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

[mhegazy]

Reported by Christian Yang (@yangchristian) in #2775 (comment)

I repro Andreas Larsen (@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' )

Playground link

[ryanlin1986]

This blocks us from using typescript 2.5, please fix it so we can upgrade.

[sandersn]

Duplicate of #17069 #17166 #15371

Fixed by #17912