InstanceOf incorrectly narrow when two type extends same base class

[yuit]

Version: master

class B { name: string }
class A { }
class C { name: string; }
var a: A | C;
if (!(a instanceof A)) {
    a;  // a has "never" type 
}

The change is a result of the PR #10216. It is followed the specification in that PR. Though I wonder whether this should get treated differently.

[yortus]

You can remove extends B from both the A and C classes and see the same behaviour, so extending the same base class doesn't seem important here.

Isn't this because C is structurally a subtype of A? So when the type guard removes A, it also removes C which it considers a subtype, leaving never as the narrowed type.

The PR #10216 addressed a very similar situation raised in #7271, but the problem and the fix addressed the situation where unrelated classes were structurally identical. This is a case where unrelated classes have a structural subtype-supertype relationship.

You can see the same behaviour with the first example in #7271 (comment).

[yuit]

@yortus thanks for correcting my example! Yes, it is because C is subtype of A but structurally not identical to A which according to PR #10216, then type C is instance of type A.

This example is slight different from the example in your comment comment in that we will treat user-defined typeguard and instanceof slightly different as @mhegazy pointed out here

[yortus]

@yuit this behaviour was the same before #10216 was merged - you can see it for example using [email protected]. #10216 tweaked the existing narrowing behaviour for structurally identical types, but I don't think it had any effect on this scenario.

[mhegazy]

That is a current design limitation with structural comparison.

[ericmorand]

So when will this bug be fixed?