Ref: https://github.com/reactjs/rfcs/pull/147#issuecomment-623740218

React version: 0.0.0-experimental-e5d06e34b

## Steps To Reproduce

1. createMutableSource that contains functions
2. getSnapshot returns one of the functions

Link to code example:

https://codesandbox.io/s/jovial-dew-1eg4t?file=/src/App.js

## The current behavior

If getSnapshot returns a function, it will run the function in render. This results in various errors.

My workaround: return an object wrapping a function, and destruct on caller.

> For now, you can work around this by returning a function that returns your function

I don't know how this works as workaround.

## The expected behavior

It would be nice if returning a function just works.

Note: this is not a rare use case in my library https://github.com/dai-shi/use-context-selector
because we often pass `[state, setState]` in a context value.
function throwInvalidHookError() {
  invariant(
    false,
    'Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for' +
      ' one of the following reasons:\n' +
      '1. You might have mismatching versions of React and the renderer (such as React DOM)\n' +
      '2. You might be breaking the Rules of Hooks\n' +
      '3. You might have more than one copy of React in the same app\n' +
      'See https:fb.me/react-invalid-hook-call for tips about how to debug and fix this problem.',
  );
}
function areHookInputsEqual(
  nextDeps: Array<mixed>,
  prevDeps: Array<mixed> | null,
) {
  if (__DEV__) {
    if (ignorePreviousDependencies) {
      
      return false;
    }
  }
  if (prevDeps === null) {
    if (__DEV__) {
      console.error(
        '%s received a final argument during this render, but not during ' +
          'the previous render. Even though the final argument is optional, ' +
          'its type cannot change between renders.',
        currentHookNameInDev,
      );
    }