Narrow doesn't work on class properties

[DetachHead]

🐞 Bug Report

Describe the bug

Narrow doesn't work on class properties

Reproduce the bug

interface Foo {
    bar: Narrow<{hi: 'there'}>
}

class Bar implements Foo {
    bar = {hi: 'there'} //error: Type 'string' is not assignable to type 'NarrowRaw<"there">'.
}

Expected behavior

the value of bar is narrowed to type {hi: 'there'}

Screenshots

Additional context

ts-toolbelt 9.5.3
typescript 4.2.0

[millsp]

Narrow only works on generic types, that is to say on parameter input:

class C<B> {
    b: F.Narrow<B>;

    constructor(b: F.Narrow<B>) {
        this.b = b
    }
}

const c = new C({a: 1})

or write a helper

const narrow = <T>(thing: Narrow<T>) => thing

interface Foo {
    bar: {hi: 'there'}
}

class Bar implements Foo {
    bar = narrow({hi: 'there'})
}

Hope this will help

[DetachHead]

is this a typescript limitation or an intentional decision?

i'm having trouble understanding how this type works tbh, how exactly does it force types at the call site to be narrowed?

[millsp]

It is some kind of limitation, we cannot narrow other things than a generic type. It has to pass through a function first. So you could as well write a very useful little utility like:

const narrow = <T>(thing: Narrow<T>) => thing

interface Foo {
    bar: {hi: 'there'}
}

class Bar implements Foo {
    bar = narrow({hi: 'there'})
}

[millsp]

Or you can always use as const assertions, but this has the inconvenient to make things deeply readonly.