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

[swift-ci]

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 #44398

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]

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]

This still occurs in the Xcode 10 toolchain.

[belkadan]

Forward-duping because there's more discussion and other incoming dups on #44398.