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.
mutateA('baz');
        root.render(
          <App
            getSnapshotFirst={getSnapshotA}
            getSnapshotSecond={getSnapshotA}
          />,
        );
        
        
      });
      
      
      mutateA('bar');
      
      
    });
    
    
    
    
    expect(Scheduler).toHaveYielded(['x: bar, y: bar']);
  });
  
  it('getSnapshot changes and then source is mutated in between paint and passive effect phase', async () => {
    const source = createSource({
      a: 'foo',
      b: 'bar',
    });
    const mutableSource = createMutableSource(source);
    function mutateB(newB) {