TypeScript Version: 4.1.0-dev

Search Terms:
operator in exception crash unhandled

Code

const hasKey = <A, K extends string | number | symbol>(
    thing: A,
    key: K,
): boolean => {
    return key in thing;
};

hasKey(123, 'hello'); // TypeError exception

Expected behavior:

TS should expect A to be an object

Actual behavior:

TS didn't detect the potential crash

Playground Link: https://www.typescriptlang.org/play?ts=4.1.0-beta#code/MYewdgzgLgBAFgQwgaQKYE8YF4YB4CCANDMjKgB5SpgAmEM0ATgJZgDmMAPjGAK4C2AI1SMuDdEJAAbAHwAKAFAxlMKHFZsAXDCJKVAawzbkhBQEptgkNNQIw2GTADee5Y1RReje4cytV6uwA3AoAviEKiCgYcgCMAEwAzMQA5HCoUlIgKWZBMAD0+TAAKugADqgAooyMIKIUwKhlUMzgCkA

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