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[SR-514] Generic type cannot construct itself with a transformed type #43131

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swift-ci opened this issue Jan 11, 2016 · 3 comments
Closed

[SR-514] Generic type cannot construct itself with a transformed type #43131

swift-ci opened this issue Jan 11, 2016 · 3 comments
Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself type checker Area → compiler: Semantic analysis

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@swift-ci
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Previous ID SR-514
Radar None
Original Reporter mschumacher (JIRA User)
Type Bug
Status Resolved
Resolution Duplicate
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, TypeChecker
Assignee None
Priority Medium

md5: 25c05faa51facf7575dd488e4683f95d

duplicates:

  • SR-1789 Swift can't infer types in generic methods that reuse part of the struct/class' parameters

Issue Description:

Example:

struct Wrapper<T> {
    let value: T
    
    func map<R>(closure: T -> R) -> Wrapper<R> {
        return Wrapper(value: closure(value))
    }
}

This code produces the error:

Cannot invoke initializer for type 'Wrapper<T>' with an argument list of type '(value: R)'

Though it makes sense that this error occurs, since within Wrapper's implementation, T is bound to a concrete type, this shouldn't happen. Expected behavior is that Wrapper<T> can be constructed with any type.

It's maybe worth noting that there is a work-around:

protocol WrapperType {
    typealias T
    
    var value: T { get }
}

extension WrapperType {
    func map<R>(closure: Self.T -> R) -> Wrapper<R> {
        return Wrapper(value: closure(self.value))
    }
}

struct Wrapper<T>: WrapperType {
    let value: T
}

But it's verbose and unintuitive.
@belkadan
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A simpler workaround: just specify <R> explicitly:

struct Wrapper<T> {
    let value: T
    
    func map<R>(closure: T -> R) -> Wrapper<R> {
        return Wrapper<R>(value: closure(value))
    }
}

But I agree that the original code should work.

@Dante-Broggi
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This still occurs in the Xcode 10 toolchain.

@belkadan
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Forward-duping because there's more discussion and other incoming dups on SR-1789.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
@hamishknight hamishknight mentioned this issue Apr 26, 2022
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Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself type checker Area → compiler: Semantic analysis
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@belkadan @swift-ci @Dante-Broggi