Open Radar

 
Summary:
In Swift 2.1 it is not possible to make an extension with a type constraint inherit from a protocol. This makes it hard to create an API where one might have a simple generic "model" object. This model could be able to specialize its behavior based on its content object type. This specialization would allow the model to be used as the data source for a table view, for example.

Steps to Reproduce:
Outlined in the attached playground:

import UIKit

public protocol TableCellObject {}

public class Model <ObjectType> {
  public typealias Section = ((header: String?, footer: String?)?, objects: [ObjectType])
  var sections: [Section]
  public init(sections: [Section]) {
    self.sections = sections
  }
}

// Compiler error is thrown here:
extension Model: UITableViewDataSource where ObjectType: TableCellObject {
}

Expected Results:
That extending an object with type constraints would not result in a compiler error.

Actual Results:
A compiler error is thrown.