spec requires type checking of for-comprehension to consider refutability

The following only compiles with the 'filter' method, however it is never called:

```scala
  object Foo
  {
    def foreach( f : ((Int,Int)) => Unit ) 
    {
      println("foreach")
      f(1,2)
    }
//    def filter ( f : ((Int,Int)) => Boolean ) =
//    {
//      println("filter")
//      this
//    }
  
    for( (a,b) <- this )
    {
      println((a,b))
    }  
  }
```