Strict.mapMaybe[WithKey] too lazy in collision contents

[sjakobi]

unordered-containers/Data/HashMap/Internal/Strict.hs

Lines 544 to 559 in 7237826

	-- | /O(n)/ Transform this map by applying a function to every value
	-- and retaining only some of them.
	mapMaybeWithKey :: (k -> v1 -> Maybe v2) -> HashMap k v1 -> HashMap k v2
	mapMaybeWithKey f = filterMapAux onLeaf onColl
	where onLeaf (Leaf h (L k v)) | Just v' <- f k v = Just (leaf h k v')
	onLeaf _ = Nothing
	onColl (L k v) | Just v' <- f k v = Just (L k v')
	| otherwise = Nothing
	{-# INLINE mapMaybeWithKey #-}
	-- | /O(n)/ Transform this map by applying a function to every value
	-- and retaining only some of them.
	mapMaybe :: (v1 -> Maybe v2) -> HashMap k v1 -> HashMap k v2
	mapMaybe f = mapMaybeWithKey (const f)
	{-# INLINE mapMaybe #-}

I think there's a problem in line 551: v' isn't forced.