"Type argument cannot be inferred" in TypeScript class using generics

Issue

I'm creating a BehaviorSubject inside a generic class in TypeScript like so:

import { BehaviorSubject } from 'rxjs/BehaviorSubject';

interface Bar {
  a: any;
}

class Foo<T extends Bar> {
  subject = new BehaviorSubject<T[]>([]);
}

Here, subject is intended to emit arrays of T. However, the (WebStorm) compiler complains about the [] array passed to the BehaviorSubject constructor, saying:

Type argument cannot be inferred by usage

I get the same result even if I try to cast the array like:

subject = new BehaviorSubject<T[]>([] as T[]); // "Type argument cannot be inferred by usage"

What may be a clue is that if the type T does not extend any other type, there is no compiler complaint:

class Foo<T> {
  subject = new BehaviorSubject<T[]>([]); // no warning here
}

Everything seems to function as expected at runtime, but I'm curious as to why I encounter this error.

Solution

This issue seems to have been fixed by an update of one or more components, as I no longer see any errors in the IDE. Move along, nothing to see!

• WebStorm 2019.3.4
• rxjs@6.6.3
• typescript@4.0.3

Answered By - Myk Willis