src/Composer/DependencyResolver/DefaultPolicy.php
protected function pruneToHighestPriorityOrInstalled(Pool $pool, array $installedMap, array $literals)
    {
        $selected = array();

        $priority = null;

        foreach ($literals as $literal) {
            $package = $pool->literalToPackage($literal);

            if (isset($installedMap[$package->id])) {
                $selected[] = $literal;
                continue;
            }

            if (null === $priority) {
                $priority = $this->getPriority($pool, $package);
            }

            if ($this->getPriority($pool, $package) != $priority) {
                break;
            }

            $selected[] = $literal;
        }

        return $selected;
    }
protected function pruneToHighestPriorityOrInstalled(Pool $pool, array $installedMap, array $literals)
    {
        $selected = array();

        $priority = null;

        foreach ($literals as $literal) {
            $package = $pool->literalToPackage($literal);

            if (isset($installedMap[$package->id])) {
                $selected[] = $literal;
                continue;
            }

            if (null === $priority) {
                $priority = $this->getPriority($pool, $package);
            }

            if ($this->getPriority($pool, $package) != $priority) {
                break;
            }

            $selected[] = $literal;
        }

        return $selected;
    }
Assumes that installed packages come first and then all highest priority packages