The type returned by Array.prototype.find() is not narrowed

[exoRift]

Bug Report

🔎 Search Terms

.find
narrow find

🕗 Version & Regression Information

5.0.2

⏯ Playground Link

Playground link with relevant code

💻 Code

type Foo = {
    discriminator: 'bar',
    value: number
} | {
    discriminator: 'baz',
    value: string
}

function assertString (param: string) {}
function assertNumber (param: number) {}

function func (param: Foo[]) {
    const found = param.find((p) => p.discriminator === 'bar')

    if (!found) return

    assertNumber(found.value) // < issue

    if (found.discriminator === 'bar') assertNumber(found.value) // < no problem
}

🙁 Actual behavior

The find does not narrow the returned type

🙂 Expected behavior

The logical flow SHOULD narrow it. Typescript IS capable of this. I understand the check is in another scope, but can't it be propagated upwards?

[MartinJohns]

Duplicate of #38945 and others.

[exoRift]

Duplicate of #38945 and others.

It's similar, but it wouldn't be fixed with what that issue calls for as that is an evaluation of the type which an x is Foo typeguard seemingly fixes. This is a narrowing of properties of the same type which is quite un-band-aid-able (and I really don't want to template this type)

[MartinJohns]

It's the same. find() has two overloads: one with a function returning just a boolean, this can't be used for narrowing because the boolean might have nothing to do with the type, and another with a function returning a type guard. The latter is used for narrowing, but you call the first overload with just a boolean.

Another related issue is #38390, although the discriminator case is not explicitly listed there.