Type predicates fail to remove subclasses from unions for assumeTrue=false.

[sandersn]

@ahejlsberg you probably already know about this one but I thought I'd confirm:

interface Error {
    e
}
declare function isError(value: any): value is Error;
class CustomError extends Error { }
function narrowTest(value: number | CustomError) {
    if(isError(value)) {
        let result: CustomError = value;
    }
    else {
        let result: number = value; // error!
    }
}

Expected: narrowing to number in the else branch.
Actual: No narrowing: value: CustomError | number in the else branch, causing an error.

I found this in lodash on DefinitelyTyped.

[ahejlsberg]

This is an effect of #10216. The issue we face here is that Error and CustomError are structurally identical from the type system's point of view, yet at runtime a CustomError is an instance of Error, but not the other way around. The implementation currently makes the assumption that when two distinct types are structurally identical, neither is an instance of the other. We could look into "cheating" and check whether one extends the other, even though technically that isn't part of the type. Maybe we just need to get over that.

[sandersn]

Hm. We certainly treat structurally identical things badly in a number of other places, so it's already a definite anti-pattern.

I should probably just update lodash's DefinitelyTyped tests, and maybe the Typescript FAQ.