CalendarCellProps for custom component too strict

[KammererTob]

When trying to build a custom CalendarCell component the type used for props is seemingly too strict (in terms of required properties).

In runtime it shows a lot of warnings in the console like:
[Vue warn]: Missing required prop: "monthOfYear"
[Vue warn]: Missing required prop: "year"

The built-in CalendarCell component does not have the issue since all props are made optional there.

To Reproduce
Steps to reproduce the behavior:

1. Go here
2. Reload and check Console

Expected behavior
To not throw any warnings about missing properties.

EDIT: Seems to be a known issue in runtime for Vue (not being able to determine the required props for discriminated unions): vuejs/core#12059

[logaretm]

Sometimes I wish we can turn off Vue's runtime prop conversion.