Sema: do not assume switch item indices align with union field indices

Resolves: #17754

1 files changed, 23 insertions, 0 deletions

diff --git a/test/behavior/switch.zig b/test/behavior/switch.zig
index d7d2f8f7b6..37cf8b7c6f 100644
--- a/test/behavior/switch.zig
+++ b/test/behavior/switch.zig
@@ -800,3 +800,26 @@ test "nested break ignores switch conditions and breaks instead" {
     // Originally reported at https://github.com/ziglang/zig/issues/10196
     try expect(0x01 == try S.register_to_address("a0"));
 }
+
+test "peer type resolution on switch captures ignores unused payload bits" {
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    const Foo = union(enum) {
+        a: u32,
+        b: u64,
+    };
+
+    var val: Foo = undefined;
+    @memset(std.mem.asBytes(&val), 0xFF);
+
+    // This is runtime-known so the following store isn't comptime-known.
+    var rt: u32 = 123;
+    val = .{ .a = rt }; // will not necessarily zero remaning payload memory
+
+    // Fields intentionally backwards here
+    const x = switch (val) {
+        .b, .a => |x| x,
+    };
+
+    try expect(x == 123);
+}