Why does this code using 'concepts' fail?

[TKD]

This fails with `dist.sample_dist() is int too nested for type matching`. Is 
there a fix?
    
    
    {.experimental.}
    
    
    Run
    
    
    import std/[random]
    
    randomize()
    
    type
      Distribution = concept dist
        dist.sample_dist() is int
      
      Die = object
        sides: int
    
    func sample_dist( dist: Distribution ) : int =
      rand(1..(dist.num_sides))
    
    func roll_dice( d: Die ) : int =
      d.sample_dist() + d.sample_dist()
    
    let six_sided = Die(sides: 6)
    echo roll_dice six_sided
    
    
    Run