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] 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