subtype.js

// @flow
type Foo = { n: number };
type Foo2 = Foo & { s: string };

type Bar = { foo: Foo };
type Bar2 = { foo: Foo2 };

function fooN(foo: Foo){ return foo.n; }
function barFooN(bar: Bar){ return bar.foo.n }

let bar2: Bar2 = { foo: { n: 2, s: "" }};
barFooN(bar2);

/*
subtype2.js:12
 12: barFooN(bar2);
     ^^^^^^^^^^^^^ function call
  3: type Foo2 = Foo & { s: string };
                       ^^^^^^^^^^^^^ property `s`. Property not found in
  5: type Bar = { foo: Foo };
                       ^^^ object type
*/