From bc7a22b3fa9030c3e8d2868f6c9a4c850535b27a Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 5 Apr 2021 07:34:45 -0700
Subject: struct: fix lack of hygiene in null-safe qref.

The expression a.?b is not being treated hygienically;
a is evaluated twice. This is only if the null-safe object
is the left most; a.b.?c is hygienic.

* share/txr/stdlib/struct.tl (qref): Add the necessary gensym
use to fix the broken case.
---
 share/txr/stdlib/struct.tl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/share/txr/stdlib/struct.tl b/share/txr/stdlib/struct.tl
index 2eecfbc7..ca3de714 100644
--- a/share/txr/stdlib/struct.tl
+++ b/share/txr/stdlib/struct.tl
@@ -211,7 +211,9 @@
     (throwf 'eval-error "~s: bad syntax" 'qref))
   (tree-case obj
     ((a b) (if (eq a 't)
-             ^(if ,b (qref ,b ,*refs))
+             (let ((s (gensym)))
+               ^(slet ((,s ,b))
+                  (if ,s (qref ,s ,*refs))))
              :))
     (x (tree-case refs
          (() ())
-- 
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