[swift-users] Ambiguity passing `Any?` to generic constructor

[Jens Alfke]

I’m making a little utility struct that needs to be able to wrap an arbitrary value, which may be an optional, and be able to tell whether it’s nil. So I declared it like this (I’ve removed the interesting/useful parts of the code leaving just the constructors and one property):

public struct check<T> {
    let actual: T?

    public init(_ a: T) {
        actual = a
    }

    public init(_ a: T?) {
        actual = a
    }

    public var isNil: Bool {
        return actual == nil
    }
}

This works fine, except when I try to use it with a value of type `Any?`:

let str: String = "Hi"
check(str).isNil		// false

let maybe: String? = nil
check(maybe).isNil		// true

let hmm: Any? = nil
check(hmm).isNil		// Error: “Ambiguous use of ‘init'"

(I agree that `Any?` is sort of a weird type; in my code it represents a value that might be a JSON object. NSJSONSerialization parses to type `Any`, but there might not be a value, so it becomes an Optional<Any> or Any?.)

This worked in Swift 2.2, but after upgrading to Swift 3 [in the latest Xcode 8 beta] the compiler started complaining.

How can I fix the `check` struct's constructor(s) to make this work?

—Jens
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