Xcode 7.2: Swift covariant assignment fails for nested generics

Originator:tapikz
Number:rdar://24135961 Date Originated:11-Jan-2016 04:26 PM
Status:Open Resolved:
Product:Developer Tools Product Version:Xcode 7.2 (7C68)
Classification:Serious Bug Reproducible:Always
 
Summary:
When assigning a more specific generic type to a less specific generic type the compiler throws an error. A simple case is 2 dimensional arrays.

The following works just fine

let views = Array<UIView>()
var objects: Array<NSObject>?

objects = views

Steps to Reproduce:
let views = Array<Array<UIView>>()
var objects: Array<Array<NSObject>>?

objects = views

Expected Results:
- Code compiles
- The assignment is successful

Actual Results:
- Cannot assign value of type ‘Array<Array<UIView>>’ to type Array<Array<NSObject>>’

Regression:
Unsure if this is a regression

Notes:
The demonstrated case is the trivial one but the issue is present for all nested generics.
ie. 

let views = Array<Dictionary<String, UIView>>()
var objects: Array<Dictionary<String, NSObject>>?

objects = views


will yield a similar error

Comments


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