function getProperty‹T, K extends keyof T›(obj: T, key: K) { return obj[key]; } let x = { a: 1, b: 2, c: 3, d: 4 }; // okay getProperty(x, "a"); // error: Argument of type 'm' isn't assignable to 'a' | 'b' | 'c' | 'd'. getProperty(x, "m");