bug: type predicates (V is T) fails on expressions (V[K] is T)

[KiaraGrouwstra]

TypeScript Version: nightly (2.2.0-dev.20161128)

Code

propIs<T, V, K extends keyof V>(type: T, name: K, obj: V): V[K] is T;

Expected behavior:
TS being totally cool with V[K] is T.

Actual behavior:
Nope, many syntax errors.

[DanielRosenwasser]

The type predicate syntax has always taken a parameter name on the left side of the is keyword. So it sounds like what you want is better parser recovery here.

[KiaraGrouwstra]

Oh, yeah, V is T itself works fine, this seems to be specifically about adding the [K] there. It does seem like storing (extends) V[K] in a new generic may form a solution here...
But apparently propIs<T, V, K extends keyof V, R extends V[K]>(type: T, name: K, obj: V): R is T; yields Cannot find parameter 'R'. I must be doing something wrong...

[DanielRosenwasser]

Well it's only the parameter that a type predicate can operate on - in other words, you write

function foo(x: any): x is string {
    // ....
}

As opposed to the following:

function foo(x: any): any is string {
    // ....
}

[KiaraGrouwstra]

I'm sorry, that would be my misinterpretation then.
In that event I would be inclined to try the following for this specific case:

propIs<T, V, K extends keyof V, R extends V[K]>(type: T, name: K, obj: V): obj[name] is T;

Still not okay though. I suppose it'd help if I had the ability to save this result to a new variable first, but I'm not confident I can do this in the type language...

[DanielRosenwasser]

You could do something like this:

propIs<T, V, K extends keyof V, R extends V[K]>(type: T, name: K, obj: V): obj is (V & Record<K, T>)

[KiaraGrouwstra]

Whoa, some out of the box thinking there, wouldn't have come up with that myself. Thanks for thinking along. :)