Cast Procedure to GC-Safe: expression cannot be cast to proc (){.closure, gcsafe.}

[LemongrabThree]

I'm trying to combine a library that's necessarily threaded with one that 
necessarily uses shared heap. So I found 
[this](https://forum.nim-lang.org/t/2457#15219) suggestion to use a shared-heap 
GC and cast each procedure to a fake GC-safe type. Except that doesn't work and 
I can't find any info on why. This code produces the problem:
    
    
    import threadpool
    
    type
      BoxedInt = ref object
        value : int
    
    proc box(value : int) : BoxedInt =
      return BoxedInt(value : value)
    
    var global : BoxedInt = box 0;
    
    proc incGlobal() : int =
      global.value += 1
      return global.value
    
    proc echoInt(i : int) : void =
      echo $i
    
    var thread : FlowVar[int] = spawn (cast[proc():int{.gcsafe.}](incGlobal))()
    awaitAndThen(thread, echoInt)
    
    
    Run

And get the following message:

`Error: expression cannot be cast to proc (): int{.closure, gcsafe.}`

So... what's going on? Is this cast-to-gcsafe business even possible at all? 
And what do I do if it's generally not allowed?