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(() =>
            PrevScheduler.unstable_flushAllWithoutAsserting(),
          ).toThrow('Invalid hook call');
        });
      });
      
      it('should error if multiple renderers of the same type use a mutable source at the same time with mutation between', () => {
        const source = createSource('one');
        const mutableSource = createMutableSource(source);
        act(() => {
          
          ReactNoop.render(
            <>
              <Component
                label="a"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
              <Component
                label="b"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
            </>,
          );
          expect(Scheduler).toFlushAndYieldThrough(['a:one']);
          const PrevScheduler = Scheduler;