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.
if (__DEV__) {
    if (current !== null && current.memoizedState !== null) {
      ReactCurrentDispatcher.current = HooksDispatcherOnUpdateInDEV;
    } else if (hookTypesDev !== null) {
      
      
      
      
      
      ReactCurrentDispatcher.current = HooksDispatcherOnMountWithHookTypesInDEV;
    } else {
      ReactCurrentDispatcher.current = HooksDispatcherOnMountInDEV;
    }
  } else {
    ReactCurrentDispatcher.current =
      current === null || current.memoizedState === null
        ? HooksDispatcherOnMount
        : HooksDispatcherOnUpdate;
  }
  let children = Component(props, secondArg);
   Check if there was a render phase update
  if (didScheduleRenderPhaseUpdateDuringThisPass) {
    
    
    let numberOfReRenders: number = 0;