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[SR-6725] @autoclosure causes ambiguity warnings that can be fixed by wrapping in identity function #49274

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weissi opened this issue Jan 9, 2018 · 1 comment
Open

[SR-6725] @autoclosure causes ambiguity warnings that can be fixed by wrapping in identity function #49274

weissi opened this issue Jan 9, 2018 · 1 comment
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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@weissi
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weissi commented Jan 9, 2018

Previous ID SR-6725
Radar rdar://problem/36376946
Original Reporter @weissi
Type Bug

Attachment: Download

Environment 
$ /Library/Developer/Toolchains/swift-4.1-DEVELOPMENT-SNAPSHOT-2017-12-30-a.xctoolchain/usr/bin/swiftc --version
Apple Swift version 4.1-dev (LLVM 0fcc19c0d8, Clang 1696f82ad2, Swift 691139445e)
Target: x86_64-apple-darwin17.4.0
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, TypeChecker
Assignee None
Priority Medium

md5: 7b0d8a53ec125efb1af7dad400438ba5

Issue Description:

The following code

public func pretendsEscaping(_ body: @escaping () -> Int) -> Int {
    return body()
}

func foo(_ value: @autoclosure () -> Int) -> Int {
    return withoutActuallyEscaping(value) { (value: @escaping () -> Int) -> Int in
        return pretendsEscaping(value)
    }
}

print(foo(42))

should just print 42 but unfortunately it doesn't compile:

$ /Library/Developer/Toolchains/swift-4.1-DEVELOPMENT-SNAPSHOT-2017-12-30-a.xctoolchain/usr/bin/swiftc test.swift 
test.swift:6:12: error: expression type '(_, (_) throws -> _) throws -> _' is ambiguous without more context
    return withoutActuallyEscaping(value) { (value: @escaping () -> Int) -> Int in
           ^~~~~~~~~~~~~~~~~~~~~~~

Adding more type annotations also doesn't seem to help.

However, just wrapping the @autoclosure value value in the identity function id makes it compile. That's surprising 🙂

public func pretendsEscaping(_ body: @escaping () -> Int) -> Int {
    return body()
}

func foo(_ value: @autoclosure () -> Int) -> Int {
    func id<T>(_ value: T) -> T {
        return value
    }
    return withoutActuallyEscaping(id(value)) { (value: @escaping () -> Int) -> Int in
        return pretendsEscaping(value)
    }
}

print(foo(42))

compiler is the latest 4.1 snapshot but the same problem exists in 4.0 (and 3.1).
@belkadan
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belkadan commented Jan 9, 2018

@swift-ci create

@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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@weissi @belkadan