Github user pnowojski commented on the issue:
https://github.com/apache/flink/pull/4705
Hmmm, I looked a little bit and it seems like there are tools doing this:
https://github.com/google/guava/issues/1640
https://github.com/WorksApplications/findbugs-plugin
https://github.com/WorksApplications/findbugs-plugin/commit/d77ca84a9dcc8a99a99a155423c5e7b707a1a5fc
It evens seems to be working based on
```
* <p>A detector to ensure that implementation (class in src/main/java)
doesn't call
* package-private method in other class which is annotated by {@code
@VisibleForTesting}.
```
If above doesn't work/fit us, we could probably easily implement our own
check for that using
[findbugs-maven-plugin](https://gleclaire.github.io/findbugs-maven-plugin/usage.html),
maybe like described
[here](https://writeoncereadmany.wordpress.com/2016/04/08/how-to-find-bugs-part-3-visiblefortesting/).
Nevertheless I think this feature should be first discussed on a dev
mailing list. I am not against it (as long as it will not significantly prolong
builds), but it would be better to discuss this with the community.
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