From 96bae851666ada20b77294212a5dcca4dbc50533 Mon Sep 17 00:00:00 2001
From: pentuppup <pentuppup@noreply.codeberg.org>
Date: Thu, 1 Oct 2026 13:05:37 -0400
Subject: [PATCH 1/2] InternPool: inherit captures from owner union in
 generated enums

---
 src/InternPool.zig      |  2 +-
 test/behavior/union.zig | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/InternPool.zig b/src/InternPool.zig
index 68d4fae575e3f02351194f3d0c3747e0631b1c24..fd467d30b303e60dfa2590c2c0606aab8ae240eb 100644
--- a/src/InternPool.zig
+++ b/src/InternPool.zig
@@ -3851,7 +3851,7 @@ pub fn loadEnumType(ip: *const InternPool, index: Index) LoadedEnumType {
         .generated_union_tag => info: {
             const owner_union: Index = @fromBackingInt(@intCast(extra_items[extra_index]));
             extra_index += 1;
-            break :info .{ .none, .empty, owner_union };
+            break :info .{ .none, ip.loadUnionType(owner_union).captures, owner_union };
         },
         _ => |n| info: {
             const zir_index: TrackedInst.Index = @fromBackingInt(@intCast(extra_items[extra_index]));
diff --git a/test/behavior/union.zig b/test/behavior/union.zig
index 083da4df4bf67ae8b6d36a4a5dd3d1394f1f8ff4..2e72103467d12f8b1d7f61b9b7ad1dabaa4b5628 100644
--- a/test/behavior/union.zig
+++ b/test/behavior/union.zig
@@ -2178,6 +2178,23 @@ test "matching captures causes union equivalence" {
     try expect(a.u == b.u);
 }
 
+test "capture used as explicit field value" {
+    const S = struct {
+        fn Union(n: u8) type {
+            return union(enum(u8)) {
+                a: u32 = n,
+                b,
+            };
+        }
+    };
+
+    comptime assert(@typeInfo(S.Union(0)).@"union".tag_type.? != @typeInfo(S.Union(1)).@"union".tag_type.?);
+
+    const s: S.Union(1) = .{ .a = 0 };
+    try expect(@backingInt(s) == 1);
+    try expect(@backingInt(@as(@typeInfo(S.Union(1)).@"union".tag_type.?, s)) == 1);
+}
+
 test "signed enum tag with negative value" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
-- 
2.54.0


From 04742ad6f58ae5860efe9b4846be2caafca079e7 Mon Sep 17 00:00:00 2001
From: pentuppup <pentuppup@noreply.codeberg.org>
Date: Thu, 1 Oct 2026 13:05:37 -0400
Subject: [PATCH 2/2] Sema: fix error on undefined tagged union values

---
 src/Sema/type_resolution.zig                  |  2 +-
 .../compile_errors/undefined_enum_value.zig   | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 test/cases/compile_errors/undefined_enum_value.zig

diff --git a/src/Sema/type_resolution.zig b/src/Sema/type_resolution.zig
index a862afa9efa3a524a3b2eaeac8dbef5c2b896f11..a6e6a7609f63a50b18dbc41eeafeb3213f06a823 100644
--- a/src/Sema/type_resolution.zig
+++ b/src/Sema/type_resolution.zig
@@ -1478,7 +1478,7 @@ pub fn resolveEnumLayout(sema: *Sema, enum_ty: Type) CompileError!void {
                 };
                 const uncoerced = try sema.resolveInlineBody(&block, value_body, zir_index);
                 const coerced = try sema.coerce(&block, int_tag_ty, uncoerced, field_val_src);
-                const val = try sema.resolveConstValue(&block, field_val_src, coerced, null);
+                const val = try sema.resolveConstDefinedValue(&block, field_val_src, coerced, null);
                 enum_obj.field_values.get(ip)[zir_field.idx] = val.toIntern();
             }
         }
diff --git a/test/cases/compile_errors/undefined_enum_value.zig b/test/cases/compile_errors/undefined_enum_value.zig
new file mode 100644
index 0000000000000000000000000000000000000000..9ed69fa2ebf4171d53d716e9271a32807f10944e
--- /dev/null
+++ b/test/cases/compile_errors/undefined_enum_value.zig
@@ -0,0 +1,19 @@
+const E = enum(u8) {
+    a = undefined,
+    b,
+};
+const U = union(enum(u8)) {
+    a: u32 = undefined,
+    b,
+};
+comptime {
+    _ = E.a;
+}
+comptime {
+    _ = U.a;
+}
+
+// error
+//
+// :2:9: error: use of undefined value here causes illegal behavior
+// :6:14: error: use of undefined value here causes illegal behavior
-- 
2.54.0