Type B|null gets narrowed to type B when calling a method that returns mixed #9293
Description
Bug report
When calling method that returns mixed on type B|null, it unexpectedly gets narrowed to type B.
Code snippet that reproduces the problem
https://phpstan.org/r/a035f8ad-afc4-438a-a948-edd349afb0ce
Expected output
The type shouldn't get narrowed at all when calling a method that returns mixed.
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