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Instanceof left-hand-side check rejects generic index access types #19298

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@mhegazy

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@mhegazy

Reported by @yangchristian in #2775 (comment)

I repro @larsenwork's issue with the following:

type CreatedOrUpdatedAt =
    { createdAt: Date | string } |
    { updatedAt: Date | string };

const datePropComp = <T extends CreatedOrUpdatedAt, K extends keyof T>(a: T, b: T, field: K) => {
    // The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.
    const dateA = a[field] instanceof Date ? a[field] : new Date(a[field]);
    const dateB = b[field] instanceof Date ? b[field] : new Date(b[field]);

    return dateA.getTime() - dateB.getTime();
};

datePropComp({ updatedAt: Date() }, { updatedAt: Date() }, 'updatedAt' )

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