Particularly the test
tests/Composer/Test/Fixtures/installer/partial-update-downgrades-non-whitelisted-unstable.test
is interesting because it verifies that an older version will be
installed on update if the new one is only present in the installed repo
or vendor dir. This was the cause of a lot of weird edge cases and
unreliable update behavior in Composer v1
Jordi Boggiano