Partial does not work well with generic guards

[RReverser]

TypeScript Version: 2.4.2

Code

(Note this is a simplified example and easy to workaround, but it gets dirtier in a real-world code)

type Obj<B> = { body: B };

function acceptPartial<O extends Obj<string>>(obj: Partial<O>) {}

function acceptFull<O extends Obj<string>>(obj: O) {
    acceptPartial<O>({
        body: obj['body']
    });
}

Expected behavior:

No type errors.

Actual behavior:

temp.ts(8,22): error TS2345: Argument of type '{ body: string; }' is not assignable to parameter of type 'Partial<O>'.

This doesn't make much sense because { body: obj['body'] } is correctly determined as { body: string } which is same as Obj<string>, and so compatible with Partial<Obj<string>> and any Partial<O> in this context.

Moreover, calling acceptPartial<Obj<string>> instead of O works which seems to mean that Partial doesn't currently "understand" that Partial<O> is just a superset of Partial<Obj<string>> when O extends Obj<string>.

[mhegazy]

The core issue here is that this is not allowed:

var p: Partial<O> = { body: "" };

Since the compiler can not verify that body is not specialized in O further.

Duplicate of #19388

[RReverser]

Well #19388 is newer so that one is technically a duplicate of this :P

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.