Excess properties not checked for unannotated function

[OliverJAsh]

TypeScript Version: 3.6.3.

Search Terms: excess property properties checks checking return type function annotation

Code

Are there plans to expand excess property checks so that this case is also covered?

type Fn = () => {
  foo: string;
};
const fn: Fn = () => ({
  foo: "foo",
  // No excess error :-(
  bar: "bar"
});

Workaround: annotate function return type or use constructor

type FnResult = {
  foo: string;
};
type Fn = () => FnResult;
const fn: Fn = (): FnResult => ({
  foo: "foo",
  // Excess error :-)
  bar: "bar"
});

[AnyhowStep]

So, this is a feature request to propagate the "freshness" flag to the function return type, and perform excess property checks when the return type contains this "freshness"?

[AnyhowStep]

Would you want this to give you an error?

function foo () {
  return { x : 1, y : 2 }
}
//OK?
//Or excess property error?
const obj : { x : number } = foo()

[AnyhowStep]

How about,

type Fn = () => {
  foo: string;
};
function blah () {
  return {
    foo: "foo",
    bar: "bar"
  }
}

//OK?
//Or excess property error?
const fn: Fn = () => blah();

//OK?
//Or excess property error?
const fn2: Fn = blah;

[AnyhowStep]

How about,

type Fn = () => { foo : string }[];

const fn : Fn = () => ["hello"].map(str => {
  return {
    foo : str,
    //OK?
    //Or excess property error?
    bar : str,
  };
});

---

Or,

type Fn = () => { foo : string }[];

//Does this freshness propagate through generics?
function identity<T> (callback : () => T) : T {
  return callback();
}

//OK?
//Or excess property error?
const fn : Fn = () => identity(() => ({
  foo: "foo",
  //Excess property
  bar: "bar"
}));

//Does this freshness propagate through generics?
function identity2<T> (t : T) : T {
  return t;
}

//OK?
//Or excess property error?
const fn2 : Fn = () => identity2({
  foo: "foo",
  //Excess property
  bar: "bar"
});

[RyanCavanaugh]

Duplicate #12632, lots of others.

[typescript-bot]

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