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[SR-1692] Custom prefix/postfix increment/decrement operators fail #44301

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swift-ci opened this issue Jun 6, 2016 · 2 comments
Open

[SR-1692] Custom prefix/postfix increment/decrement operators fail #44301

swift-ci opened this issue Jun 6, 2016 · 2 comments
Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself type checker Area → compiler: Semantic analysis

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@swift-ci
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swift-ci commented Jun 6, 2016

Previous ID SR-1692
Radar None
Original Reporter erica (JIRA User)
Type Bug
Environment

Swift 3 on OS X and Linux

Additional Detail from JIRA
Votes 1
Component/s Compiler
Labels Bug, TypeChecker
Assignee None
Priority Medium

md5: 76ece653521e2f9c0d42a9eb83ea7fe9

Issue Description:

Although ++ is removed from the language, you'd think you could add it back in
@belkadan
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belkadan commented Jun 6, 2016

I'm pretty sure this is an overload ordering question, and I'm not convinced the compiler is wrong. If something is a more specific overload and marked unavailable, silently dropping back to the less-specific overload might be counterintuitive. (It also means the behavior of existing code could change silently if something is marked unavailable between releases.)

@swift-ci
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Comment by erica sadun (JIRA)

This is the behavior under the latest beta:

// THIS WORKS
prefix operator ++*
postfix operator ++*
prefix operator --*
postfix operator --*

@discardableResult
prefix public func ++* <T: Integer>(x: inout T) -> T {
    x = x + 1; return x
}
@discardableResult
postfix public func ++* <T: Integer>(x: inout T) -> T {
    defer{ x = x + 1 }; return x
}
@discardableResult
prefix public func --* <T: Integer>(x: inout T) -> T {
    x = x - 1; return x
}
@discardableResult
postfix public func --* <T: Integer>(x: inout T) -> T {
    defer{ x = x - 1 }; return x
}

And this does not work:

// THIS DOESN'T WORK
prefix operator ++
postfix operator ++
prefix operator --
postfix operator --

@discardableResult
prefix public func ++ <T: Integer>(x: inout T) -> T {
    x = x + 1; return x
}
@discardableResult
postfix public func ++ <T: Integer>(x: inout T) -> T {
    defer{ x = x + 1 }; return x
}
@discardableResult
prefix public func -- <T: Integer>(x: inout T) -> T {
    x = x - 1; return x
}
@discardableResult
postfix public func -- <T: Integer>(x: inout T) -> T {
    defer{ x = x - 1 }; return x
}

// Bug report filed: https://bugs.swift.org/browse/SR-1692
// Uncomment for errors, making sure to comment the version below
let a: Int8 = 5
//let b = ++a
//let c = a++
//print(a, b, c) // 7, 6, 6

However, individual type overloads do work:

// This works
@discardableResult
prefix public func ++(x: inout Int) -> Int { x = x + 1; return x }
@discardableResult
prefix public func ++(x: inout UInt) -> UInt { x = x + 1; return x }
@discardableResult
prefix public func --(x: inout Int) -> Int { x = x - 1; return x }
@discardableResult
prefix public func --(x: inout UInt) -> UInt { x = x - 1; return x }

@discardableResult
postfix public func ++(x: inout Int) -> Int { defer { x = x + 1 }; return x }
@discardableResult
postfix public func ++(x: inout UInt) -> UInt { defer { x = x + 1 }; return x }
@discardableResult
postfix public func --(x: inout Int) -> Int { defer { x = x - 1 }; return x }
@discardableResult
postfix public func --(x: inout UInt) -> UInt { defer { x = x - 1 }; return x }

var j = 5
print(++j, j, j++, j, --j, j, j--, j) // 6 6 6 7 6 6 6 5

@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. compiler The Swift compiler in itself type checker Area → compiler: Semantic analysis
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@belkadan @swift-ci