Feature request: infer type predicates

[pelotom]

TypeScript Version: 2.4.2

It would be really nice if certain boolean-returning functions could be inferred to be type predicates. For example, in RxJS one of the signatures for the filter method is

function filter<T, S extends T>(
  this: Observable<T>,
  predicate: (value: T, index: number) => value is S,
  thisArg?: any
): Observable<S>;

It's very useful to filter on an observable and thereby have the resulting observable's type be refined:

type Action =
  | { type: 'FOO'; fooField: number }
  | { type: 'BAR'; barField: string }

action$
  .filter(action => action.type === 'FOO')
  .mergeMap(({ fooField }) => ...)

however this won't work because the lambda action => action.type === 'FOO' is not inferred to have the type (action: Action) => action is { type: 'FOO'; fooField: number }, even though the compiler should know this is true.

[aluanhaddad]

See also #12798

[mhegazy]

Duplicate of #5101, #14891. Please see #5101 (comment) for more details. if you do have a proposal in mind on how this can be accomplished for the general case feel free to post it to #5101.

[mhegazy]

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