Swift 4 - Generic Type Conversion issue - Cannot convert to expected type '(_) -> Void'

Question

I am using Xcode 9.2.

I am receiving the error:

Cannot convert value of type '(ValueType2) -> Void' to expected argument type '(_) -> Void'

I have the following code:

import UIKit

enum Value<ValueType> {

    case hasValue(ValueType)

}

class BaseClass<ValueType> {

    var value: Value<ValueType>?

    func f1<ValueType2>(closure: @escaping (ValueType, @escaping (ValueType2) -> Void) -> Void ){

        let s1 = SubClass<ValueType2>()

        // The point of the generic type conversion issue
        // Cannot convert value of type '(ValueType2) -> Void' to expected argument type '(_) -> Void'

        closure(value!, s1.f2)

    }

}

class SubClass<ValueType>: BaseClass<ValueType> {

    func f2(value: ValueType) -> Void { /* DO SOMETHING */ }

}

I am unsure whether this is related to the Swift bug being fixed in Swift 4.1:

Swift Generics: Cannot convert value of type to expected argument type

As autocomplete knows the class's correct generic type parameter & function's parameter - ValueType2 - in both cases.

Any ideas on how to overcome this error?

Thanks

It looks like it might be an instance of that bug. If I replace all instances of `ValueType2` with a concrete type like `Int` the error goes away. — JeremyP, Mar 08 '18 at 10:54
I also noticed that if I remove ValueType generic parameter from the f1(closure:) method, the compiler issue goes away. It seems the compiler does not like two generic types in a closure. — Mark Jarecki, Mar 08 '18 at 11:36

score 2 · Accepted Answer · answered Mar 08 '18 at 11:43

In your f2, the type of closure is @escaping (ValueType, @escaping (ValueType2) -> Void) -> Void.

The type of the first parameter is ValueType. But you are using it as:

closure(value!, s1.f2)

The type of the first argument value! is Value<ValueType>, not ValueType.

If you change the type of closure to @escaping (Value<ValueType>, @escaping (ValueType2) -> Void) -> Void, your code would compile without any issue.

You may have found some issue about generics, but at least, your code example is not describing the issue.

Perfect. I'd been staring at the answer for ages. Thank you! — Mark Jarecki, Mar 08 '18 at 18:12

Swift 4 - Generic Type Conversion issue - Cannot convert to expected type '(_) -> Void'

1 Answers1