Version

3.2.0-beta.7

Reproduction link

sfc playground

this is inline example that does not work an raise the error

// [1,2,3] inline array
v-for="v,k in [1,2,3]"

the following will work as expected ( when arr = [1,2,3] )

v-for="v,k in arr"

Steps to reproduce

open sfc

1. click inc button 3 times, when v-memo trigger a change the error will show.

What is expected?

no error

What is actually happening?

Uncaught (in promise): can't access property "el", oldVNode is undefined

I noticed another bug in your example. If you declare an array using const instead of let/var then following error occurs

Uncaught (in promise): Cannot read property 'emitsOptions' of null

are you sure its not the error you get on the 4th click?

ahh yes, it shows up on 4th click, nevermind then

the root cause is:

return (_ctx, _cache) => {
  return (_openBlock(), _createElementBlock(_Fragment, null, [
    _createElementVNode("button", {
      onClick: _cache[0] || (_cache[0] = $event => (count.value++))
    }, " inc " + _toDisplayString(count.value), 1 /* TEXT */),
    (_openBlock(), _createElementBlock(_Fragment, null, _renderList([1,2,3], (id, index, ___, _cached) => {
      const _memo = ([count.value < 3 ? true : count.value])

+ 	  we will lose the `dynamicChidren` (the `currentBlock` is empty) because return the cached vnode here.
      if (_cached && _cached.key === id && _isMemoSame(_cached.memo, _memo)) return _cached
      const _item = _createVNode(__import_1__.default, {
        count: count.value,
        index: index,
        key: id
      }, null, 8 /* PROPS */, ["count", "index"])
      _item.memo = _memo
      return _item
    }, _cache, 1), 64 /* STABLE_FRAGMENT */))
  ], 64 /* STABLE_FRAGMENT */))
}

it will be interesting to see if this also related to this root cause https://github.com/vuejs/vue-next/issues/4262

@lidlanca

withMemo works fine because it catches the block tree.

see :

export function withMemo(
  memo: any[],
  render: () => VNode<any, any>,
  cache: any[],
  index: number
) {
  const cached = cache[index] as VNode | undefined
  if (cached && isMemoSame(cached.memo!, memo)) {
    // make sure to let parent block track it when returning cached
    if (isBlockTreeEnabled > 0 && currentBlock) {
      currentBlock.push(cached)
    }
    return cached
  }
  const ret = render()
  ret.memo = memo
  return (cache[index] = ret)
}