TYPO3 Core

Given the following composer.json

{
    "require": {
        "typo3/cms": "^7.6.18" 
    },
    "extra": {
        "typo3/cms": {
            "cms-package-dir": "{$vendor-dir}/typo3/cms",
            "web-dir": "web" 
        }
    }
}

When I do composer require typo3/cms dev-master

I expect the web directory to have a typo3 folder with a symlink to sysext inside.

What actually happens:

I get the error message:

The symlink source "/test/vendor/typo3/cms/ty
po3/sysext" is not available.

This is due to the fact, that web/typo3/sysext will be deleted, which is a folder inside the TYPO3 sources,
as the 7.6 installation created a symlink from web/typo3 to the sources directory.