Thank you for creating this excellent composer plugin!

Commit 7d3d824 introduced at least one PHP 7.1+ feature, a nullable type, in

public function __construct(
        IOInterface $io,
        Config $config,
        ProcessExecutor $executor,
        Filesystem $fs,
        FallbackInterface $fallback = null,
        ?VersionConverterInterface $versionConverter = null
    ) {
        $this->io = $io;
        ...
   }

Despite this breaking change the foxy version number did not bump to 2.0.0 and composer.json still requires only php >= 5.3.3:

{
    ...
    "require": {
        "php": ">=5.3.3",
        ...
    },
    ...

This is the first error encountered when running composer with PHP < 7.1. Maybe there are more.

Petr