Warn when calling dispatch() from useEffect() cleanup function on unmounting

<!-- Note: if the issue is about documentation or the website, please file it at: https://github.com/reactjs/reactjs.org/issues/new --> **Do you want to request a *feature* or report a *bug*?** bug **What is the current behavior?** action dispatched in return callback of `useEffect` seem to not work **If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:** https://codesandbox.io/s/5yqmo128v4 only foo -> baz is logged ```javascript import React, { useState, useEffect, useReducer } from "react"; import ReactDOM from "react-dom"; import "./styles.css"; function reducer(state, action) { console.log("bar", action); // not logged // debugger return state; } function Foo({ value }) { const [state, dispatch] = useReducer(reducer, {}); useEffect( () => { return () => { console.log("foo"); // debugger dispatch({ type: "foo" }); // debugger console.log("baz"); }; }, [state, value] ); return <p>{value}</p>; } function App() { const [value, set] = useState(0); return ( <div className="App"> <button onClick={() => set(value + 1)}>INC</button> {value % 2 ? <Foo value={value} /> : null} </div> ); } const rootElement = document.getElementById("root"); ReactDOM.render(<App />, rootElement); ``` **What is the expected behavior?** bar is logged in console (foo -> baz -> bar`action`) **Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?** react: "16.7.0-alpha.2", react-dom: "16.7.0-alpha.2"