[SR-11472] Generic parameter inference problem

[jeremyabannister]

Previous ID	SR-11472
Radar	None
Original Reporter	@jeremyabannister
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	None
Priority	Medium

md5: 3094e6c47b27e8d301bf4f2a2ef9c7e9

duplicates:

• [SR-1789] Swift can't infer types in generic methods that reuse part of the struct/class' parameters #44398

Issue Description:

This code compiles with Swift 5:

struct A <X> {
    
    let dict: [Int: X]
    
    init (dict: [Int: X]) {
        self.dict = dict
    }
    
    func map <Y> (_ transform: (X)->Y) -> A<Y> {
        return A<Y>(dict: dict.mapValues { transform($0) })
    }
}

but this code does not:

struct A <X> {
    
    let dict: [Int: X]
    
    init (dict: [Int: X]) {
        self.dict = dict
    }
    
    func map <Y> (_ transform: (X)->Y) -> A<Y> {
        return A(dict: dict.mapValues { transform($0) })
    }
}

The only difference is that I have removed the explicit type from the construction of A in the map method. Of course the generic parameter is already specified in two ways, one is the return type and the other is the fact that the transform spits out values of type Y. What's going on here?

[belkadan]

Yep. :-( See the discussion on #44398.