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.
return callbacksB.length;
      },
      set valueA(newValue) {
        revision++;
        valueA = newValue;
        callbacksA.forEach(callback => callback());
      },
      get valueA() {
        return valueA;
      },
      set valueB(newValue) {
        revision++;
        valueB = newValue;
        callbacksB.forEach(callback => callback());
      },
      get valueB() {
        return valueB;
      },
      get version() {
        return revision;
      },
    };
  }
  function createSource(initialValue) {
    const callbacks = [];
    let revision = 0;
    let value = initialValue;
    return {
      subscribe(callback) {