fixes #451

compiler/semstmts.nim

@@ -146,7 +146,9 @@ proc discardCheck(result: PNode) =
       if result.typ.kind == tyNil:
         fixNilType(result)
       else:
-        localError(result.info, errDiscardValue)
+        var n = result
+        while n.kind in skipForDiscardable: n = n.lastSon
+        localError(n.info, errDiscardValue)
 
 proc semIf(c: PContext, n: PNode): PNode = 
   result = n

tests/reject/tneedsdiscard.nim

@@ -0,0 +1,12 @@
+discard """
+  line: 10
+  errormsg: "value returned by statement has to be discarded"
+"""
+
+proc p =
+  var f: TFile
+  echo "hi"
+  
+  open(f, "arg.txt")
+  
+p()