Infer type guard from predicate for functions accepting type guards.

[martaver]

It would be great to avoid the ceremony of having to declare separate type guards for functions accepting a type guard as an argument.

Consider functions like Array.filter or Observable.filter being used to filter a union type with discriminant:

type FooBar = { type: 'foo' } | { type: 'bar' };
const isFoo = <(fb: FooBar) => fb is { type: 'foo'}>(fb => fb.type == "foo");
const fooBar$: Observable<FooBar> = { /* ... */ };
const foo$ = fooBar$.filter(isFoo); //We can use the type guard 'isFoo' to narrow the type to Foo.

However this is quite a lot of wiring for something that anywhere else is quite straight forward. In an if/else or switch block, a type guard isn't necessary... Typescript infers the type from the predicate:

if(fb.type === 'foo') {
    //'fb' is { type: 'foo' }
}

Having to declare the type guard also requires us to 'repeat' out the shape of the type which can be quite painful if it isn't a type alias or class... i.e. we have to explicitly write `fb is { type: 'foo' }.

Typescript should be able to infer that the correct type is { type: 'foo' } from the supplied predicate.

In such an ideal world, we should be able to simply write:

const foo$ = fooBar$.filter(fb => fb.type === 'foo');

If Typescript requires us to explicitly opt-in the function as a type-guard, then it should be possible to do so without explicitly repeating the type in the declaration. Maybe something like:

const foo$ = fooBar$.filter(<fb is>(fb => fb.type === 'foo'));

[jcalz]

Duplicate of #10734 and #14891 and #5101 (with a relevant comment)

[DanielRosenwasser]

Thanks for finding that @jcalz

[martaver]

To clarify, I'm looking for a general solution that doesn't require explicit definition of the narrowed guard type, not just an overload to Array.filter...

[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.

[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.