Why composer hacks don't help when testing against PHP 8

readme.md

• composer.json: our project
• ext-v*.json: composer.json for MongoDB ODM in version 1 and 2
• restrictive-v*.json: composer.json for package restrictive about its php constraint

Assume we're running on PHP 8 and have ext-something installed. This presents a set of uninstallable depdendencies;

1. Using --ignore-platform-reqs won't work, as that will cause the installation of vendor/extension 2.0 even though ext-other is missing
2. Using config.platform with PHP locked to 7.4 won't work, as that would install vendor/restrictive 1.0 which is not compatible with PHP 8 even though there's a newer release that is compatible

The only way to test this is by changing our own constraint to ^7.2 || ^8.0. However, if vendor/extension were to also advertise a PHP constraint of ^7.2, we would never end up with a properly testable system.

composer.json

{
  "require": {
    "php": "^7.2",
    "vendor/extension": "^1 || ^2",
    "vendor/restrictive": "^1 || ^2"
  }
}

ext-v1.json

{
  "require": {
    "ext-something": "*"
  }
}

odm-v2.json

{
  "require": {
    "ext-other": "*"
  }
}

restrictive-v1.json

{
  "require": {
    "php": "~7.4.2"
  }
}

restrictive-v2.json

{
  "require": {
    "php": "~8.0.0"
  }
}