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[SR-8391] Subscript declaration can't use only nested type of generic parameter #50917

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natecook1000 opened this issue Jul 27, 2018 · 2 comments
Open

[SR-8391] Subscript declaration can't use only nested type of generic parameter #50917

natecook1000 opened this issue Jul 27, 2018 · 2 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@natecook1000
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Previous ID SR-8391
Radar None
Original Reporter @natecook1000
Type Bug
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: 87f28a6c6c81239df3d0d7c16e641669

Issue Description:

I can define a generic method that only uses nested/associated types of the generic parameter, like the example below. The type C isn't used explicitly in the method signature, but C.Element and C.Index are:

extension Collection {
  func index<C: Collection>(_ i: Index, _ j: C.Index) -> C.Element
    where Element == C
  {
    return self[i][j]
  }
}

That same declaration doesn't work when converted to a subscript, which seems like an unnecessary limitation:

extension Collection {
  subscript<C: Collection>(i: Index, j: C.Index) -> C.Element
    where Element == C
  {
    get {
      return self[i][j]
    }
  }
}
// error: generic parameter 'C' is not used in function signature
//    subscript<C: Collection>(i: Index, j: C.Index) -> C.Element
@belkadan
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I'm not sure you should be allowed to declare the function either. There's no way to call it. cc @DougGregor, @huonw

@belkadan
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Oh, sorry, I misread the declaration. You're adding additional constraints on Self.Element rather than introducing a new type, but we don't have member-level syntax for that.

extension Collection where Element: Collection {
  func index(_ i: Index, _ j: Element.Index) -> Element.Element {
    return self[i][j]
  }
  subscript<Element: Collection>(i: Index, j: Element.Index) -> Element.Element {
    get {
      return self[i][j]
    }
  }
}

@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
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@belkadan @natecook1000