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 rerenderDeferredValue(value, config);
    },
    useTransition(
      config: SuspenseConfig | void | null,
    ): [(() => void) => void, boolean] {
      currentHookNameInDev = 'useTransition';
      warnInvalidHookAccess();
      updateHookTypesDev();
      return rerenderTransition(config);
    },
    useMutableSource<Source, Snapshot>(
      source: MutableSource<Source>,
      getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
      subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
    ): Snapshot {
      currentHookNameInDev = 'useMutableSource';
      warnInvalidHookAccess();
      updateHookTypesDev();
      return updateMutableSource(source, getSnapshot, subscribe);
    },
    useOpaqueIdentifier(): OpaqueIDType | void {
      currentHookNameInDev = 'useOpaqueIdentifier';
      warnInvalidHookAccess();
      updateHookTypesDev();
      return rerenderOpaqueIdentifier();
    },
    unstable_isNewReconciler: enableNewReconciler,
  };
}