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.
});
    expect(Scheduler).toHaveYielded(['Parent: 1', 'Child: 2', 'Commit: 1, 2']);
    await act(async () => {
      
      root.render(<App parentConfig={configB} childConfig={configB} />);
      
      expect(Scheduler).toFlushAndYieldThrough(['Parent: 2']);
      
      
      
      Scheduler.unstable_runWithPriority(
        Scheduler.unstable_IdlePriority,
        () => {
          source.valueB = '3';
        },
      );
      expect(Scheduler).toFlushAndYieldThrough([
        
        'Child: 2',
        'Commit: 2, 2',
      ]);
      
      
      
      
      if (gate(flags => flags.new)) {
        
        
        expect(Scheduler).toFlushAndYieldThrough([
          'Parent: 3',