title: "Cannot convert value of type 'T'..." in generics
body: <p>Cannot understand how to treat the types of generics and how to use the arguments with the undefined types with libraries: </p>
  
  
  
  <pre><code>func cellWith&lt;T&gt;(value: T) -&gt; String {         // I expect Int, Double or String 
                                                 // type value as argument  
  
      let fm = NSNumberFormatter()               // Double type argument will be
      fm.numberStyle = .CurrencyStyle            // processed like "currency"
  
      if value.self is Double {               // In case value have type "Double", 
                                              // like 20_000.00
          return(fm.stringFromNumber(value))  // I expect return $20,000.00
                                     ^~~~~
  // ERROR: Cannot convert value of type 'T' to expected argument type 'NSNumber'
  
      } else {
          return("bla-bla-bla")
      }
  }
  </code></pre>