Hello from WordPress land with crazy edge case.

The user of one of plugins had reported following error:

Fatal error: Call to undefined method Composer\Autoload\ClassLoader::setPsr4() in C:\xampp\htdocs\mywebsite.com\wp-content\plugins\mailchimp-top-bar\vendor\composer\autoload_real.php on line 33

The working theory is that another plugin present in the system is also using Composer autoload, but at old pre-PSR-4 version.

The logical attempt was to try optimized autoload, but turns out that despite all the classes getting compiled into classmap file, the PSR-4 instructions are still retained and loaded (thus still causing crash).

cc @dannyvankooten @coenjacobs