public class TestBlankClass {}

public class Option<U, S> {
    public var onExecution: ([U], S) -> S
    
    public init(
        onExecution: @escaping ([U], S) -> S
    ) {
        self.onExecution = onExecution
    }
    
    public func execute() {
        let change: S = self.onExecution([], TestBlankClass()) // error:'([U], S) -> S' is not convertible to '@lvalue ([U], S) -> S'
    }
}

let change: S = self.onExecution([], TestBlankClass() as! S) // compiles