Summary

There's a change in behaviour between Composer 2.1 and 2.2, where dependencies which were previously resolved without problem, now no longer get resolved.

Basic premise is a repo which has a committed composer.lock file and uses on-the-fly updates in the CI runs via GH Actions.
These updates used to work fine, but are now broken and failing builds.

I've set up a reproduction sample here: https://github.com/jrfnl/composer-2.2-bug-poc

And the changed behaviour and full -vvv output can be seen in the GH Actions output: https://github.com/jrfnl/composer-2.2-bug-poc/actions/runs/1616471281

My composer.json:

{
  "name": "jrfnl/composer-2.2-bug-poc",
  "description": "Proof of concept",
  "license": "GPL-2.0-or-later",
  "require": {
    "php": ">=5.6"
  },
  "require-dev": {
    "yoast/wp-test-utils": "^1.0.0"
  },
  "config": {
    "platform": {
      "php": "5.6.40"
    }
  }
}

When I run this command on PHP 8.0:

composer require --dev phpunit/phpunit:"^7.5" --no-update --ignore-platform-req=php
composer update yoast/wp-test-utils --with-dependencies --ignore-platform-req=php

I get the following output:

./composer.json has been updated

Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpunit/phpunit[7.5.0, ..., 7.5.20] require sebastian/exporter ^3.1 -> found sebastian/exporter[3.1.0, ..., 3.1.4] but these were not loaded, likely because it conflicts with another require.
    - Root composer.json requires phpunit/phpunit ^7.5 -> satisfiable by phpunit/phpunit[7.5.0, ..., 7.5.20].

Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.

And I expected this to happen:

The update to succeed without problems.

Other notes:

Additional things I've tried:

• --with-dependencies vs --with-all-dependencies does not make a difference.
• Specifying the versions in the update command didn't help.
• Using --with ... in the update command doesn't help.
• Using --no-install instead of --no-update in the first command doesn't help.

Work-around which works

... but may not play nice with caching in GH Actions as much:

composer remove --dev yoast/wp-test-utils
composer require --dev yoast/wp-test-utils:"^1.0.0" phpunit/phpunit:"^7.5" --update-with-dependencies --ignore-platform-reqs