From 456f3c026b3d6fb289f42e442b202e0be152c9d3 Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Wed, 5 Feb 2025 19:08:05 +0000
Subject: [PATCH] Sema: fix crash on `@tagName` of undefined enum literal

Resolves: #20826
---
 src/Sema.zig                                              | 2 +-
 .../compile_errors/tagName_on_undef_enum_literal.zig      | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)
 create mode 100644 test/cases/compile_errors/tagName_on_undef_enum_literal.zig

diff --git a/src/Sema.zig b/src/Sema.zig
index ad144bf013..af077c5150 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -21401,7 +21401,7 @@ fn zirTagName(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air
     try operand_ty.resolveLayout(pt);
     const enum_ty = switch (operand_ty.zigTypeTag(zcu)) {
         .enum_literal => {
-            const val = try sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, operand, undefined);
+            const val = (try sema.resolveDefinedValue(block, operand_src, operand)).?;
             const tag_name = ip.indexToKey(val.toIntern()).enum_literal;
             return sema.addNullTerminatedStrLit(tag_name);
         },
diff --git a/test/cases/compile_errors/tagName_on_undef_enum_literal.zig b/test/cases/compile_errors/tagName_on_undef_enum_literal.zig
new file mode 100644
index 0000000000..653f23db59
--- /dev/null
+++ b/test/cases/compile_errors/tagName_on_undef_enum_literal.zig
@@ -0,0 +1,8 @@
+comptime {
+    const undef: @Type(.enum_literal) = undefined;
+    _ = @tagName(undef);
+}
+
+// error
+//
+// :3:18: error: use of undefined value here causes undefined behavior