From ba6abd71c2d3664bc658cf5ce55b5c53052dc720 Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Tue, 12 Aug 2025 23:18:05 +0100
Subject: [PATCH] llvm: unions which are equivalent to enums are not by-ref

The LLVM backend lowers unions where all fields are zero-bit as
equivalent to their backing enum, and expects them to have the same
by-ref-ness in at least one place in the backend, probably more.

Resolves: #23577
---
 src/codegen/llvm.zig   |  2 +-
 test/behavior/cast.zig | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index bccb67ec3c..1aa8e86a7f 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -12777,7 +12777,7 @@ fn isByRef(ty: Type, zcu: *Zcu) bool {
         },
         .@"union" => switch (ty.containerLayout(zcu)) {
             .@"packed" => return false,
-            else => return ty.hasRuntimeBits(zcu),
+            else => return ty.hasRuntimeBits(zcu) and !ty.unionHasAllZeroBitFieldTypes(zcu),
         },
         .error_union => {
             const payload_ty = ty.errorUnionPayload(zcu);
diff --git a/test/behavior/cast.zig b/test/behavior/cast.zig
index 740d123170..7eee20e3e0 100644
--- a/test/behavior/cast.zig
+++ b/test/behavior/cast.zig
@@ -2722,3 +2722,18 @@ test "@intFromFloat vector boundary cases" {
     try S.doTheTest();
     try comptime S.doTheTest();
 }
+
+test "coerce enum to union with zero-bit fields through local variables" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+
+    const E = enum(u1) { foo, bar };
+    const U = union(E) { foo, bar };
+
+    var runtime: E = undefined;
+    runtime = .foo;
+
+    var result: U = undefined;
+    result = runtime;
+
+    try expect(result == .foo);
+}