Submitted by 0x52
AlgebraPool.sol#L626-L673
Exact output functionality does not exist for fee on transfer tokens.
Exact output swaps are signaled by setting amountRequired to a negative number. When calling AlgebraPool#swapSupportingFeeOnInputTokens amount required is set to the difference in token balances before and after which will always be positive. Since amountRequired cant be negative, the exact output functionality is impossible to use for those tokens.
Fee on transfer tokens are messy and there is no standard implementation to query the fee from the token contract. Ultimately support for these tokens should be added through either user inputs (i.e. allowing high slippage) or a specialized router.
vladyan18 (QuickSwap & StellaSwap) acknowledged
