From fecd28371d5c0e25357dfb91f75c8790f8b4155b Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Wed, 6 May 2026 11:21:40 +0100
Subject: [PATCH] Sema: fix crash bitcasting undefined to bitpack type

Resolves: https://codeberg.org/ziglang/zig/issues/31944
---
 src/Sema/bitcast.zig            | 2 ++
 test/behavior/packed-struct.zig | 6 ++++++
 test/behavior/packed-union.zig  | 6 ++++++
 3 files changed, 14 insertions(+)

diff --git a/src/Sema/bitcast.zig b/src/Sema/bitcast.zig
index 6d3da8daf4ec2a51b3f2c36b55812863e70f8d8a..40a29e183bd4324aedc297ab1cba278c987e63fa 100644
--- a/src/Sema/bitcast.zig
+++ b/src/Sema/bitcast.zig
@@ -565,6 +565,7 @@ const PackValueBits = struct {
                 },
                 .@"packed" => {
                     const backing_int_val = try pack.primitive(ty.bitpackBackingInt(zcu));
+                    if (backing_int_val.isUndef(zcu)) return pt.undefValue(ty);
                     return pt.bitpackValue(ty, backing_int_val);
                 },
             },
@@ -659,6 +660,7 @@ const PackValueBits = struct {
                 },
                 .@"packed" => {
                     const backing_int_val = try pack.primitive(ty.bitpackBackingInt(zcu));
+                    if (backing_int_val.isUndef(zcu)) return pt.undefValue(ty);
                     return pt.bitpackValue(ty, backing_int_val);
                 },
             },
diff --git a/test/behavior/packed-struct.zig b/test/behavior/packed-struct.zig
index a851cfc509093051b61036a119735bc270418a92..ad0107dcf1c9437bec0fac7a97f434e0983d8a83 100644
--- a/test/behavior/packed-struct.zig
+++ b/test/behavior/packed-struct.zig
@@ -1240,3 +1240,9 @@ test "packed struct store of comparison result" {
     try expect(result2.a);
     try expect(!result2.b);
 }
+
+test "initialize packed struct field to undefined at comptime" {
+    const S = packed struct(u8) { x: u8 };
+    const val: S = .{ .x = undefined };
+    _ = val;
+}
diff --git a/test/behavior/packed-union.zig b/test/behavior/packed-union.zig
index fec142665737c166abd8ac921bec555cef85a530..b0850392aa51efd60cac8af9403656bc07bae9a3 100644
--- a/test/behavior/packed-union.zig
+++ b/test/behavior/packed-union.zig
@@ -219,3 +219,9 @@ test "packed union equality" {
     try S.doTest(x, y);
     comptime try S.doTest(x, y);
 }
+
+test "initialize packed union field to undefined at comptime" {
+    const U = packed union(u8) { x: u8 };
+    const val: U = .{ .x = undefined };
+    _ = val;
+}
-- 
2.54.0