Why need to set public before override viewDidLoad in a public access control viewController

The error message is fairly explicit:

Overriding instance method must be as accessible as the declaration it overrides.

This means that a method must not have a lower access level than the method it overrides.

For example given this class:

public class Superclass {
    internal func doSomething() {
        ...
    }
}

You cannot then override doSomething with a method that is less accessible than interal. e.g.

public class Subclass : Superclass {
    // error
    private override func doSomething() {
    }
}

You can however override a method and make it more accessible:

public class Subclass : Superclass {
    public override func doSomething() {
        // You can even call the internal method in the superclass
        super.doSomething()
    }
}

The reference documentation has lots of detail on this, but seems to leave this relationship to implication.