vi.assertType

code:ts

import { assertType } from 'vitest'

function concat(a: string, b: string): string

function concat(a: number, b: number): number

function concat(a: string | number, b: string | number): string | number

assertType<string>(concat('a', 'b'))

assertType<number>(concat(1, 2))

// @ts-expect-error wrong types

assertType(concat('a', 2))