[Vue 3] Support nullable Function props

[AverageHelper]

• I have searched through existing issues
• I have read through docs
• I have read FAQ
• I have tried restarting VS Code or running Vetur: Restart VLS

Info

• Platform: macOS
• Vetur version: 0.33.1
• VS Code version: 1.55.2

Problem

Conversion of type 'FunctionConstructor' to type 'PropType<OnSelectCallback | null>' may be a mistake because neither type sufficiently overlaps with the other.

Reproducible Case

https://github.com/AverageHelper/prop-type-function-or-null

• Set "strict": true in the tsconfig that manages the vue file in question.
• Create a function callback signature type to use as a prop type
• Add a prop whose type is Function and default is null
• Augment the function type with a PropType that includes your custom callback signature and null
• Observe error

[AverageHelper]

This issue only occurs when the PropType annotation contains null. Vetur takes a normal function signature there just fine.

Since other prop types allow being augmented with null, and Vue has supported adding function signatures to props since #9733, I'm fairly certain this syntax should work.

Nullable function props are useful over events in the case where a component should behave differently based on the existence of a callback function. For example, a component may in some contexts be interactive, perhaps indicating in its UI that it is interactive, and calling a callback function passed by its parent component. In other contexts, the same component may only display data, but formatted in the same manner as the interactive version. I don't know a way to have a component check that its parent component handles an event, so function props have been a good way for us to make that check.

[yoyo930021]

It's problem in vue 3 TypeScript type information.
Please report to vue-next project

[AverageHelper]

Issue been reported at vue-next#3760.