Error : Unsupported
declare strict_types in
../vendor/magento/inventory-composer-installer while install the extension using composer in Magento 2.3.0.
I had the same issue, it is due to the version of PHP you are using, and the version of PHP required for Composer.
My CLI was using PHP 5.4, but I had to run it on 7.2.
So instead of running something like:
php7.2 /path/to/composer upgrade
Hope this helps. I was confused on this for a little while, as the error doesn't seem related to the PHP version, but seems like more of a bug with the installation.