[vue-tsc] Generic Props gets Inferred as `unknown`

[unrevised6419]

Vue - Official extension or vue-tsc version

3.0.5

VSCode version

1.102.3

Vue version

3.5.18

TypeScript version

5.8.0

System Info

System:
    OS: macOS 26.0
    CPU: (8) arm64 Apple M1
    Memory: 267.22 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.15.0 - ~/.nvm/versions/node/v22.15.0/bin/node
    Yarn: 2.4.3 - ~/.nvm/versions/node/v22.15.0/bin/yarn
    npm: 11.3.0 - ~/.nvm/versions/node/v22.15.0/bin/npm
    pnpm: 9.15.9 - ~/.nvm/versions/node/v22.15.0/bin/pnpm
  Browsers:
    Brave Browser: 138.1.80.125
    Chrome: 138.0.7204.184
    Safari: 26.0

package.json dependencies

{
  "dependencies": {
    "vue": "^3.5.18"
  },
  "devDependencies": {
    "@tsconfig/node22": "^22.0.1",
    "@types/node": "^22.14.0",
    "@vitejs/plugin-vue": "^5.2.3",
    "@vue/tsconfig": "^0.7.0",
    "npm-run-all2": "^7.0.2",
    "typescript": "~5.8.0",
    "vite": "^6.2.4",
    "vite-plugin-vue-devtools": "^7.7.2",
    "vue-tsc": "^3.0.5"
  }
}

Steps to reproduce

Having this component

LocalScope.vue

<script setup lang="ts" generic="T">
defineOptions({ inheritAttrs: false });
defineProps</* @vue-ignore */ T>();
</script>

<template>
	<slot v-bind="$attrs as T" />
</template>

And this usage

<script setup lang="ts">
import LocalScope from './LocalScope.vue';
</script>

<template>
  <LocalScope #default="{ foo, bar }" :foo="42" bar="baz">
    {{ foo }} {{ bar }}
  </LocalScope>
</template>

With vue-tsc@3, running npx vue-tsc --build, we have such results

src/App.vue:6:27 - error TS2339: Property 'foo' does not exist on type 'unknown'.

6   <LocalScope #default="{ foo, bar }" :foo="42" bar="baz">
                            ~~~

src/App.vue:6:32 - error TS2339: Property 'bar' does not exist on type 'unknown'.

6   <LocalScope #default="{ foo, bar }" :foo="42" bar="baz">
                                 ~~~


Found 2 errors.

What is expected?

With vue-tsc@2, running npx vue-tsc --build, we no errors

What is actually happening?

With vue-tsc@3, running npx vue-tsc --build, we have such results

src/App.vue:6:27 - error TS2339: Property 'foo' does not exist on type 'unknown'.

6   <LocalScope #default="{ foo, bar }" :foo="42" bar="baz">
                            ~~~

src/App.vue:6:32 - error TS2339: Property 'bar' does not exist on type 'unknown'.

6   <LocalScope #default="{ foo, bar }" :foo="42" bar="baz">
                                 ~~~


Found 2 errors.

Link to minimal reproduction

https://github.com/unrevised6419/issue-vue-tsc-3

Any additional comments?

This happens with Vue extension v3 also. Downgrading to v2, works fine.

vuejs/rfcs#505 (comment)

2025-08-07.19-16-47.mov

[KazariEX]

It was caused by #5424. See playground.

[unrevised6419]

Oh, thanks @KazariEX

Closing in favor of #5159

[KazariEX]

Let's open it for now to see if there are other solutions.