Several days ago the HHVM job of the Wikibase Travis CI build started to fail, see e.g. https://travis-ci.org/wikimedia/mediawiki-extensions-Wikibase/jobs/498420867
The error reported (see below) stems from the recently merged change in MediaWiki https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/485341/
Catchable fatal error: Argument 1 passed to Mediawiki\Logger\LogCapturingSpi::__construct() must implement interface Mediawiki\Logger\Spi, MediaWiki\Logger\LegacySpi given in /home/travis/build/wikimedia/phase3/tests/phpunit/MediaWikiLoggerPHPUnitTestListener.php on line 28
The error seems interesting, as MediaWiki\Logger\LegacySpi actually implements Mediawiki\Logger\Spi.
I started to suspect there is some weird bug in HHVM, which causes the error. Interestingly, Travis seem to run HHVM 3.24, whereas WMF Jenkins CI, same as WMF production, run HHVM 3.18. This could support, or not, the bug in HHVM explanation hypothesis.