From 5382c84ed22519a600642f5cf3b1d8c2769050d6 Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Wed, 21 Oct 2026 16:33:39 +0100
Subject: [PATCH] AstGen: error on tag capture in `else` prong of `switch`

Resolves: https://codeberg.org/ziglang/zig/issues/37325
---
 lib/std/zig/AstGen.zig                            |  6 ++++++
 test/behavior/switch.zig                          |  3 +--
 test/behavior/switch_loop.zig                     |  3 +--
 .../compile_errors/tag_capture_in_else_prong.zig  | 15 +++++++++++++++
 4 files changed, 23 insertions(+), 4 deletions(-)
 create mode 100644 test/cases/compile_errors/tag_capture_in_else_prong.zig

diff --git a/lib/std/zig/AstGen.zig b/lib/std/zig/AstGen.zig
index fd6e48314901787bedbf74ba8118af6b7b09de36..0ca2397a64bced3b64f6781732296f7956c7c7db 100644
--- a/lib/std/zig/AstGen.zig
+++ b/lib/std/zig/AstGen.zig
@@ -7731,6 +7731,12 @@ fn switchExpr(
             const tag_name = try astgen.identAsString(tag_token);
             try astgen.detectLocalShadowing(payload_sub_scope, tag_name, tag_token, tag_slice, .@"switch tag capture");
 
+            if (case_node.toOptional() == else_case_node and case.inline_token == null) {
+                return astgen.failTokNotes(tag_token, "cannot capture tag of 'else' prong", .{}, &.{
+                    try astgen.errNoteTok(ident, "first capture in 'else' prong always captures 'switch' operand", .{}),
+                });
+            }
+
             assert(any_has_tag_capture);
             has_tag_capture = true;
 
diff --git a/test/behavior/switch.zig b/test/behavior/switch.zig
index 25eb5159357c06134f6b8e2f3fa5276528b7d4c4..8631be2150679ef66aaf6f53d4ad98eb5f9bcd9f 100644
--- a/test/behavior/switch.zig
+++ b/test/behavior/switch.zig
@@ -1186,8 +1186,7 @@ test "switch with tag capture" {
                     _ = &tag;
                     comptime unreachable;
                 },
-                else => |un, tag| {
-                    try expect(tag == .c);
+                else => |un| {
                     try expect(un == .c);
                     try expect(un.c == 0xFF);
                 },
diff --git a/test/behavior/switch_loop.zig b/test/behavior/switch_loop.zig
index f45ef3ef4113b493fc434f75ba1fe5c6a0111945..6169baef0e02039e042576bae568066668a16138 100644
--- a/test/behavior/switch_loop.zig
+++ b/test/behavior/switch_loop.zig
@@ -416,8 +416,7 @@ test "switch loop with tag capture" {
                     _ = &tag;
                     return error.AnalyzedNoreturnProng;
                 },
-                else => |un, tag| {
-                    try expect(tag == .c);
+                else => |un| {
                     try expect(un == .c);
                     if (un.c == 0xFF) continue :label .a;
                     if (un.c == 0x00) break :label false;
diff --git a/test/cases/compile_errors/tag_capture_in_else_prong.zig b/test/cases/compile_errors/tag_capture_in_else_prong.zig
new file mode 100644
index 0000000000000000000000000000000000000000..3b32d49bc6737de25a5a8fce924f64a0dcbd938f
--- /dev/null
+++ b/test/cases/compile_errors/tag_capture_in_else_prong.zig
@@ -0,0 +1,15 @@
+const Foo = union(enum) { a: u32, b: u32 };
+export fn entry() void {
+    const foo: Foo = .{ .a = 123 };
+    switch (foo) {
+        else => |payload, tag| {
+            _ = payload;
+            _ = tag;
+        },
+    }
+}
+
+// error
+//
+// :5:27: error: cannot capture tag of 'else' prong
+// :5:18: note: first capture in 'else' prong always captures 'switch' operand
-- 
2.54.0