Problem

Fatal error: Class UpdateHelper\ComposerPlugin_composer_tmp1 contains 2 abstract methods and must therefore be declared abstract or implement the remaining methods (Composer\Plugin\PluginInterface::deactivate, Composer\Plugin\PluginInterface::uninstall) in phar:///bin/composer/src/Composer/Plugin/PluginManager.php(221) : eval()'d code on line 11

Solution

rm -rf vendor
rm -f composer.lock
composer install

• https://stackoverflow.com/a/69248316/4810183