From 172945de31d152e362636daca3fb968481d96bdb Mon Sep 17 00:00:00 2001
From: Grzegorz Adam Hankiewicz <gradha@imap.cc>
Date: Tue, 9 Jul 2013 23:31:13 +0200
Subject: [PATCH] Avoids usefulFact recursion with nil parameter. Fixes #518.

---
 compiler/guards.nim | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/compiler/guards.nim b/compiler/guards.nim
index f02a53684d..c43e918d9c 100644
--- a/compiler/guards.nim
+++ b/compiler/guards.nim
@@ -187,7 +187,8 @@ proc usefulFact(n: PNode): PNode =
     #   if a:
     #     ...
     # We make can easily replace 'a' by '2 < x' here:
-    result = usefulFact(n.sym.ast)
+    if n.sym.ast != nil:
+      result = usefulFact(n.sym.ast)
   elif n.kind == nkStmtListExpr:
     result = usefulFact(n.lastSon)