From 274949fdbb7a98d374d5e994110dbfd639c70acf Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus@klausecker.de>
Date: Mon, 29 Jun 2026 22:12:49 +0200
Subject: [PATCH] wasm: lower `packed union` backing integer type correctly

`IntType.fromType` would previously set the signedness of any backing int
of a `packed union` to `unsigned` unconditionally which caused comparisons
of packed unions with large, signed backing integers to miscompile.
Packed unions are now lowered correctly to their actual backing int type.
---
 src/codegen/wasm/CodeGen.zig    | 16 ++++------------
 test/behavior/packed-struct.zig | 13 +++++++++++++
 test/behavior/packed-union.zig  | 13 +++++++++++++
 3 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/src/codegen/wasm/CodeGen.zig b/src/codegen/wasm/CodeGen.zig
index 8d203139b8854c4913902087f033d26af520c60d..284bce953a0590c6c9a602db78714aa51ad349b4 100644
--- a/src/codegen/wasm/CodeGen.zig
+++ b/src/codegen/wasm/CodeGen.zig
@@ -2527,18 +2527,10 @@ const IntType = struct {
                 .f16, .f32, .f64, .f80, .f128, .c_longdouble => unreachable,
                 .anyopaque, .void, .type, .comptime_int, .comptime_float, .noreturn, .null, .undefined, .enum_literal, .generic_poison => unreachable,
             },
-            .struct_type => {
-                const loaded_struct = ip.loadStructType(ty_index);
-                switch (loaded_struct.layout) {
-                    .auto, .@"extern" => unreachable,
-                    .@"packed" => ty_index = loaded_struct.packed_backing_int_type,
-                }
-            },
-            .union_type => return switch (ip.loadUnionType(ty_index).layout) {
-                .auto, .@"extern" => unreachable,
-                .@"packed" => .{ .is_signed = false, .bits = @intCast(ty.bitSize(zcu)) },
-            },
-            .enum_type => ty_index = ip.loadEnumType(ty_index).int_tag_type,
+            .enum_type,
+            .struct_type,
+            .union_type,
+            => ty_index = Type.fromInterned(ty_index).backingIntType(zcu).toIntern(),
             .error_set_type, .inferred_error_set_type => return .{ .is_signed = false, .bits = zcu.errorSetBits() },
             else => unreachable,
         };
diff --git a/test/behavior/packed-struct.zig b/test/behavior/packed-struct.zig
index 986c15415486bf9c23b7b879a9d305e1a3cfaed1..96585d056cdf4c13b1aff3610ad08088fc0bf61c 100644
--- a/test/behavior/packed-struct.zig
+++ b/test/behavior/packed-struct.zig
@@ -1261,3 +1261,16 @@ test "convert from/to backing int" {
     try S.doTheTest(.{ .a = 123, .b = .y, .c = 0.23 });
     try comptime S.doTheTest(.{ .a = 123, .b = .y, .c = 0.23 });
 }
+
+test "equality with wide backing integer" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // https://codeberg.org/ziglang/zig/issues/35982
+
+    const S = packed struct(i200) {
+        x: u200,
+        fn doTheTest(s: @This(), int: i200) !void {
+            try expect(s == @as(@This(), @bitCast(int)));
+        }
+    };
+    try S.doTheTest(.{ .x = (1 << 200) - 1 }, -1);
+    try comptime S.doTheTest(.{ .x = (1 << 200) - 1 }, -1);
+}
diff --git a/test/behavior/packed-union.zig b/test/behavior/packed-union.zig
index 44090943d487e3b52beae89a32baf5033d034a3d..4fd25526ce4a37deef7233b5b71120742604ce80 100644
--- a/test/behavior/packed-union.zig
+++ b/test/behavior/packed-union.zig
@@ -241,3 +241,16 @@ test "convert from/to backing int" {
     try U.doTheTest(.{ .a = 123 });
     try comptime U.doTheTest(.{ .a = 123 });
 }
+
+test "equality with wide backing integer" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // https://codeberg.org/ziglang/zig/issues/35982
+
+    const U = packed union(i200) {
+        x: u200,
+        fn doTheTest(s: @This(), int: i200) !void {
+            try expect(s == @as(@This(), @bitCast(int)));
+        }
+    };
+    try U.doTheTest(.{ .x = (1 << 200) - 1 }, -1);
+    try comptime U.doTheTest(.{ .x = (1 << 200) - 1 }, -1);
+}
-- 
2.54.0