Readonly breaks reactive class instance

[johannes-z]

Version

2.7.0-beta.5

Reproduction link

https://github.com/johannes-z/repro_vite-plugin-vue2-templates/tree/bug/vue-2.7-beta-readonly

See src/dev.ts

Steps to reproduce

git clone https://github.com/johannes-z/repro_vite-plugin-vue2-templates/
git checkout bug/vue-2.7-beta-readonly
pnpm install
pnpm run dev

What is expected?

this.data.complex.log() should log log to console.

What is actually happening?

this.data.complex.log is not a function is printed.

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Passing a complex object (e.g. instance of an ES6 class) to readonly(...) breaks the prototype. If you remove readonly and only return data, it works.