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[SR-5676] Can't downcast custom generics structs, only arrays #48246

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swift-ci opened this issue Aug 11, 2017 · 1 comment
Open

[SR-5676] Can't downcast custom generics structs, only arrays #48246

swift-ci opened this issue Aug 11, 2017 · 1 comment
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compiler The Swift compiler in itself feature A feature request or implementation improvement

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@swift-ci
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Previous ID SR-5676
Radar None
Original Reporter mhuusko5 (JIRA User)
Type Improvement
Additional Detail from JIRA
Votes 1
Component/s Compiler
Labels Improvement, LanguageFeatureRequest
Assignee None
Priority Medium

md5: 9017f57f186bfe5c62b488e8053e40a6

relates to:

  • SR-11682 Inconsistent inheritance behavior for generic classes.

Issue Description:

protocol Pet: class {}
protocol Dog: Pet {}

let pets: [Pet] = ...

if let pets = pets as? [Dog] { .. } // works

struct Food<T> { }

let food: Food<Pet> = ...

if let food = food as? Food<Dog> { .. } // doesn't work

Perhaps the same kind of mapping could be applied to custom types automatically, or at least machinery exposed that custom types can buy into to get this syntax.
@belkadan
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Yep. Nothing in Swift is covariant at the moment besides the primary collection types, because it isn't always correct to allow downcasting (or upcasting). A simple example is a struct that refers to a reference-semantics NSArray, where it assumes everything in the array matches the generic type.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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compiler The Swift compiler in itself feature A feature request or implementation improvement
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@belkadan @swift-ci