Array.isArray() type guard doesn't recognize ReadonlyArray<T>

[jrockwood]

TypeScript Version: 2.6.1

Code

interface StringRecord {
    readonly message: string | ReadonlyArray<string>; 
}

const record: StringRecord = { message: 'item' };
const message: string = Array.isArray(record.message) ? record.message.join('') : record.message;

Expected behavior:
The Array.isArray() type guard check should detect that record.message is a string. Note that if the message field is changed to string | string[] then everything works as expected. It's only when it's a ReadonlyArray<T> that it fails to compile. I can definitely add a as string cast, but that's kind of ugly and potentially dangerous if I change the data type later.

Actual behavior:
An error is reported: error TS2322: Type 'string | ReadonlyArray<string>' is not assignable to type 'string'.

[ghost]

Looks like a duplicate of #17002.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.