purposes.

@NevinBR
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NevinBR commented Jan 28, 2020

The documentation for AnyHashable states:

The `AnyHashable` type forwards equality comparisons and hashing operations to an underlying hashable value, hiding its specific underlying type.

This strongly implies that the hashing operations will be forwarded to the wrapped value, and thus the result will be the same and unmodified therefrom.

@xwu
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xwu commented Jan 28, 2020

The documentation is out of date. See: https://bugs.swift.org/browse/SR-7496

@beccadax
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@swift-ci create

@lorentey
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While AnyHashable does often call the underlying value's hash(into🙂 implementation, it does not guarantee to preserve the hash values of any type. (Even if AnyHashable happens to preserve hash values in the current stdlib, please do not assume that this will always be the case – we may need to switch to a different hash encoding in a future release to prevent hash collisions between values of different underlying types.)

Please see the links below for more details:

https://forums.swift.org/t/why-isn-t-anyhashable-implemented-in-pure-swift/33124/7
#29498

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella
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@xwu @lorentey @beccadax @NevinBR