Bug report

The following error is no longer picked up by the latest 0.12 Docker image. When running PHPStan against our codebase using the 0.12 image from 2 weeks ago the baseline matches the result. When using the 0.12 image pulled today the error is no longer in the result:

I believe this has started happening sometime yesterday.

Code snippet that reproduces the problem

https://phpstan.org/r/c8caef76-2a0b-4e64-9017-29b59f86afd6

Docker run command:

docker run \
	--rm \
	-v $(pwd):/app \
	phpstan/phpstan:0.12 \
	analyse \
	-c phpstan.neon \
	--level=0 \
	src/ \
    > $OUTPUT

The specific method that causes the error: Overriding class: public function toString(string $delimiter = PHP_EOL): string Overridden class: public function toString(string $delimiter = PHP_EOL): string

Expected output

Error not being detected:

Parameter #1 $parameter (string) of method                             
<          OverridingClass::toString() is not  
<          contravariant with parameter #1 $parameter (string|null) of method     
<          OverriddenClass::toString().