Affirmative conclusion from a negative premise
Incorrect application of a syllogism by deriving an affirmative (positive) conclusion from a negative premise.
For example ◈:
All squares are rectangles. No circles are squares. ∴ All circles are rectangles.
One fundamental rule for all syllogistic and similar inferences is that if one of the premises is a negative, the conclusion can only also be a negative statement. There are no valid syllogism forms that contradict this rule. Trying to construct one is a formal fallacy.
Furthermore, if both premises are negative, no conclusion can be inferred at all (☞ Fallacy of exclusive premises).