async type narrowing #62624

function getValue(): string | null {
    return "theValue"
}

async function doWork(): Promise<void> { }

/**
 * Type-narrowing is applied. `x` satisfies the constraint `string`.
 */
function doSomethingSync(): string {
    function fail(): never {
        throw new Error()
    }

    const x: string | null = getValue()
    if (x === null) fail()
    return x satisfies string
}

/**
 * Type-narrowing is applied. `x` satisfies the constraint `string`.
 */
async function doSomethingAsyncWithInlineFail(): Promise<string> {
    const x: string | null = getValue()
    if (x === null) {
        await doWork()
        throw new Error()
    }
    return x satisfies string
}

/**
 * Type-narrowing is not applied. `x` does not satisfy the constraint `string` as it may be of type `null`.
 */
async function doSomethingAsyncWithAsyncFunctionFail(): Promise<string> {
    async function fail(): Promise<never> {
        await doWork()
        throw new Error()
    }

    const x: string | null = getValue()
    if (x === null) await fail()
    return x satisfies string
}