[comagoosie]

One nuance missing from the article is that since branded / tagged types extend from the base type, callers can still see and use string methods, which may not be what you want.

Equality can be problematic too. Imagine an Extension type, one could compare it with ".mp4" or "mp4", which one is correct?

Opaque types (that extend from `unknown` instead of T) work around these problems by forcing users through selector functions.