feat: Strict type children in slots

active-rfcs/0000-slots-strict-type.md

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+- Start Date: 2025-01-08
+- Target Major Version: 3.x
+- Reference Issues: https://github.com/vuejs/core/issues/8015, https://github.com/tobiasdiez/storybook-vue-addon/issues/74
+- Implementation PR: (leave this empty)
+
+# Summary
+
+Allow for tools to enforce strict children type for Slots, by allowing to describe what type of children can be used in the slot.
+
+# Basic example
+
+```tsx
+const slots = defineSlots<{
+  default: () => VNode[] // any VNODE
+  foo: () => HTMLInputElement // single <input/>
+  foos: () => HTMLInputElement[] // multiple <input/>
+  foo2: () => [HTMLInputElement, HTMLInputElement] // 2 <input/> allowed, error if children different than 2
+  bar: () => SlotComponent<{ test: string }> // any Component with `test: string` prop
+  baz: () => SlotComponent<{ test: string }, Comp> // Comp with `test: string` passed
+}>()
+```
+
+`Tab.vue`
+
+```vue
+<script lang="ts">
+import type TabItem from './TabItem.vue'
+defineSlots<{
+  default: () => (typeof TabItem)[]
+}>()
+</script>
+```
+
+`Comp.vue`
+
+```vue
+<template>
+  <Tabs>
+    <TabItem />
+  </Tabs>
+  <!--@ts-expect-error not valid children-->
+  <Tabs>
+    <input/>
+  </Tags>
+</template>
+```
+
+# Motivation
+
+Allow greater control for library creators to specify what can be used for children.
+
+# Detailed design
+
+This is the bulk of the RFC. Explain the design in enough detail for somebody
+familiar with Vue to understand, and for somebody familiar with the
+implementation to implement. This should get into specifics and corner-cases,
+and include examples of how the feature is used. Any new terminology should be
+defined here.
+
+This only affects Typescript declaration, `SlotComponent` can be renamed to something else,
+without a specific type to allow granualy defining the required props and the Component, it will become quite difficult to extract it in Typescript while maintainnig the correct type.
+
+### Returns
+
+Strict numered of children:
+
+- Single
+- Array
+- Tuple
+
+Type:
+
+- Component
+- HTMLElement
+- SlotComponent
+
+# Drawbacks
+
+The main draw back is this is to be mostly use by tooling, vue language tools and others can get this information and provide better DX.
+
+# Alternatives
+
+This does not need a Vue core change, since `defineSlots` does not strict check the types, this is to be mostly adopted by tools.
+
+# Adoption strategy
+
+Can be adopted as soon as there's support for Vue language tools.
+
+# Unresolved questions
+
+- `SlotComponent` naming?