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[SR-10031] Constraints involving a Class & Protocol constraint don't type check correctly #52434

tjw opened this issue Mar 1, 2019 · 1 comment


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@tjw tjw commented Mar 1, 2019

Previous ID SR-10031
Radar None
Original Reporter @tjw
Type Bug
Status Resolved
Resolution Duplicate

Xcode 10.2b3

macOS 10.14.3

Additional Detail from JIRA
Votes 0
Labels Bug
Assignee None
Priority Medium

md5: 1806abf12b1c5a5c1e2a1eb6d9bd8796


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

Issue Description:

protocol P {}
func f<C: Collection>(objects: C) where C.Element : NSObject & P {}
let objects: [NSObject & P] = []
_ = f(objects: objects)

results in:

class-and-protocol-constraint.swift:24:5: error: global function 'f(objects:)' requires that 'NSObject' conform to 'P'
_ = f(objects: objects)
class-and-protocol-constraint.swift:21:6: note: where 'C.Element' = 'NSObject'
func f<C: Collection>(objects: C) where C.Element : NSObject & P {}

This also fails in Xcode 10.1, but with a different and stranger error message.

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@belkadan belkadan commented Mar 4, 2019

This is correct behavior; the elements of the collection are not a particular concrete type conforming to P, which means static and initializer requirements of P are not available.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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