rxjs: Error in project with TS 2.6 and strictFunctionTypes

RxJS version: 5.5.2

Code to reproduce:

In tsconfig.json:

{
  "compilerOptions": {
    "skipLibCheck": false,
    "strictFunctionTypes": true
  }
}

import * as Rx from "rxjs";

Expected behavior:

Compiles successfully.

Actual behavior:

Compiler error:

../../node_modules/rxjs/scheduler/VirtualTimeScheduler.d.ts(22,22): error TS2415: Class 'VirtualAction<T>' incorrectly extends base class 'AsyncAction<T>'.
  Types of property 'work' are incompatible.
    Type '(this: VirtualAction<T>, state?: T | undefined) => void' is not assignable to type '(this: AsyncAction<T>, state?: T | undefined) => void'.
      The 'this' types of each signature are incompatible.
        Type 'AsyncAction<T>' is not assignable to type 'VirtualAction<T>'.
          Property 'index' is missing in type 'AsyncAction<T>'

Additional information:

Workaround: disable strictFunctionTypes OR enable skipLibCheck

I believe this is happening because work is not defined as a method (rather, it’s defined as a property), so its parameters are checked contravariantly. If it was defined as a method, the parameters would be checked covariantly.

This could be fixed by defining work as a method in both classes, or defining it as (this: AsyncAction<T>, state?: T) => void in VirtualAction.