Inferred type predicate in filter function not getting inferred

[smoky scaffold]

Hey all, just trying to figure out why this filter function isn't getting its type predicate inferred. I've set up a simple example on the playground

[daring yacht]

advdanny#0

Playground Link

Preview:```ts
let arrMaybePromise: (string | Promise<string>)[] = []

// works
const arrPromises = arrMaybePromise.filter(
e => e instanceof Promise
)

// doesn't work
const arrNotPromises = arrMaybePromise.filter(
e => !(e instanceof Promise)
)
...```

[crisp oasis]

for !(e instanceof Promise) to work as a general typeguard, ts would need to have negated types (to say, "not Promise") which ts currently doesn't have

or be generic and Exclude Promise from the input type, which wouldn't be inferred afaik

(something like this)

[daring yacht]

that_guy977#0

Playground Link

Preview:ts ... function notPromise<T>(x: T): x is Exclude<T, Promise<unknown>> { return !(x instanceof Promise) } const arrNotPromises_ = arrMaybePromise.filter(notPromise) // ^? ...

[smoky scaffold]

Oh so conceptually because I can't do is (not Promise) in a type predicate, then there's no type predicate that can be inferred from e => !(e instanceof Promise)

[crisp oasis]

right

[smoky scaffold]

Thanks, that makes sense. I was getting narrowing mixed up with type predicates it seems

Thanks @crisp oasis