types: allow (almost) anything in VNodeChildrenArrayContents

[KaelWD]

What problem does this feature solve?

The Vue runtime allows pretty much anything to be put into the children array, and simply strips away boolean | null | undefined:

vue/src/core/vdom/helpers/normalize-children.js

Line 48 in 48acf71

if (isUndef(c) || typeof c === 'boolean') continue

The types on the other hand only allow string, VNode, or another nested array, which means we can't use short-circuit expressions to conditionally render things.

// Doesn't work - `false` is not allowed
return h('div', data, [
  this.enableSomething && h(Something)
])

// We have to do
const children: VNodeChildrenArrayContents = []
this.enableSomething && children.push(h(Something))
return h('div', data, children)

// Or
return h('div', data, [
  this.enableSomething ? h(Something) : []
])

What does the proposed API look like?

Add a separate type for createElement that also allows boolean | null | undefined, that way vnode.children can still be the current normalised version.

[KaelWD]

This is needed even more since #8946 was merged, now you can't even do

h('div', [this.$slots.default])