Test.ts

interface Foo {
  x: number | string
}

interface Bar {
  x: number
}

function main(): void {
  const foo: Foo = {x: 10}
  if (typeof foo.x === 'string') {
    return
  }

  // Compile Error!
  //
  // Type 'Foo' is not assignable to type 'Bar'.
  //   Types of property 'x' are incompatible.
  //     Type 'string | number' is not assignable to type 'number'.
  //       Type 'string' is not assignable to type 'number'.
  const bar: Bar = foo

  console.log(foo, bar)
}

main()