title: How do I get around this "Subject incorrectly extends Observable" error in TypeScript 2.4 and RxJS 5.x?
body: <p>When I compile, I get the following compiler error in the RxJS declaration files:</p>
  
  <pre><code>node_modules/rxjs/Subject.d.ts(16,22): error TS2415: Class 'Subject&lt;T&gt;' incorrectly extends base class 'Observable&lt;T&gt;'.
    Types of property 'lift' are incompatible.
      Type '&lt;R&gt;(operator: Operator&lt;T, R&gt;) =&gt; Observable&lt;T&gt;' is not assignable to type '&lt;R&gt;(operator: Operator&lt;T, R&gt;) =&gt; Observable&lt;R&gt;'.
        Type 'Observable&lt;T&gt;' is not assignable to type 'Observable&lt;R&gt;'.
           Type 'T' is not assignable to type 'R'.
  </code></pre>
  
  <p>What's going on here, and how do I get around this without downgrading to TypeScript 2.3 or earlier?</p>