Releation between parameter and result note proveable by static checker

hi,
following code:

imports system.diagnostics.contracts
  
  public class class1
      public shared function removeempty(byval comp string()) string()
          contract.requires(comp isnot nothing)
          contract.ensures(contract.result(of string())() isnot nothing)
          contract.ensures(contract.result(of string()).length <= comp.length)
          dim ret string()
          dim max integer
          max = 0
          redim ret(comp.length - 1)
          each str string in comp
              contract.assert(ret.length > max)
              ret(max) = str
              if not string.isnullorempty(str) then
                  max += 1
              end if
          next
          contract.assert(max <= comp.length)
          redim preserve ret(max - 1)
          contract.assert(ret.length <= comp.length)
          contract.assert(ret isnot nothing)
          contract.assert(ret.length <= comp.length)
          return ret
      end function
  end class
  

generates warning:
class1.vb(13,13): warning : contracts: assert unproven
class1.vb(21,9): warning : contracts: assert unproven

since have each on base array (var comp), counter valid entries (max) can't greater length of new array (ret) since must smaler or eqal base array.
miss contracts, in order static checker able prove ?

it seems checker failing infer loop invariant "max <= ", iteration variable vb compiler introduces in compilation.
i try fix problem inference.
meanwhile, 1 way fix code above use loop (so iteration variable explicit) , add assertion contract.assert(max <= i)

thanks,
f

DevLabs  >  Code Contracts