type union on props casts empty string to true

[geoffgscott]

Vue version

3.5.13

Link to minimal reproduction

https://play.vuejs.org/#eNqVUk2P0zAQ/SuDL1mk3XQRnEJbCVAl4AArQOLiS5pME+86tmU73aKQ/87YSdqEj5U42TPvefxm5nXsjTHpsUWWsbUrrDAeHPrWbLkSjdHWwzvdGDhY3UCSrkIQ6MkZ7sDiAfqRMUBcFVo5D42rYBPwq+Q9Sqnhu7ayfJY8v1DQWm0nUgDWq0EGCaDAY2Nk7pEigHWUksUnG87iyRlkFo9oHZa7JbDaAoR6sxrsmnlHHx9Eld47rajrLlTmrKDSQqL9bLwgYZxlEJGA5ST98WPMedvi9ZQvaiwe/pK/d6eQ4+zOokN7RM7OmM9thX6Ad18/4YnuZ7DRZSuJ/QT4BZ2WbdA40N62qiTZM15U+yEuR6jqm9udPCo3NRWEBmYf+ZzRwsJU/9X6Re7L9FV8x1VPU5x88LttQOaqohV4KnOxUAclHoTCO6uNW3jl9cUJJoKbOXUddcR1ZrDXWmKu4Cc4b6mzAC02n40AMUYuad1eRbM95apSHKGQuXOke587mj9kU9xBUtTUEpZJNnq1J3z1ny8XOs8VFtak0PkfcrimodqwhRpFVfsMXtzemhONC+BRlL6eJxpaklAZ5K3Xs8SN1ybQBtY+Lx4qq8kuN4WWYVqVRVQE0Urpx1Hs8OmfZIvlQKVJRpms/wUafHFB

Steps to reproduce

When doing a boolean | string prop an empty string (which is nullish) is cast to a true value. If the union is reversed this does not happen.

What is expected?

Empty strings should not be cast at all to a type: boolean.

What is actually happening?

Empty strings are cast to a type: true when the boolean is the first item in the union

System Info

Any additional comments?

No response

[jacekkarczmarczyk]

https://vuejs.org/guide/components/props.html#boolean-casting

[jh-leong]

This behavior is expected. Closing as explained above.