Open Radar

 
Summary:
Given Box<T>, I would like to make Box conform to Printable such that if T is Printable, Box uses the value’s description property; if it is Reflectable, it uses the reflect(value).summary; and otherwise it just makes something up. Unfortunately, despite the fact that this could (I think) be determined from a given T, I can’t (modulo the `as?` operator—see Notes, below).

Further, calling a function which is generic on T doesn’t work either—if I provide describe() specializations for Printable, Reflectable, and arbitrary T, it always calls the arbitrary T one.

Example code can be found here: https://gist.github.com/robrix/80b14ad4257f96c339e8


Steps to Reproduce:
1. Make a type which is generic on a parameter T.
2. Try to make it Printable in either manner described above.
3. Print an instance of it, whose value is of a type which conforms to Printable.


Expected Results:
I expected the resulting string to indicate that the code path had been selected based on the knowledge that T for this instance was Printable.


Actual Results:
Instead, the string was the arbitrary one.


Regression:
N/A


Notes:
This could be resolved by extensions parameterized on types, universal function-call syntax as described in rdar://17405411, or by specializing the call site based on the value of T. I do not think `as?` is viable due to the limitation requiring @objc (which isn’t used in the stdlib), and also due to the limitation prohibiting struct/enum/tuple checks.