无法推断通用参数'T'：数组参数

Question

我有一个通用函数，它接受符合Protocol Printable的记录。

class Printer
{
   func printRecords<T: Record where T: Printable>(records: [T], completion: ((e: NSError?) -> Void)? )
   {
       // do print stuff here
       var data = NSMutableData()
       for r in records
       {
          data.appendData(r.printableData)
       }

       let error = self.printData(data)
       completion?(e: error)
   }
}

以下是Record和Printable声明和实施：

protocol Printable
{
   var printableData: NSData { get }
}

public class Record: NSManagedObject
{
   // property declaration here  
}

public class Person: Record
{
   // property declaration here  
}

public class Address: Record
{
   // property declaration here  
}

// implement Printabl
extension Address: Printable
{
   var printData: NSData 
   { 
     // format print data here...
     .
     .
     .

     return data 
   }
}

但是当我调用这样的方法时：

// was initially inserted into coredata via
// NSEntityDescription.insertNewObjectForEntityForName("Address", inManagedObjectContext: context) as! Address
let address: Address = ...

// This line: Generic Parameter 'T' Could not be Inferred
printer.printRecords([address], completion: { (e: NSError?) in
   // handle print completion here
})

我收到了错误：

  

无法推断通用参数'T'