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 "carview.php?tsp=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.