Narrowing to remove undefined from optional object properties

[ethanresnick]

TypeScript Version: 3.2.0-dev.201xxxxx

Search Terms:
narrowing optional required property undefined

Code

interface Message {
  to: string;
  from: string;
}

interface MessageOptionalFrom {
  to: string;
  from?: string;
}

declare function send(data: Message | Message[]): Promise<void>;

function test(x: MessageOptionalFrom) {
    // This doesn't narrow because of 
    // https://github.com/Microsoft/TypeScript/issues/21732
    if (!("from" in x)) {
        throw new Error("");
    }
    
    // But this doesn't narrow either :(
    if (typeof x.from === "undefined") {
        throw new Error("");
    }

    // Or this :(
    if (x.from == null) {
        throw new Error("");
    }

    return send(x);
}

Expected behavior:
One of these approaches to narrowing should work such that, at the bottom of the function, x should be (MessageOptionalFrom & { from: string }), and the call to send should succeed.

Actual behavior:

Argument of type 'MessageOptionalFrom' is not assignable to parameter of type 'Message | Message[]'.
  Type 'MessageOptionalFrom' is not assignable to type 'Message'.
    Property 'from' is optional in type 'MessageOptionalFrom' but required in type 'Message'.

Playground Link:
http://www.typescriptlang.org/play/index.html#src=interface%20Message%20%7B%0D%0A%20%20to%3A%20string%3B%0D%0A%20%20from%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20MessageOptionalFrom%20%7B%0D%0A%20%20to%3A%20string%3B%0D%0A%20%20from%3F%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Adeclare%20function%20send(data%3A%20Message%20%7C%20Message%5B%5D)%3A%20Promise%3Cvoid%3E%3B%0D%0A%0D%0A%2F%2F%20Irl%2C%20this%20function%20would%20set%20the%20default%20%60from%60%2C%20%0D%0A%2F%2F%20or%20error%20if%20that's%20not%20possible%2C%20then%20send.%0D%0A%2F%2F%20For%20now%2C%20it%20always%20errors%20if%20from%20is%20missing.%0D%0Afunction%20test(x%3A%20MessageOptionalFrom)%20%7B%0D%0A%20%20%20%20%2F%2F%20This%20doesn't%20narrow%20because%20of%20%0D%0A%20%20%20%20%2F%2Fhttps%3A%2F%2Fgithub.com%2FMicrosoft%2FTypeScript%2Fissues%2F21732%0D%0A%20%20%20%20if%20(!(%22from%22%20in%20x))%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20new%20Error(%22%22)%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20%0D%0A%20%20%20%20%2F%2F%20But%20this%20doesn't%20narrow%20either%20%3A(%0D%0A%20%20%20%20if%20(typeof%20x.from%20%3D%3D%3D%20%22undefined%22)%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20new%20Error(%22%22)%3B%0D%0A%20%20%20%20%7D%0D%0A%0D%0A%20%20%20%20%2F%2F%20Or%20this%20%3A(%0D%0A%20%20%20%20if%20(x.from%20%3D%3D%20null)%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20new%20Error(%22%22)%3B%0D%0A%20%20%20%20%7D%0D%0A%0D%0A%20%20%20%20%2F%2F%20x%20should%20be%20(MessageOptionalFrom%20%26%20%7B%20from%3A%20string%20%7D)%20here.%0D%0A%20%20%20%20%2F%2F%20Call%20below%20should%20work%2C%20therefore%3B%20errors%20instead.%0D%0A%20%20%20%20return%20send(x)%3B%0D%0A%7D

Related Issues:
#21732, but only covers one case.

[weswigham]

Currently when a member of a type is narrowed, it is not reflected in the narrowed type of the containing type.

[Laubeee]

I have one more example

class A {
  prop?: number;
}

function test(a: A) {
  a.prop = 0;
  let prop2: number | undefined = 0;
  (async() =>  {
    a.prop += 1;
    prop2 += 1;
  })();
}

prop2 is correctly narrowed, but a.prop throws object possibly undefined HOWEVER the optional property is correctly narrowed if I remove the async keyword.

[ethanresnick]

Realized that the root issue here is the same as #10065 and its various duplicates. Would be really nice to see this addressed. Since #10065 was closed and locked, I guess this is a decent place to track...

@Laubeee Your issue looks like something different (possibly #30625)