react-router: "Warning: Automatically setting basename using is deprecated" errors

Version

2.0.0

Test Case

https://github.com/tomduncalf/react-router-base-issue (issue is not reproducable in JSBin, attempted at http://jsbin.com/poqige/edit?html,js,output, so I suspect the issue is related to ES6/Babel/webpack module bundling/loading somehow)

Steps to reproduce

1. Add a <base href="http://whatever"> tag to the head of a page containing react-router’s HTML using the default browserHistory
2. Load the page - history.js will output 2 errors, Warning: Automatically setting basename using <base href> is deprecated and will be removed in the next major release. The semantics of <base href> are subtly different from basename. Please pass the basename explicitly in the options to createHistory
3. Change the react-router history to use history where history is const history = useRouterHistory(createHistory)({ basename: 'http://whatever' }), which should fix the problem (as basename is now explicitly passed)
4. Reload the page

Expected Behavior

Console errors do not appear

Actual Behavior

2 console errors still appear

Additional notes

I have done a bit of digging into this, and I believe what is going on is something like:

• history 2.x warns if the page has a base tag and no basename option is passed in. However, we are passing a basename option, so this should not be an issue.

• However, https://github.com/reactjs/react-router/blob/master/modules/index.js#L31 exports https://github.com/reactjs/react-router/blob/master/modules/hashHistory.js. This export (Babel) compiles down to:

  import _hashHistory from './hashHistory';
  export { _hashHistory as hashHistory };
  

• https://github.com/reactjs/react-router/blob/master/modules/hashHistory.js#L3 contains export default createRouterHistory(createHashHistory)

• https://github.com/reactjs/react-router/blob/master/modules/createRouterHistory.js#L10 creates a new history without any options: history = useRouterHistory(createHistory)() so I guess is where the error comes from

(the same is also true of https://github.com/reactjs/react-router/blob/master/modules/browserHistory.js)

So because this export is actually an import then an export, the import triggers a call to createRouterHistory for the default export of hashHistory, which in turn creates a new history without the options we need to pass to it to avoid an error.

A few options I can think of are:

1. Ignore the errors, they only show in dev (not ideal)
2. Fix the issue on the history module end somehow, e.g. by passing a flag to silence these errors
3. Fix the issue in react-router, e.g. make hashHistory export a function that calls createRouterHistory rather than calling it directly, or to somehow pass the options through

Would be great to get your input on this, first time I have looked in the react-router codebase otherwise I might have dug deeper and tried to fix 😃