Why I think default expressions for default parameters are not a good idea.

gistfile1.txt

<?php

enum LoggerType: string
{
    case DB = 'db';
    case FILE = 'file';
    case CLOUD = 'cloud';

    public function getLogger(): LoggerInterface
    {
        return match($this)
        {
            static::DB => new DatabaseLogger(),
            static::FILE => new FileLogger(),
            static::CLOUD => new CloudLogger()
        };
    }
}

interface LoggerInterface
{
    public function log(string $x): LoggerInterface;
}

// Assume DatabaseLogger, etc. exist and implement LoggerInterface 

class A
{
    protected ?LoggerInterface $log;

    public function withLogger(LoggerInterface|LoggerType $a = new DatabaseLogger): static
    {
        if($a instanceof LoggerInterface)
        {
            $this->log = $a;
            return $this;
        }

        $this->log = $a->getLogger();
        return $this;
    }
}

(new A)->withLogger((default)->log('B'));

The reason this is problematic is because class A's owner today could swap $a = new DatabaseLogger for $a = LoggerType::DB and downstream no callers code would break. With the RFC being proposed on default that would no longer be the case.