src/Composer/Installer.php
private function getCurrentPackages($installedRepo)
    {
        if ($this->locker->isLocked()) {
            try {
                return $this->locker->getLockedRepository(true)->getPackages();
            } catch (\RuntimeException $e) {
                // fetch only non-dev packages from lock if doing a dev update fails due to a previously incomplete lock file
                return $this->locker->getLockedRepository()->getPackages();
            }
        }

        return $installedRepo->getPackages();
    }
private function getCurrentPackages($installedRepo)
    {
        if ($this->locker->isLocked()) {
            try {
                return $this->locker->getLockedRepository(true)->getPackages();
            } catch (\RuntimeException $e) {
                // fetch only non-dev packages from lock if doing a dev update fails due to a previously incomplete lock file
                return $this->locker->getLockedRepository()->getPackages();
            }
        }

        return $installedRepo->getPackages();
    }
Loads the most "current" list of packages that are installed meaning from lock ideally or from installed repo as fallback
@param  RepositoryInterface $installedRepo
@return array