From 3543f283208951514c0fdb45b0dde7393e5e40c7 Mon Sep 17 00:00:00 2001
From: Ian Johnson <ian@ianjohnson.dev>
Date: Thu, 5 Sep 2024 23:05:21 -0400
Subject: [PATCH] std.math.big.int: fix shiftRight sign handling

Closes #21311

The sign of the result `r` needs to be initialized before the correction
`r.addScalar(r.toConst(), -1)`, or the intended end result could be off
by 2 (depending on the original sign of `r`).
---
 lib/std/math/big/int.zig      | 2 +-
 lib/std/math/big/int_test.zig | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/std/math/big/int.zig b/lib/std/math/big/int.zig
index 2c656033e91b..2d38517661a8 100644
--- a/lib/std/math/big/int.zig
+++ b/lib/std/math/big/int.zig
@@ -1202,6 +1202,7 @@ pub const Mutable = struct {
         llshr(r.limbs[0..], a.limbs[0..a.limbs.len], shift);
 
         r.len = a.limbs.len - full_limbs_shifted_out;
+        r.positive = a.positive;
         if (nonzero_negative_shiftout) {
             if (full_limbs_shifted_out > 0) {
                 r.limbs[a.limbs.len - full_limbs_shifted_out] = 0;
@@ -1210,7 +1211,6 @@ pub const Mutable = struct {
             r.addScalar(r.toConst(), -1);
         }
         r.normalize(r.len);
-        r.positive = a.positive;
     }
 
     /// r = ~a under 2s complement wrapping semantics.
diff --git a/lib/std/math/big/int_test.zig b/lib/std/math/big/int_test.zig
index 17652179f599..2e0ccc96c17b 100644
--- a/lib/std/math/big/int_test.zig
+++ b/lib/std/math/big/int_test.zig
@@ -2083,6 +2083,15 @@ test "shift-right negative" {
     try a.shiftRight(&a, 1);
     a.setSign(true);
     try testing.expect(try a.to(u64) == 0x8000000000000000);
+
+    var arg7 = try Managed.initSet(testing.allocator, -32767);
+    defer arg7.deinit();
+    a.setSign(false);
+    try a.shiftRight(&arg7, 4);
+    try testing.expect(try a.to(i16) == -2048);
+    a.setSign(true);
+    try a.shiftRight(&arg7, 4);
+    try testing.expect(try a.to(i16) == -2048);
 }
 
 test "sat shift-left simple unsigned" {