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[SR-10076] [Swift 5] Generic parameter cannot get correct type with generic override constraint #52478

swift-ci opened this issue Mar 10, 2019 · 2 comments


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@swift-ci swift-ci commented Mar 10, 2019

Previous ID SR-10076
Radar None
Original Reporter andyl (JIRA User)
Type Bug
Status Resolved
Resolution Duplicate

Error is reproduced with macOS 10.14.3 with Xcode 10.2 beta 4 (Swift 5) and does not happen with Xcode 10.1 (Swift 4.2)

Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug, 5.0Regression
Assignee @slavapestov
Priority Medium

md5: fc407436e0f058c43c437612c34564ad


  • SR-4206 Override checking does not properly enforce requirements

Issue Description:

In Swift 5, when we override a method that takes in a generic constraint that adds on another generic constraint, when we call the methods from subclass objects with superclass type. An example snippet is provided below to show this behaviour.

I was able to reproduce this with the latest Swift 5 beta from Xcode 10.2 beta 4. This behaviour does not happen in Swift 4.

class A {}
class SubA<T>: A {}

class B {
  func test<E: A>(_: E) {}
class SubB<T>: B {
  override func test<E>(_: E) where E: SubA<T> {

let subA = SubA<Int>(), subB = SubB<Int>()
let a: A = subA, b: B = subB

subB.test(subA) // works, prints Int
b.test(subA) // works, prints Int
b.test(a) // fails, prints Swift._SwiftObject (instead of Int)
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@belkadan belkadan commented Mar 14, 2019

That shouldn't even build; it's not a valid override. An override always has to be able to accept anything that the original method can accept.

cc @slavapestov

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@slavapestov slavapestov commented Mar 14, 2019

I have several dupes of this. We need to check generic constraints when checking for a valid override; in general we should not allow adding new constraints via a 'where' clause. I'll consolidate the dupes later.

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