Incorrect any type distilled from extended generic after typeof check

[dennis-c]

TypeScript Version: 4.1.2 and 4.2.0-dev.20201202

Search Terms: extended generic typeof any

Code

type Identifier = string;

interface Identifiable {
	readonly id: Identifier;
}

class DropdownItem<ID extends Identifier> implements Identifiable {
	public readonly id: ID;
	public readonly name: string;

	public constructor(id: ID, name: string) {
		this.id = id;
		this.name = name;
	}
}

const defaultItemGetCells = (item: Identifiable): string[] => {
	if (item instanceof DropdownItem) {
		return [ item.id, item.name ]; //item.id is incorrectly typed as any
	} else {
		return [ item.id ]; //item.id is correctly typed as string
	}
};

Expected behavior:
The type of item.id on line 19 is correctly identified as a string (just like on line 21). It should always be at least a string, even is no generic is passed to DropdownItem (And I can't pass a generic because it's plain old javascript). If I remove the name field from the DropdownItem, it infers correctly.

Actual behavior:
The type of item.id on line 19 is incorrectly identified as any, this causes our typescript-eslint no any rule to kick in.

Playground Link: Example

Related Issues: I could find nothing that looked similar

[MartinJohns]

Workaround:

const itemId = item.id;
if (item instanceof DropdownItem) {
	return [ itemId, item.name ];
} else {
	return [ itemId ];
}

Side-note:

Using instanceof on classes containing only public members is not a safe operation. Such classes a structurally typed, so an object literal is implicitly assignable, and in this case instanceof fails "unexpectedly".

[RyanCavanaugh]

I think we should be using the constraint type to instantiate DropdownItem instead of any here

[yuvalbl]

Hey, any updates on this one?
I've encountered this issue as well - here is a simplified version of my scenario (which also using instanceof).

@MartinJohns I didn't understand your side-note, the following still fails even if I add a private field to MyClass

interface BaseT {
  baseProp: number;
}
export class MyClass<T extends BaseT> {
  propA: T;
}

const obj1 = {}; // simpified version, assume factory function which return type any

// should show a compilation error, since baseProp is number, but it doesn't!
// will show compilation error when "propA: T" by "propA: BaseT"
if (obj1 instanceof MyClass && obj1.propA.baseProp === 'aa') {
}

[kevincox]

This appears to be a duplicate of #17253