Version 7 introduced a bug with ContainerBuilder::writeProxiesToFile().

Previous versions of ProxyFactory had a bool $writeProxiesToFile property.
Since 7.0, these checks were updated from if ($this->writeProxiesToFile) ... to if ($this->proxyDirectory) ....
No issues here.

Prior to 7.0, ProxyFactory was created in ContainerBuilder by new ProxyFactory($this->writeProxiesToFile, $this->proxyDirectory).
Since 7.0, ProxyFactory is created in ContainerBuilder by new ProxyFactory($this->proxyDirectory).
No issues here.

Until you get to ContainerBuilder::writeProxiesToFile(bool $writeToFile, string $proxyDirectory = null):
The writeProxiesToFile parameter essentially has no effect if proxyDirectory has anything other than null.

In other words, writeProxiesToFile(false, 'tmp/proxies') has exactly the same effect as writeProxiesToFile(true, 'tmp/proxies') or even

$builder->writeProxiesToFile(true, 'tmp/proxies');
/* other code */
if ($dontWriteProxiesAnymore) {
    $builder->writeProxiesToFile(false);
}

or

$builder->writeProxiesToFile($resultOfSomeLogic, $someProxyDirectory);

Therefore, to fix this bug, I believe this line should be updated to

$this->proxyDirectory = $writeToFile ? $proxyDirectory : null;

Thoughts?