Support taxed tokens transfers in the ERC4626 contract

Scope ERC4626

Motivation Make the standard more inclusive and usable across the hugely-diversified DeFi ecosystem by supporting taxed tokens transfers.

Description Implement a token transfer library that takes into account tokens with transfer taxes or non-standard behaviours (example: a user calls deposit(...amount = 10) but the actual deposit is 9 TKN because of a 10% transfer tax.

Example

function transferTokens(
        IERC20 token,
        address from,
        address to,
        uint256 amount
    ) internal returns (uint256 originalBalance, uint256 received) {
        if (token.balanceOf(from) < amount) revert TransferHelper__Insufficient_Token_Balance(from, address(token));

        if (token.allowance(from, address(this)) < amount)
            revert TransferHelper__Insufficient_Token_Allowance(from, address(this), address(token));

        // computes transferred balance for tokens with tax on transfers
        originalBalance = token.balanceOf(to);
        token.safeTransferFrom(from, to, amount);

        received = token.balanceOf(to) - originalBalance;
    }

Now instead of SafeERC20.transferFrom(...) one could use the function above using the received value instead of the user-inputted amount.

Source: TransferHelper.sol

[frangio]

I’m okay with making _deposit and _withdraw internal. Wouldn’t add special behavior for taxed tokens.

[OxMarco]

ok perfect, if that is not going to largely impact the gas costs I think it could be useful to implement them for flexibility