From ab3ac1e6701431ae7dea99b23852e36b369d6b87 Mon Sep 17 00:00:00 2001
From: Veikka Tuominen <git@vexu.eu>
Date: Wed, 27 Sep 2023 10:41:03 +0300
Subject: [PATCH] Value: fix assertion failure when mutating extern union

Closes #17292
---
 src/value.zig           |  6 +++++-
 test/behavior/union.zig | 13 +++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/value.zig b/src/value.zig
index 279f52e3e0..e7029369e7 100644
--- a/src/value.zig
+++ b/src/value.zig
@@ -398,7 +398,11 @@ pub const Value = struct {
             },
 
             .un => |un| Tag.@"union".create(arena, .{
-                .tag = un.tag.toValue(),
+                // toValue asserts that the value cannot be .none which is valid on unions.
+                .tag = .{
+                    .ip_index = un.tag,
+                    .legacy = undefined,
+                },
                 .val = un.val.toValue(),
             }),
 
diff --git a/test/behavior/union.zig b/test/behavior/union.zig
index 47e0b2f345..bc3a7b044d 100644
--- a/test/behavior/union.zig
+++ b/test/behavior/union.zig
@@ -1662,3 +1662,16 @@ test "union with 128 bit integer" {
         }
     }
 }
+
+test "memset extern union at comptime" {
+    const U = extern union {
+        foo: u8,
+    };
+    const u = comptime blk: {
+        var u: U = undefined;
+        @memset(std.mem.asBytes(&u), 0);
+        u.foo = 0;
+        break :blk u;
+    };
+    try expect(u.foo == 0);
+}