Can not call the function approve() of the USDT contract

Support Contracts
2

Hi @Skyge,

USDT approve doesn’t comply with the ERC20 standard.

I modified Test.sol to use SafeERC20 and this worked in Remix on a testnet (I haven’t tried on mainnet):

Test.sol

pragma solidity ^0.5.0;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v2.4.0/contracts/token/ERC20/IERC20.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v2.4.0/contracts/token/ERC20/SafeERC20.sol";

contract Test {

    using SafeERC20 for IERC20;

    function tokenAllowAll(address asset, address allowee) public {
        IERC20 token = IERC20(asset);

        if (token.allowance(address(this), allowee) != uint256(-1)) {
            token.safeApprove(allowee, uint256(-1));
        }
    }
}

ERC20 Verifier

I used https://erc20-verifier.openzeppelin.com/ to verify USDT and got the following report showing approve was non-standard:

Contract TetherToken
== ERC20 functions definition ==
[x] transfer (address, uint256) -> (bool)
[x] approve (address, uint256) -> (bool)
[x] transferFrom (address, address, uint256) -> (bool)
[✓] allowance (address, address) -> (uint256)
[✓] balanceOf (address) -> (uint256)

== Custom modifiers ==
[✓] No custom modifiers in ERC20 functions

== ERC20 events ==
[✓] Transfer (address, address, uint256)
[✓] Approval (address, address, uint256)

== ERC20 getters ==
[✓] totalSupply () -> (uint256)
[x] decimals () -> (uint8)
[✓] symbol () -> (string)
[✓] name () -> (string)

== Allowance frontrunning mitigation ==
[x] increaseAllowance (address, uint256) -> (bool)
[x] decreaseAllowance (address, uint256) -> (bool)

== Balance check in approve function ==
[✓] approve function should not check for sender's balance

USDT

https://etherscan.io/address/0xdac17f958d2ee523a2206206994597c13d831ec7#code

    /**
    * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
    * @param _spender The address which will spend the funds.
    * @param _value The amount of tokens to be spent.
    */
    function approve(address _spender, uint _value) public onlyPayloadSize(2 * 32) {

        // To change the approve amount you first have to reduce the addresses`
        //  allowance to zero by calling `approve(_spender, 0)` if it is not
        //  already 0 to mitigate the race condition described here:
        //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
        require(!((_value != 0) && (allowed[msg.sender][_spender] != 0)));

        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
    }