From d3056114f6f17bb4723cccfa4dd578ddafb909a9 Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus@klausecker.de>
Date: Fri, 24 Jul 2026 16:50:07 +0200
Subject: [PATCH] Sema: disallow unreachable `else` prong for tagged unions
 with nonexhaustive tag types

---
 src/Sema.zig                                  |  2 +-
 ...n_with_nonexhaustive_tag_is_exhaustive.zig | 56 +++++++++++++++++++
 2 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 test/cases/compile_errors/switch_on_union_with_nonexhaustive_tag_is_exhaustive.zig

diff --git a/src/Sema.zig b/src/Sema.zig
index 6dafdeb0ee6c53fb6f116272e28c641239573cff..a52b11b506a71f7d4c4c734dbded00d5f5996a40 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -11375,7 +11375,7 @@ fn validateSwitchBlock(
 
             if (has_else) {
                 if (all_tags_handled) {
-                    if (item_ty.isNonexhaustiveEnum(zcu)) {
+                    if (operand_ty.isNonexhaustiveEnum(zcu)) {
                         if (has_under) return sema.fail(
                             block,
                             else_prong_src,
diff --git a/test/cases/compile_errors/switch_on_union_with_nonexhaustive_tag_is_exhaustive.zig b/test/cases/compile_errors/switch_on_union_with_nonexhaustive_tag_is_exhaustive.zig
new file mode 100644
index 0000000000000000000000000000000000000000..fc289fb55fb2c7fb0db1dd990fea16f431509595
--- /dev/null
+++ b/test/cases/compile_errors/switch_on_union_with_nonexhaustive_tag_is_exhaustive.zig
@@ -0,0 +1,56 @@
+const E = enum(u8) {
+    a,
+    b,
+    _,
+};
+const U = union(E) {
+    a,
+    b,
+};
+fn foo() U {
+    return undefined;
+}
+
+export fn entry1() void {
+    const u = foo();
+    switch (u) {
+        .a => {},
+    }
+}
+export fn entry2() void {
+    const u = foo();
+    switch (u) {
+        .a => {},
+        .b => {},
+        else => {},
+    }
+}
+export fn entry3() void {
+    const u = foo();
+    switch (u) {
+        .a => {},
+        .b => {},
+        _ => {},
+    }
+}
+export fn entry4() void {
+    const u = foo();
+    switch (u) {
+        .a => {},
+        else => {},
+        _ => {},
+    }
+}
+
+// error
+//
+// :16:5: error: switch must handle all possibilities
+// :3:5: note: unhandled enumeration value: 'b'
+// :1:11: note: enum 'tmp.E' declared here
+// :25:14: error: unreachable else prong; all cases already handled
+// :30:5: error: '_' prong only allowed when switching on non-exhaustive enums
+// :33:9: note: '_' prong here
+// :30:5: note: consider using 'else'
+// :38:5: error: '_' prong only allowed when switching on non-exhaustive enums
+// :41:9: note: '_' prong here
+// :38:5: note: consider using 'else'
-- 
2.54.0