Array#includes argument type is too narrow

[aquark]

TypeScript Version: my project uses 3.5.3 but I can see the issue is still present in the master branch.

Search Terms: Array includes

Code

const b: string = 'b'
['a' as const].includes(b)

Expected behavior:
It should compile. Specifically, the type definition for Array#includes should allow for any searchElement type:

interface Array<T> {
    /**
     * Determines whether an array includes a certain element, returning true or false as appropriate.
     * @param searchElement The element to search for.
     * @param fromIndex The position in this array at which to begin searching for searchElement.
     */
    includes(searchElement: unknown, fromIndex?: number): boolean;
}

It is type-safe to ask whether an array contains any value for the same reason it is type-safe to compare any two types with ===. The example above is a toy example but this pattern shows up all the time in real-world use, e.g. to check if a string is a member of a union of strings.

Actual behavior:
Argument of type 'string' is not assignable to parameter of type '"a"'

Related Issues: None found

[jcalz]

Duplicate of #26255

[aquark]

Seems like this keeps coming up because there are common idioms which cannot be expressed without unsafe casts. I hear the argument that TS should prevent stupid errors, at the same time it should also try to support idiomatic code naturally. I would argue that until bivariant argument types are supported, it would be better to give this a less restrictive type which supports all the standard use cases, rather than encourage casting. Compare Java's Collection#contains for example.

[jcalz]

Relevant SO answer about this

[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.