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[SR-11920] Bogus Warning When Invalid Protocol is Used in Existential Position #54339

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CodaFi opened this issue Dec 6, 2019 · 7 comments
Closed

[SR-11920] Bogus Warning When Invalid Protocol is Used in Existential Position #54339

CodaFi opened this issue Dec 6, 2019 · 7 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation good first issue Good for newcomers type checker Area → compiler: Semantic analysis

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@CodaFi
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CodaFi commented Dec 6, 2019

Previous ID SR-11920
Radar rdar://problem/57715257
Original Reporter @CodaFi
Type Bug
Status Resolved
Resolution Done
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, DiagnosticsQoI, StarterBug, TypeChecker
Assignee None
Priority Medium

md5: 6a0184667d6dc91a903498b162ead4ab

Issue Description:

protocol P { associatedtype T }
class C : P {}

func i<T: C>(_ x : T?) -> Bool {
  //  warning: checking a value with optional type 'T?' against dynamic type 'P' succeeds whenever the value is non-nil; did you mean to use '!= nil'?
  return x is P
}

This warning's text makes no sense. `P` is not a dynamic protocol and may not be used in existential position. The wording of this diagnostic needs to be amended not to make reference to dynamism when a protocol may not be used in existential position.
@CodaFi
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CodaFi commented Dec 6, 2019

@swift-ci Create

@swift-ci
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Comment by Tapan Thaker (JIRA)

@CodaFi: I am not clear on the meaning of "dynamic protocol" here. Could you please explain or link me to a reference, if possible.

@CodaFi
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CodaFi commented Dec 13, 2019

To be honest, I’m not sure why that particular description is used. The a protocol without self or associated type constraints has nothing to do with dynamism... That said, there is a dynamic cast involved, so perhaps that was the original intent.

@swift-ci
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Comment by Ziyuan Zhao (JIRA)

I'll try to fix this

@slavapestov
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I think this behaves correctly. Since the class C conforms to P, it is true that you can replace the ‘is’ check with ‘!= nil’.

@CodaFi
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CodaFi commented Jan 2, 2020

The diagnostic's wording is still bogus. Its text needs to reflect the the actual situation that's occurred in source. Re-opening this.

@LucianoPAlmeida
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Resolved by #40811
CodaFi (JIRA User) I'm resolving, but if you still think the wording can improve we can reopen again =]

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation good first issue Good for newcomers type checker Area → compiler: Semantic analysis
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@slavapestov @CodaFi @LucianoPAlmeida @swift-ci