Array Includes incorrectly uses generic type

[willyboy]

Bug Report

Array.includes should not be inferring type. Especially when the type is a string and the array is defined by a union of strings.
The point of using includes is to see if something exists in an array. If the thing we are looking for is a string in an array with a union type of strings, we get an error like, "argument of type string is not assignable to parameter of type 'a' | 'b'".

🔎 Search Terms

array, includes

🕗 Version & Regression Information

I'm on 4.5.5

⏯ Playground Link

Playground link with relevant code

💻 Code

let thing: string = 'random string that is not hardcoded';
const allowedStrings: ('a' | 'b') = ['a', 'b']

if (!allowedStrings.includes(thing)) {
    // do something
}

🙁 Actual behavior

An error is thrown preventing us from using includes to see if our string is in the array.

🙂 Expected behavior

No error is thrown as we don't know if the string is in the array or not until at runtime.

[RyanCavanaugh]

Lacking something like #14520, this is the intended behavior.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[willyboy]

@RyanCavanaugh How is this the intended behavior? "Includes" is basically runtime type checking. It shouldn't even be inferring a type.