There's a Glasgow extension that gets you to this:
treeFold :: (a -> a -> a) -> Tree a -> a
treeFold f = \case
Leaf {} -> id
Node {} -> f `on` treeFold f
Or maybe this if parens are needed:
treeFold :: (a -> a -> a) -> Tree a -> a
treeFold f = \case
(Leaf {}) -> id
(Node {}) -> f `on` treeFold f
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