From 143144604497b5cc7c70009f9f567117d35b944d Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Fri, 6 Mar 2026 16:30:56 +0000
Subject: [PATCH] behavior: disable some tests under the C backend targeting
 MSVC

The bugs here actually exist on master branch too, but they are being
caught by the new static assertions which check type size and alignment.
It turns out that MSVC's struct/union "pack" pragma and its "align"
declspec interact in undocumented ways which are extremely problematic
for generated code. Solving this will require changing how the C backend
lowers various types; the disabled tests are all tagged unions, but
there are also issues with structs with underaligned fields which the
behavior tests just happen to not currently be triggering.
---
 test/behavior/align.zig | 1 +
 test/behavior/union.zig | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/test/behavior/align.zig b/test/behavior/align.zig
index a59bed56ba..550d7008b4 100644
--- a/test/behavior/align.zig
+++ b/test/behavior/align.zig
@@ -18,6 +18,7 @@ test "global variable alignment" {
 test "large alignment of local constant" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest; // flaky
+    if (builtin.zig_backend == .stage2_c and builtin.target.abi == .msvc) return error.SkipZigTest;
 
     const x: f32 align(128) = 12.34;
     try std.testing.expect(@intFromPtr(&x) % 128 == 0);
diff --git a/test/behavior/union.zig b/test/behavior/union.zig
index 9badc9dabd..faa3f42ea5 100644
--- a/test/behavior/union.zig
+++ b/test/behavior/union.zig
@@ -148,6 +148,7 @@ const err = @as(anyerror!Agg, Agg{
 const array = [_]Value{ v1, v2, v1, v2 };
 
 test "unions embedded in aggregate types" {
+    if (builtin.zig_backend == .stage2_c and builtin.target.abi == .msvc) return error.SkipZigTest;
     switch (array[1]) {
         Value.Array => |arr| try expect(arr[4] == 3),
         else => unreachable,
@@ -2022,6 +2023,7 @@ test "runtime union init, most-aligned field != largest" {
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_c and builtin.target.abi == .msvc) return error.SkipZigTest;
 
     const U = union(enum) {
         x: u128,