prop with `type: [String, Number]` is inferred as `string | number | Date`

[07akioni]

Version

3.2.9

Reproduction link

codesandbox.io

this is a regression, 3.2.8 has no error

Steps to reproduce

see link

What is expected?

string | number

What is actually happening?

string | number | Date

[edison1105]

cause by https://github.com/vuejs/vue-next/pull/4518/files#diff-dbacb533411ec164eec5335d9310a44a911751dafc3f72c6879a6cdaf795be93R115