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[SR-4075] Array covariance does not apply to function subtyping #46658

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swift-ci opened this issue Feb 25, 2017 · 3 comments
Open

[SR-4075] Array covariance does not apply to function subtyping #46658

swift-ci opened this issue Feb 25, 2017 · 3 comments
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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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@swift-ci
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Previous ID SR-4075
Radar None
Original Reporter mroi (JIRA User)
Type Bug
Environment 
Michael@jupiter:~ > swiftc --version
Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1)
Target: x86_64-apple-macosx10.9
Additional Detail from JIRA
Votes 1
Component/s Compiler
Labels Bug, TypeChecker
Assignee None
Priority Medium

md5: f83b18da5d48933c1170230c46a5db8d

Issue Description:

class Super {}; class Sub: Super {}
func f(_: [Super]) {}
let test: ([Sub]) -> Void = f

Since functions are contravariant in their arguments and arrays are covariant in their element type, I would expect the assignment to work. However, it does not:

<stdin>:3:29: error: cannot convert value of type '([Super]) -> ()' to specified type '([Sub]) -> Void'
@CodaFi
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CodaFi commented Feb 25, 2017

Arrays are invariant but we privilege certain array element upcasts.

@swift-ci
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Comment by Michael Roitzsch (JIRA)

Oh, sorry, I am not aware of such details. Please advise if I should change the title of this issue.

@belkadan
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There's no reason why this couldn't work. Once we added the general array-of-concrete-type-to-array-of-protocol-type conversion we already gave up on trying to distinguish these cases. We should be able to treat array element upcasts like any other conversion.

@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 @CodaFi @swift-ci