This article is updated on October 19th, 2021 to reflect recent developments on this subject.
Composer is a great tool for Drupal and in general PHP developers, but in some cases, it might take some effort to accomplish what you want. I tried to install Drupal 9 (dev version) and Drush 10 today and here's what I had to do to make it work.
I'm using Docker4Drupal for my local environment, so the first step was to configure a new project according to the documentation. After that, I just cloned Drupal 9 using GIT and executed composer install. Starting with Drupal 8.8, we have the template projects provided directly by Drupal core so we don't have to use drupal-composer/drupal-project anymore.
Now the fun part. To install Drush 10 in theory you just have to execute:
composer require drush/drush:^10
But if you do that you will see the following error:
That looks scary!
Fortunately, fixing it is easy. This is happening because of symfony/var-dumper component. Drupal 9 has the following constraint:
while Drush 10 has this one:
"symfony/var-dumper": "^3.4 || ^4.0",
Running composer install after you clone Drupal core will install version v5.0.0 of symfony/var-dumper. That's not good because Drush is not supporting that version.
The fix is to find the common ground and add the following line to the composer.json file:
Version 4.x is supported by both Drupal 9 and Drush 10, so you will now be able to install the latest version of Drush. After adding this line first downgrade var-dumper and then install Drush:
composer update symfony/var-dumper composer require drush/drush:^10
Note: This will probably be fixed soon by Drush using the same versions of symfony/var-dumper as Drupal, but let this blog post be of educational value.
You can also read my blog posts on How to upgrade Drupal 8 to 9 and Upgrading to Drupal 9 and Composer 2.
October 19th, 2021 update
Drupal 9 is stable and installing Drush is no longer an issue. Drush 10 can be installed on the latest Drupal 9.3 development version, and also the latest Drush 11.x-dev version can be installed as well.
About the Author
Goran Nikolovski is a senior web developer who loves working with Drupal, React, and React Native. He is the founder of this website and he enjoys sharing his knowledge. Need help with anything? Get in touch.