From 6ccd48a59d516f37e61770a4337259c6eb9a5abf Mon Sep 17 00:00:00 2001
From: heew <yanwen.he@geely.com>
Date: Thu, 27 Jun 2024 16:53:45 +0800
Subject: [PATCH] how to fix the USDT swaping problem

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+---
+layout: post
+title: "USDT Does Not Comply With ERC20 Standard"
+date: 2024-06-27 15:12:29 +0800
+categories: jekyll update
+---
+
+### Problem Introduction
+
+I am running into this erros when i swap USDT to DAI by uniswap. The tool i am using is `foundry`.
+
+```solidity
+    ├─ [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`
+
+```javascript
+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.
+
+```javascript
+   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](https://etherscan.io/address/0xdAC17F958D2ee523a2206206994597C13D831ec7#code)
+
+```javascript
+    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.
+
+```javascript
+interface IERC20 {
+    function approve(address spender, uint256 value) external;
+}
+```
+
+I already try it, it really works. The code is in [USDTSwapWithoutStandardERC20.t.sol](https://github.com/heeween/foundryProject/blob/main/uniswap/test/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.
+
+2. Use safeERC20 library
+
+```javascript
+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](https://github.com/heeween/foundryProject/blob/main/uniswap/test/USDTSwapWithSafeERC20.t.sol)