This looks like a Resharper bug to me. Is it?
Yes :
RSRP-78748 False 'conversion does not exist' (UIntPtr)
using System; class A { static void Main() { E? x = 0; UIntPtr z = (UIntPtr)x; } } enum E { }
It is a known spec devation.
Not fixed as of 2013-03-05.