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.
defaultSubscribe,
      );
      let result = `x: ${first}, y: ${second}`;
      if (getSnapshotFirst === getSnapshotSecond) {
        
        
        if (first !== second) {
          result = 'Oops, tearing!';
        }
      }
      React.useEffect(() => {
        Scheduler.unstable_yieldValue(result);
      }, [result]);
      return result;
    }
    const root = ReactNoop.createRoot();
    await act(async () => {
      root.render(
        <App
          getSnapshotFirst={getSnapshotA}
          getSnapshotSecond={getSnapshotB}
        />,
      );
    });
    
    expect(Scheduler).toHaveYielded(['x: foo, y: bar']);
    await act(async () => {
      ReactNoop.discreteUpdates(() => {