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).toFlushAndYieldThrough(['a0', 'b0']);
        
        
        mutateA('a1');
        mutateB('b1');
        
        
        
        
        
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_IdlePriority,
          () => {
            mutateA('a0');
            mutateB('b0');
          },
        );
        
        expect(Scheduler).toFlushUntilNextPaint(['c']);
        
        
        expect(Scheduler).toFlushUntilNextPaint(['a0']);
        expect(root).toMatchRenderedOutput('a0b0c');
        
        expect(Scheduler).toFlushAndYield([]);
        expect(root).toMatchRenderedOutput('a0b0c');
      });
    },