Vue version

3.2.47

Link to minimal reproduction

New reproduction with runtime error: codesandbox.io

typescriptlang.org

Steps to reproduce

• Use shallowRef with a generic type params that's a union : shallowRef<{a:1} | {b:2}>({a:1})
• Or use shallowRef and specify argument's type with as: shallowRef({a:1} as {a:1} | {b:2})

What is expected?

The return type should be ShallowRef<{a:1} | {b:2}>.

What is actually happening?

The return type is ShallowRef<{a:1}> | ShallowRef<{b:2}>

System Info

System:
    OS: Linux 6.1 Ubuntu 22.10 22.10 (Kinetic Kudu)
    CPU: (16) x64 AMD Ryzen 9 5900HX with Radeon Graphics
    Memory: 10.10 GB / 30.75 GB
    Container: Yes
    Shell: 3.5.1 - /usr/bin/fish
  Binaries:
    Node: 14.20.0 - ~/.local/share/nvm/v14.20.0/bin/node
    Yarn: 1.22.19 - ~/.local/share/nvm/v14.20.0/bin/yarn
    npm: 8.17.0 - ~/.local/share/nvm/v14.20.0/bin/npm
  Browsers:
    Chrome: 110.0.5481.177
    Firefox: 110.0.1

Any additional comments?

This breaks Vue's watch function. The first argument of watch's callback becomes a Ref instead of the value, which will cause a runtime error because Property 'value' does not exist on type '{ a: 1; } | { b: 2; }'

The reproduction link provides a good example of this. If you have any questions just ask me.

I've fixed the issue on my fork and the CI actions completed successfully.

You need to change
function shallowRef<T extends object>(value: T): T extends Ref ? T : ShallowRef<T>;

to
function shallowRef<T extends object>(value: T): [T] extends [Ref] ? T : ShallowRef<T>;

This matches ref's type:
function ref<T extends object>(value: T): [T] extends [Ref] ? T : Ref<UnwrapRef<T>>;

I think this bug is present in other parts of the code but I'm not sure right now.