Inspired by #10421

The in operator could trivially be seen as a type guard:

interface A {
  x: number;
}
interface B {
  y: string;
}

let q: A | B = ...;
if ('x' in q) {
  // q: A
} else {
  // q: B
}

Basically, for a n in x where n is a string literal or string literal type and x is a union type, the "true" arm narrows to types which have an optional or required property n, and the "false" arm narrows to types which have an optional or missing property n.