public protocol MySelectionManager {
    
    associatedtype SelectionValue : Hashable
    
    mutating func select(_ value: Self.SelectionValue)
    mutating func deselect(_ value: Self.SelectionValue)
    func isSelected(_ value: Self.SelectionValue) -> Bool
}


struct MyBinding<Value> {
    
    var wrappedValue: Value {
        get { getValue() }
        nonmutating set { setValue(newValue) }
    }
    
    let getValue: () -> Value
    let setValue: (Value) -> Void    
}


extension MyBinding where Value: MySelectionManager {


    func isSelected(_ value: Value.SelectionValue) -> MyBinding<Bool> {
        MyBinding(
            getValue: { self.wrappedValue.isSelected(value) }, // error: Cannot convert value of type 'Bool' to closure result type '_'
            setValue: { isSelected in
                if isSelected {
                    self.wrappedValue.select(value)
                } else {
                    self.wrappedValue.deselect(value)
                }
            }
        )
    }


}

extension MyBinding where Value: MySelectionManager {


    func isSelected(_ value: Value.SelectionValue) -> MyBinding<Bool> {
        MyBinding<Bool>( // add <Bool> explicitly
            getValue: { self.wrappedValue.isSelected(value) },
            setValue: { isSelected in
                if isSelected {
                    self.wrappedValue.select(value)
                } else {
                    self.wrappedValue.deselect(value)
                }
            }
        )
    }


}