See description from here:
https://teal-playground.netlify.app/?c=bG9jYWwgdHlwZSBYID0gcmVjb3JkCiAgeDogbnVtYmVyCmVuZAoKbG9jYWwgZnVuY3Rpb24gdGVzdCgpOiBYCiAgcmV0dXJuIHsgeSA9IDEgfQplbmQKCnRlc3QoKQ%3D%3D
Looks like there's a quirk in the playground — you need to edit something (e.g. prest Enter) so that the compiler refreshes, and then the "y" will appear underlined in red and when you hover it, it does say unknown field y