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[SR-7008] Subclass existential doesn't satisfy generic class constraints #49556

karwa opened this issue Feb 15, 2018 · 1 comment

[SR-7008] Subclass existential doesn't satisfy generic class constraints #49556

karwa opened this issue Feb 15, 2018 · 1 comment


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@karwa karwa commented Feb 15, 2018

Previous ID SR-7008
Radar None
Original Reporter @karwa
Type Bug
Status Resolved
Resolution Duplicate

Xcode 9.2

Apple Swift version 4.1 (swiftlang-902.0.34 clang-902.0.30)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: 10ba50906ce8e750485046f8da2fb431


  • SR-55 non-@objc protocol existentials do not conform to their own protocol type

Issue Description:


class AClass {}
protocol AProto { var value: Int { get } }

class Box<T> { 
  private var _boxed: T
  init(_ b: T) { _boxed = b } 
extension Box where T: AClass { 
  var boxed: T { 
    get { return _boxed }
    set { _boxed = newValue } 

class One: AClass, AProto { var value: Int { return 1 } }
class Two: AClass, AProto { var value: Int { return 2 } }

let a: Box<AClass & AProto> = Box(One())
print(a.boxed) // error: 'AClass & AProto' is not convertible to 'AnyObject'
a.boxed = Two()

let b: AClass & AProto = One()
print(b is AClass) // 'true'

See discussion at:

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@swift-ci swift-ci commented Feb 19, 2018

Comment by Marc Palmer (JIRA)

I completely understand how Swift experts may say "you're holding it wrong" but I think there is a much wider and more important point here that this doesn't look wrong at all and hence any APIs built like this will be very hard for developers to reason about.

I am still struggling to internalise this nuance with existentials, and I've developed in many languages (C, C++, Object Pascal, ObjC, JS, 68000 ASM) for over 30 years!

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