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.
ReactCurrentBatchConfig.suspense = config === undefined ? null : config;
    try {
      setValue(value);
    } finally {
      ReactCurrentBatchConfig.suspense = previousConfig;
    }
  }, [value, config]);
  return prevValue;
}
function startTransition(setPending, config, callback) {
  const priorityLevel = getCurrentPriorityLevel();
  runWithPriority(
    priorityLevel < UserBlockingPriority ? UserBlockingPriority : priorityLevel,
    () => {
      setPending(true);
    },
  );
  runWithPriority(
    priorityLevel > NormalPriority ? NormalPriority : priorityLevel,
    () => {
      const previousConfig = ReactCurrentBatchConfig.suspense;
      ReactCurrentBatchConfig.suspense = config === undefined ? null : config;
      try {
        setPending(false);
        callback();
      } finally {
        ReactCurrentBatchConfig.suspense = previousConfig;
      }
    },
  );