无法推断通用参数“T”，通用错误

Question

I want to create a function to choose which implementation is good, like that:

// As a BaseProtocol
protocol BaseProtocol {
    
}

// SeniorProtocol with associatedtype
protocol SeniorProtocol {
    associatedtype S
}

// First implementation of SeniorProtocol
struct SeniorImpl1:SeniorProtocol {
    typealias S = BaseProtocol
    init() {}
}

// Second implementation of SeniorProtocol
struct SeniorImpl2:SeniorProtocol {
    typealias S = BaseProtocol
    init() {}
}

// The function 
func whichImpl<T: SeniorProtocol>() -> T{
    if Int.random(in: 0 ... 5) < 3 {
        return SeniorImpl1() as! T
    }
    else {
        return SeniorImpl2() as! T
    }
}

Finally, when I run

var c = whichImpl()

I got this error: “Generic parameter 'T' could not be inferred”. It seems that the compiler does't know what T is. How can I solve it? I just want to do what the code written in whichImpl()

Answer 1

Since your method returns a value of the generic type T, then you have to let Swift know which concrete type you want your c variable to be to make Swift's type checking system works. In your case, simply give your c a concrete type like this will make the error pass away.

var c: Int = whichImpl()