Bug in Swift regarding non public resultBuilders in public init?

I just hit a strange compilation error I have not seen before using a public init, which in turn used a non-public ResultBuilder - this was an error on my part. However, Swift actually compiled this. I believe this to be a bug, since if I declared any type, e.g. a struct as internal and try to that type in a public init, Swift will produce a compilation error. But it did not for a non public ResultBuilder.

In another package I tried to use this public init which uses the non public ResultBuilder, and the compiler produced this compilation error:

And googling the error swift "with sufficient availability for this call site" gives no result other than Swift repo, so seems like not many people have seen this error.

I had the same problem today. As expected the LogBuilder needs to be public if accessed outside of its own domain. This make sense but probably the compiler needs an improvement so that it can detect which is the real issue here.