When adding a new testcase to src/test/resources/input, you can run the tests and find in
the test output the current result as a diff to src/test/resources/expected_output. Once
you've reproduced the problem you want to tackle and changed the genjavadoc code accordingly,
it is time to populate src/test/resources/expected_output.
genjavadoc is tested with many different versions of scala, and
src/test/resources/expected_output is expected to correspond to the oldest supported version
of scala (check .github/workflows/validate.yml or build.sbt for the supported scala version matrix).
When changes in Scala cause changes in the expected genjavadoc output, you add them as patches
in the respective patch in src/test/resources/expected_output. Remember to clean between
invocations of sbt with different versions.
Usually you can just copy-paste the patch from sbt ++2.12.3 clean test into,
src/test/resources/patches/2.12.3.patch, changing the +++ line to look exactly like the
--- line.