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[SR-4377] Swift 3.1 regression with conditional casting to protocol #46956

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NachoSoto opened this issue Mar 27, 2017 · 11 comments
Closed

[SR-4377] Swift 3.1 regression with conditional casting to protocol #46956

NachoSoto opened this issue Mar 27, 2017 · 11 comments
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@DougGregor
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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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@NachoSoto
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Previous ID SR-4377
Radar rdar://problem/31301668
Original Reporter @NachoSoto
Type Bug
Status Resolved
Resolution Done

Attachment: Download

Environment

  • Swift version 3.1 (swiftlang-802.0.48 clang-802.0.38)

  • Swift version 4.0.2 (swiftlang-900.0.69.1 clang-900.0.38)

  • Swift version 4.1 (swiftlang-902.0.38 clang-902.0.31)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, 3.1Regression, TypeChecker
Assignee @DougGregor
Priority Medium

md5: 0121f0ca456189104247c42e9e89b1b8

is duplicated by:

  • SR-4376 Casting an existential to another existential type for a protocol in a different module erroneously 'always succeeds'

Issue Description:

See screenshots.
The setup:

  • module A:
protocol R: Error {}
  • module B:
enum E: Error, R { case a }
  • module C:
let e: Error = E.a
let _: R? = e as? R

Expected result (worked in Swift 3.0.x): cast compiles and succeeds.
Actual result: Error "Conditional cast from 'Error' to 'ReportableError' always succeeds".
@jckarter
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@swift-ci create

@DougGregor
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I'm failing to reproduce this with exactly the same setup. Argh!

@NachoSoto
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That sucks :/
Some details I left out:

  • R has a couple properties.

  • E's conformance to R is in an extension.

  • E also conforms to another protocol in a separate module.

@NachoSoto
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I attached some screenshots from the real setup. Hopefully you see something I'm not thinking off that's making the difference.
`APIClient.Error` is `E` in my example, and `ReportableError` is `R`.

@NachoSoto
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FYI this is still an issue with Swift 4.0.2 (the workaround still works though):

func cast<T, U>(_ x: T, to: U.Type) -> U? {
    return x as? U
}

if let reportableError = cast(error.error, to: ReportableError.self) { ... }

@jckarter
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jckarter commented Nov 9, 2017

What was your workaround?

@NachoSoto
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@jckarter the one you gave me back then 🙂 (I updated my comment).

@jckarter
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jckarter commented Nov 9, 2017

Sorry, it's been a while. So the cast(to🙂 form works, but as? doesn't? Does it vary by optimization level?

@NachoSoto
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That's correct. And I just checked with optimization and as? still produces Conditional cast from 'Error' to 'ReportableError' always succeeds.

@NachoSoto
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This still reproduces in Swift 4.1.

@DougGregor
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Appears fixed in Swift 5.1

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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@DougGregor DougGregor

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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@jckarter @NachoSoto @DougGregor