I think this is correct. You've said "Array conforms to JSONValue when its element type conforms to JSONValue", but in this case the element type is JSONValue, which does not itself conform to JSONValue. Unfortunately, Swift only allows you to have one or the other; either [JSONValue] can conform to JSONValue or [String] and [Int] can conform to JSONValue, but not both.

@jckarter, anything to add?

Comment by Charlie Monroe (JIRA)

Thanks for the explanation. My major confusion was caused by the ObjC bridge - as I was initially hoping to get this working for validation of JSON objects without realizing that dynamically querying conditional conformance is not supported yet, I had added conformance to JSONValue to the Cocoa array and dictionary classes which resulted in:

[1, "Joe"] is JSONValue // warning: Test always OK
let arrObj: JSONValue = [1, "Joe"] // error: Contextual type 'JSONValue' cannot be used with array literal

Which was very confusing at the moment. However, it turns out that the is check used the ObjC bridge for the test as removing conformance of the NS* classes removed the always-OK warning.

With #15415 this now gives an error that more directly highlights the underlying problem (https://bugs.swift.org/browse/SR-55):

conditional_conformance_as.swift:29:12: error: value of type '[JSONValue]' does not conform to expected dictionary value type 'JSONValue'
  "Hello": [1 as JSONValue, "Joe"]
           ^~~~~~~~~~~~~~~~~~~~~~~
                                   as! JSONValue

As such, I'm going to call this resolved; what do you think charlieMonroe (JIRA User)?

Comment by Charlie Monroe (JIRA)

Thanks, @huonw!