Version

2.7.14

Reproduction link

codesandbox.io

(this is a typescript bug, it's not visible on the sfc playground)

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}>

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>>;

This issue is the same issue as vuejs/core#7852