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.
}
}
function updateOpaqueIdentifier(): OpaqueIDType | void {
  const id = updateState(undefined)[0];
  return id;
}
function rerenderOpaqueIdentifier(): OpaqueIDType | void {
  const id = rerenderState(undefined)[0];
  return id;
}
function dispatchAction<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
) {
  if (__DEV__) {
    if (typeof arguments[3] === 'function') {
      console.error(
        "State updates from the useState() and useReducer() Hooks don't support the " +
          'second callback argument. To execute a side effect after ' +
          'rendering, declare it in the component body with useEffect().',
      );
    }
  }
  const eventTime = requestEventTime();
  const suspenseConfig = requestCurrentSuspenseConfig();
  const lane = requestUpdateLane(fiber, suspenseConfig);
  const update: Update<S, A> = {
    eventTime,
    lane,