type A = {x:string} | {x:number} | {x: symbol}

type B = {x:string} | {x:number} | {x: boolean}
                                       // ^ literal type here
type C = {x:string} | {x:number}

type X = {x:string | number}
// `X extends B` but not A or C


type X_A = X extends A ? "X extends A" : "X not extends A";
//   ^? X not extends A

type X_B = X extends B ? "X extends B" : "X not extends B";
//   ^? X extends B

type X_C = X extends C ? "X extends C" : "X not extends C";
//   ^? X not extends C