Problem Introduction

I am running into this erros when i swap USDT to DAI by uniswap. The tool i am using is foundry.

    ├─ [24953] 0xdAC17F958D2ee523a2206206994597C13D831ec7::approve(SimpleSwap: [0x5615dEB798BB3E4dFa0139dFa1b3D433Cc23b72f], 1000)
    │   ├─ emit Approval(owner: 0x3f5CE5FBFe3E9af3971dD833D26bA9b5C936f0bE, spender: SimpleSwap: [0x5615dEB798BB3E4dFa0139dFa1b3D433Cc23b72f], value: 1000)
    │   └─ ← [Stop]
    └─ ← [Revert] EvmError: Revert

Here is my test code in SimpleSwap.t.sol

IERC20 tokenIn = IERC20(USDT);
uint256 balOfWhale = tokenIn.balanceOf(USDT_WHALE);
tokenIn.approve(address(simpleSwap), 1000);
uint256 amountOut = simpleSwap.swap(USDT, DAI, 1000);

After google the result, i know that the USDT does not complement the approve function same as ERC20 standard smart contract. The approve function in USDT does not have the return value. But the standard interface in ERC20 have a bool return value.

   function approve(address _spender, uint _value) public onlyPayloadSize(2 * 32) {
        require(!((_value != 0) && (allowed[msg.sender][_spender] != 0)));
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
   }

Code above from ethescan USDT smart contract

    function approve(address spender, uint256 value) public virtual returns (bool) {
        address owner = _msgSender();
        _approve(owner, spender, value);
        return true;
    }

The solution

So the return value indeed is different between USDT and ERC20 standard. But why the evm fails? That is because evm will check the return data size when invoking a function of smart contract. So there are two ways to solove this problem.

  1. Don’t use standard ERC20 approve interface Instead of import IERC20 from OpenZepplin, we write our own IERC20.
interface IERC20 {
    function approve(address spender, uint256 value) external;
}

I already try it, it really works. The code is in USDTSwapWithoutStandardERC20.t.sol

But if we write our own IERC20. Then other tokens that apply ERC20 standard will fails. So the second way should be the best practice to solve this problem.

  1. Use safeERC20 library
import "../lib/openzeppelin-contracts/contracts/token/ERC20/utils/SafeERC20.sol";
contract SimpleSwapTest is Test {
    using SafeERC20 for IERC20;

    function testSwapFromUSDTToDAI() public {
      ...
      IERC20 tokenIn = IERC20(USDT);
      tokenIn.forceApprove(address(simpleSwap), 1000);
      ...
    }
}

As the documentation of forceApprove in SafeERC20. The extension of ERC20 use encodeCall to bypass the return value checking in Ethereum. The full code is in here USDTSwapWithSafeERC20.t.sol