Type guard fails to narrow type in specific situations with (indirectly) recursive types

[marijnh]

TypeScript Version: 3.8.3

Search Terms: guard recursive

Code

(This is very weird code, result of reducing a real-world situation to the minimum complexity necessary to reduce this.)

class A<Value> {
  prop!: (x: Value) => void
}
class B<Value> {
  prop!: A<Value>
  is<T>(): this is B<T> { return true }
}

let b: B<any> = null as any
if (b.is<string>()) {
  // b will still be of type B<any> here
}

Expected behavior:

The conditional would, due to the type guard on the is method, narrow the type of b to B<string>.

Actual behavior:

b remains of type B<any>. But, when the line that defines A.prop is commented out, it does correctly get narrowed to B<string>. If its type is changed to just Value, or some function type that doesn't involve Value, the problem also goes away.

Playground Link: here

Related Issues: none that I found

[RyanCavanaugh]

This works as expected for me at your Playground link. Is there more configuration needed to reproduce the problem?

[marijnh]

Apologies, I pasted a slightly incorrect playground link. Updated to this one