Problem understanding a lowered mutation score

[vmassol]

Hi, on XWiki our build just failed because of a lowered mutation score.

Indeed we added a new default method to an interface (without adding any test):

    default List<String> getGroup()
    {
        return Collections.emptyList();
    }

When executing descartes we see:

And in issues.html we don't see it:

Any idea why this is not visible in the pseudo tested/partially tested list? This seems like a pseudo tested one (ie. whatever the return value the tests don't catch it), no?

Thanks

[oscarlvp]

The default method is not covered and therefore not partially-tested or pseudo-tested.
By definition, a method is classified as pseudo-tested or partially-tested if it is covered by at least one test case and no transformation was detected (pseudo) or some transformations were detected (partially).
When a new method is included, then, new mutants are created. If the new method is not covered, then the new mutants are also not covered and they count as not detected in the PIT score, thus the score lowers.
A better score should not count uncovered mutants. See STAMP-project/pitmp-maven-plugin#42