Why does `k in t.keys.toSeq.sorted` works but `k in t.keys.toSeq.sorted()` does not.

[treeform]

    import tables, sequtils, algorithm
    
    var t: Table[int, int]
    
    # this works:
    for k in t.keys.toSeq.sorted:
      echo $k
    
    # this does not?
    for k in t.keys.toSeq.sorted():
      echo $k
    
    
    Run

A very strange error in the second case: 
    
    
    Error: undeclared field: 'keys'
    
    
    Run

Is this some sort of iterator/macro bug or a feature?

[https://play.nim-lang.org/#ix=2c4I](https://play.nim-lang.org/#ix=2c4I)