From ae09800483c3a5df8d31e400b7a3be87a9dc5814 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Fri, 16 May 2025 19:59:40 -0700
Subject: arith: logtrunc: issues with short-circuiting to zero.

* arith.c (logtrunc): Firstly, the correct condition
for trivially returning zero is bits <= 0, not bits < 0.
Secondly, we don't want to do this upfront. In the
case we have a struct with user-defined arithmetic,
the method must be called. It might return a different
kind of zero.
---
 arith.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arith.c b/arith.c
index ad7e3e69..a0d2ee10 100644
--- a/arith.c
+++ b/arith.c
@@ -3867,21 +3867,20 @@ bad:
 val logtrunc(val a, val bits)
 {
   val self = logtrunc_s;
-  cnum an, bn;
-  val b;
   const cnum num_mask = (NUM_MAX << 1) | 1;
 
   if (!fixnump(bits))
     goto bad2;
 
-  bn = c_n(bits);
-
-  if (bn < 0)
-    return zero;
 
   switch (type(a)) {
     mp_err mpe;
+    cnum an, bn;
+    val b;
   case NUM:
+    bn = c_n(bits);
+    if (bn <= 0)
+      return zero;
     an = c_n(a);
     if (bn <= NUM_BIT) {
       cnum mask = num_mask >> (NUM_BIT - bn);
@@ -3890,6 +3889,9 @@ val logtrunc(val a, val bits)
     a = bignum(an);
     /* fallthrough */
   case BGNUM:
+    bn = c_n(bits);
+    if (bn <= 0)
+      return zero;
     b = make_ubignum();
     if ((mpe = mp_trunc(mp(a), mp(b), bn)) != MP_OKAY)
       do_mp_error(self, mpe);
-- 
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