Composer does not update anymore when trying to update to 8.4

Question

when I do an update with "Composer", the following error is displayed :

www.s1biose.com@vps000000:~/public_html$ composer update

Cannot create cache directory /home/www.s1biose.com/.composer/cache/repo/https---packages.drupal.org-8/, or directory is not writable. Proceeding without cache
Cannot create cache directory /home/www.s1biose.com/.composer/cache/repo/https---packagist.org/, or directory is not writable. Proceeding without cache
Cannot create cache directory /home/www.s1biose.com/.composer/cache/files/, or directory is not writable. Proceeding without cache
Gathering patches for root package.
> DrupalProject\composer\ScriptHandler::checkComposerVersion
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: don't install drupal/core 8.4.0-rc2
    - Conclusion: don't install drupal/core 8.4.0-rc1
    - Conclusion: don't install drupal/core 8.4.0-beta1
    - Conclusion: don't install drupal/core 8.4.0-alpha1|remove symfony/event-dispatcher v2.8.27|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.27|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.26|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.25|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.24|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.23|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.22|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.21|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|remove symfony/yaml v2.8.27|install symfony/serializer v3.2.10|install symfony/serializer v3.2.11|install symfony/serializer v3.2.12|install symfony/serializer v3.2.13|install symfony/serializer v3.2.9|install symfony/serializer v3.3.8|install symfony/serializer v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.20|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.19|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.18|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.17|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.16|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.15|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.14|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.13|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.12|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.11|install symfony/event-dispatcher v3.2.0|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.10|install symfony/event-dispatcher v3.2.0|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.0-RC1|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.9|install symfony/event-dispatcher v3.2.0|install symfony/event-dispatcher v3.2.0-RC2|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.0-RC1|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.8|install symfony/event-dispatcher v3.2.0|install symfony/event-dispatcher v3.2.0-RC1|install symfony/event-dispatcher v3.2.0-RC2|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.0-RC1|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.7|install symfony/event-dispatcher v3.2.0|install symfony/event-dispatcher v3.2.0-RC1|install symfony/event-dispatcher v3.2.0-RC2|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.0-BETA1|install symfony/event-dispatcher v3.3.0-RC1|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1|don't install symfony/event-dispatcher v2.8.6|install symfony/event-dispatcher v3.2.0|install symfony/event-dispatcher v3.2.0-BETA1|install symfony/event-dispatcher v3.2.0-RC1|install symfony/event-dispatcher v3.2.0-RC2|install symfony/event-dispatcher v3.2.1|install symfony/event-dispatcher v3.2.10|install symfony/event-dispatcher v3.2.11|install symfony/event-dispatcher v3.2.12|install symfony/event-dispatcher v3.2.13|install symfony/event-dispatcher v3.2.2|install symfony/event-dispatcher v3.2.3|install symfony/event-dispatcher v3.2.4|install symfony/event-dispatcher v3.2.5|install symfony/event-dispatcher v3.2.6|install symfony/event-dispatcher v3.2.7|install symfony/event-dispatcher v3.2.8|install symfony/event-dispatcher v3.2.9|install symfony/event-dispatcher v3.3.0|install symfony/event-dispatcher v3.3.0-BETA1|install symfony/event-dispatcher v3.3.0-RC1|install symfony/event-dispatcher v3.3.1|install symfony/event-dispatcher v3.3.2|install symfony/event-dispatcher v3.3.3|install symfony/event-dispatcher v3.3.4|install symfony/event-dispatcher v3.3.5|install symfony/event-dispatcher v3.3.6|install symfony/event-dispatcher v3.3.7|install symfony/event-dispatcher v3.3.8|install symfony/event-dispatcher v3.3.9
    - Conclusion: don't install drupal/core 8.4.0-alpha1
    - Conclusion: don't install drupal/core 8.5.x-dev
    - Installation request for drush/drush ^8.1 -> satisfiable by drush/drush[8.1.0, 8.1.1, 8.1.10, 8.1.11, 8.1.12, 8.1.13, 8.1.14, 8.1.2, 8.1.3, 8.1.4, 8.1.5, 8.1.6, 8.1.7, 8.1.8, 8.1.9, 8.8.x-dev, 8.x-dev].
    - drupal/commerce 2.x-dev requires drupal/core ~8.4 -> satisfiable by drupal/core[8.4.0-alpha1, 8.4.0-beta1, 8.4.0-rc1, 8.4.0-rc2, 8.4.x-dev, 8.5.x-dev].
    - drupal/commerce 2.x-dev requires drupal/core ~8.4 -> satisfiable by drupal/core[8.4.0-alpha1, 8.4.0-beta1, 8.4.0-rc1, 8.4.0-rc2, 8.4.x-dev, 8.5.x-dev].
    - Conclusion: don't install drupal/core 8.4.x-dev
    - Installation request for drupal/commerce 2.x-dev -> satisfiable by drupal/commerce[2.x-dev].

What is going on ?

It worked before

Answer 1

I think the relevant error is Your requirements could not be resolved to an installable set of packages.

I had this too when I tried to update to 8.4.

If you're trying to run Drupal 8.4 you will need to use Drush 9, not Drush 8.

Change the drush line to:

"drush/drush": "~9",

And try again.

P.S. It's better to run composer update drupal/core and composer update drupal/mymodule one-by-one than to run composer update by itself. composer update alone will update symfony dependencies before they have been tested for compatibility with core, which can break things.

Answer 2

I updated to Drupal 8.4 this way and that resolved composer requirement conflicts vis a vis Drupal and Drush, but Drush 9 was unable to recognize the running Drupal site as a running Drupal site:

 [error]  The command could not be executed successfully (returned: 

  [Exception]        
  Bootstrap failed.  

Now that https://github.com/drush-ops/drush/pull/2800 is in, dropping back to "drush/drush": "^8.0" worked for composer dependencies and for Drush to actually find the site.

Answer 3

Requirement : PHP 7.1
Explanation :
1. Doctrire, a dependancy of Drupal core, requires PHP 7.1. Doctrine uses the latest PHP 7.1 syntax to declare the methods' return type eg. function my_function() : void {...}
2. webflo breaks the chain of dependencies
3. Drupal 8.4 requires Drush 9.0

If you have PHP 7.1 on your server you can do the following:

1. edit the composer.json file: "drupal/core": "~8.4" and "drush/drush": "~9.0"
2. delete composer.lock
3. delete vendor
4. update webflo: composer require "webflo/drupal-finder ~1.0.0"
5. update the database: ../vendor/bin/drush updb -y
6. update the entities schemas: ../vendor/bin/drush entup -y