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).toFlushAndYield([
          
          'Child: 3',
          'Commit: 3, 3',
        ]);
      } else {
        
        
        expect(Scheduler).toFlushAndYield([
          'Parent: 3',
          'Child: 3',
          'Commit: 3, 3',
        ]);
      }
    });
  });
  
  it('should not tear with newly mounted component when updates were scheduled at a lower priority', async () => {
    const source = createSource('one');
    const mutableSource = createMutableSource(source);
    let committedA = null;
    let committedB = null;
    const onRender = () => {
      if (committedB !== null) {
        expect(committedA).toBe(committedB);
      }
    };
    function ComponentA() {