Issues with type narrowing in if-else statements

[abhayvatsa]

Hi there!

Thanks for making this, been enjoying this library.

I am running into an issue with the type narrowing for if-else statements. I'm not sure if I'm doing something wrong, but this issue was mentioned in #5. I expect that a type guard on result.ok would narrow the union type appropriately in the else block. Instead I see an error like this:
Type 'string | number' is not assignable to type 'string'. Type 'number' is not assignable to type 'string'.

Currently on typescript 4.2.4

Here are two examples that are more or less the same.

import { Ok, Err } from 'ts-results'

const result = Ok(1) as Ok<number> | Err<string>

if (result.ok) {
  const val: number = result.val
} else {
  const val: string = result.val
}

import { Ok, Result } from 'ts-results'

const result = Ok(1) as Result<number, string>

if (result.ok) {
  const val: number = result.val
} else {
  const val: string = result.val
}

[vultix]

Alright, I think I've tracked this one down. As far as I can tell, the type narrowing only works when strictNullChecks is turned on in your tsconfig file.

Will you try turning on strictNullChecks and let me know if that fixes the issue? I'm guessing this is a bug in the typescript compiler itself

[vultix]

Here's a super simple replication in the typescript playground

[vultix]

Looks like this is a known bug: microsoft/TypeScript#10564

[abhayvatsa]

@vultix. Wow, thanks this worked. Appreciate the fast response. Would it make sense to make a note of this somewhere in the docs?

[vultix]

I added a note in the README. I'm going to close this issue for now, hopefully this gets fixed in Typescript soon