exactOptionalPropertyTypes breaks v-bind with defineProps in some cases

Vue version

3.5.27

Link to minimal reproduction

https://github.com/DerZade/vue-tsc-exact-optional-prop-types-error

Steps to reproduce

1. Create a project with npm create vue@latest
2. Turn on exactOptionalPropertyTypes in your tsconfig.app.json
3. Create a component with a separate type of defineProps:

   <template><!-- irrelevant --></template>
   
   <script lang="ts" setup>
   export type CompProps = {
       somethingOptional?: string; // the optional (?) is important
   };
   
   const props = defineProps<CompProps>();
   </script>
   

4. Create wrapper component for first component and re-use the props type in its defineProps:

   <template>
       <Comp v-bind="props" /> <!-- This v-bind will error -->
   </template>
   
   <script setup lang="ts">
   import Comp from './Comp.vue';
   import type { CompProps } from './Comp.vue';
   
   const props = defineProps<CompProps>();
   </script>
   

What is expected?

vue-tsc not to error :D

What is actually happening?

$ npm run type-check

> vue-tsc-exact-optional-prop-types@0.0.0 type-check
> vue-tsc --build

src/App.vue:3:6 - error TS2379: Argument of type '{ somethingOptional: string; }' is not assignable to parameter of type '{ readonly somethingOptional?: string; } & VNodeProps & AllowedComponentProps & ComponentCustomProps & Record<string, unknown>' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
  Type '{ somethingOptional: string; }' is not assignable to type '{ readonly somethingOptional?: string; }' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
    Types of property 'somethingOptional' are incompatible.
      Type 'string | undefined' is not assignable to type 'string'.
        Type 'undefined' is not assignable to type 'string'.

3     <Comp v-bind="props" />
       ~~~~


Found 1 error.

System Info

System:
    OS: Linux 6.18 Fedora Linux 42 (Workstation Edition)
    CPU: (16) x64 AMD Ryzen 7 7840U w/ Radeon  780M Graphics
    Memory: 44.07 GB / 60.62 GB
    Container: Yes
    Shell: 5.2.37 - /bin/bash
  Binaries:
    Node: 22.19.0 - /home/jonas/.nvm/versions/node/v22.19.0/bin/node
    Yarn: 1.22.22 - /usr/local/sbin/yarn
    npm: 11.7.0 - /home/jonas/.nvm/versions/node/v22.19.0/bin/npm
    pnpm: 10.12.1 - /home/jonas/.local/share/pnpm/pnpm
    bun: 1.1.42 - /home/jonas/.bun/bin/bun
    Deno: 2.5.4 - /home/jonas/.deno/bin/deno
  Browsers:
    Chrome: 144.0.7559.59
    Firefox: 147.0
    Firefox Developer Edition: 147.0

Any additional comments?

This might be a duplicate of https://github.com/vuejs/core/issues/6532 and I reported this issue initially as https://github.com/vuejs/language-tools/issues/5947#

Looks like the defineProps<T> returns the optional properties of T with | undefined instead of keeping it actually optional with the ?. So { foo?: string } becomes { foo: string | undefined }, which will make it incompatible if exactOptionalPropertyTypes is enabled, therefore it is not accepted in the v-bind.

Start with the linked minimal reproduction: enable exactOptionalPropertyTypes in tsconfig.app.json and reproduce the v-bind error in App.vue using the Comp.vue and defineProps examples. Trace the prop types produced for defineProps and consumed by v-bind, then compare core issue #6532 and language-tools issue #5947. Done means the reproduction type-checks without the reported error and has regression coverage.