Swift 4: Non-nominal type 'T' does not support explicit initialization

Question

I've written an extension that searches a Collection for an object of a certain type.

extension Collection {
    /// Finds and returns the first element matching the specified type or nil.
    func findType<T>(_ type: T.Type) -> Iterator.Element? {
        if let index = (index { (element: Iterator.Element) in
            String(describing: type(of: element)) == String(describing: type) }) {
            return self[index]
        }
        return nil
    }
}

Now in Xcode 9 / Swift 4, the snippet type(of: element)) is underlined with error

Non-nominal type 'T' does not support explicit initialization

The error is strange because I'm not initializing an object.

This answer https://stackoverflow.com/a/46114847/2854041 suggests that perhaps it's a type issue - did the String(describing:) initializer change in Swift 4?

Why would you do `String(describing: type(of: element)) == String(describing: type)`, when you can directly compare Type variables and there's also `is` for checking types? — Dávid Pásztor, Sep 20 '17 at 20:45

Dávid Pásztor · Answer 1 · 2017-09-21T06:08:35.750

You shouldn't be using String(describing:) to compare values and especially shouldn't be using it to compare types. Swift have built in methods for both. For checking if a variable is of a certain type, you can use the is keyword.

Moreover, you can also take advantage of the built in first(where:) method and check the type inside the closure.

extension Collection {
    /// Finds and returns the first element matching the specified type or nil.
    func findType<T>(_ type: T.Type) -> Iterator.Element? {
        return self.first(where: {element in element is T})
    }
}

Test data:

let array: [Any] = [5,"a",5.5]
print(array.findType(Int.self) ?? "Int not found")
print(array.findType(Double.self) ?? "Double not found")
print(array.findType(Float.self) ?? "Float not found")
print(array.findType(String.self) ?? "String not found")
print(array.findType(Bool.self) ?? "Bool not found")

Output:

5
5.5
Float not found
a
Bool not found

Vini App · Accepted Answer · 2017-09-20T21:08:09.350

Here is the error I am getting

Its getting confused with the type(of: and the argument type.

After changing T.Type argument name. Its working :

extension Collection {
    /// Finds and returns the first element matching the specified type or nil.
    func findType<T>(_ typeT: T.Type) -> Iterator.Element? {
        if let index = (index { (element: Iterator.Element) in
        String(describing: type(of: element)) == String(describing: typeT) }) {
            return self[index]
        }
        return nil
    }
}

Swift 4: Non-nominal type 'T' does not support explicit initialization

2 Answers2