fix issue #1581

compiler/vmgen.nim

@@ -1126,6 +1126,7 @@ proc checkCanEval(c: PCtx; n: PNode) =
   # we need to ensure that we don't evaluate 'x' here:
   # proc foo() = var x ...
   let s = n.sym
+  if {sfCompileTime, sfGlobal} <= s.flags: return
   if s.kind in {skVar, skTemp, skLet, skParam, skResult} and 
       not s.isOwnedBy(c.prc.sym) and s.owner != c.module:
     cannotEval(n)