Affirmative conclusion from a negative premise

Affirmative conclusion from a negative premise (illicit negative) is a formal fallacy that is committed when a categorical syllogism has a positive conclusion, but one or two negative premises.

No fish are dogs, and no dogs can fly, therefore all fish can fly.

The only thing that can be properly inferred from these premises is that some things that are not fish cannot fly, provided that dogs exist.

Or:

We don't read that trash. People who read that trash don't appreciate real literature. Therefore, we appreciate real literature.