title: Existing ivar 'title' for unsafe_unretained property 'title' must be __unsafe_unretained
body: <p>I'm just getting to grips with Objective-C 2.0</p>
  
  <p>When I try to build the following in Xcode it fails. The error from the compiler is as follows:</p>
  
  <p><strong>Existing ivar 'title' for unsafe_unretained property 'title' must be __unsafe_unretained.</strong></p>
  
  <pre><code>// main.m
  #import &lt;Foundation/Foundation.h&gt;
  #import "Movie.h"
  int main (int argc, const char * argv[]){
      Movie *movie = Movie.new;
  
      NSLog(@"%@", movie);
  
      return 0;
  }
  
  // movie.h
  #import &lt;Foundation/Foundation.h&gt;
  
  @interface Movie : NSObject{
      NSString *title;
      int year;
      int rating;
  }
  
  @property(assign) NSString *title;
  @property(assign) int rating;
  @property(assign) int year;
  
  @end
  
  #import "Movie.h"
  
  @implementation Movie;
  
  @synthesize title; // this seems to be issue - but I don't understand why?
  @synthesize rating;
  @synthesize year;
  
  @end
  </code></pre>
  
  <p>Can anybody explain where I've gone wrong?</p>