openzeppelin-contracts/contracts/mocks/SafeERC20Helper.sol

pragma solidity ^0.4.24;

import "../token/ERC20/IERC20.sol";
import "../token/ERC20/SafeERC20.sol";

contract ERC20FailingMock {
  uint256 private _allowance;

  function transfer(address, uint256) public returns (bool) {
    return false;
  }

  function transferFrom(address, address, uint256) public returns (bool) {
    return false;
  }

  function approve(address, uint256) public returns (bool) {
    return false;
  }

  function allowance(address, address) public view returns (uint256) {
    return 0;
  }
}

contract ERC20SucceedingMock {
  uint256 private _allowance;

  function transfer(address, uint256) public returns (bool) {
    return true;
  }

  function transferFrom(address, address, uint256) public returns (bool) {
    return true;
  }

  function approve(address, uint256) public returns (bool) {
    return true;
  }

  function setAllowance(uint256 allowance_) public {
    _allowance = allowance_;
  }

  function allowance(address, address) public view returns (uint256) {
    return _allowance;
  }
}

contract SafeERC20Helper {
  using SafeERC20 for IERC20;

  IERC20 private _failing;
  IERC20 private _succeeding;

  constructor() public {
    _failing = IERC20(new ERC20FailingMock());
    _succeeding = IERC20(new ERC20SucceedingMock());
  }

  // Using _failing

  function doFailingTransfer() public {
    _failing.safeTransfer(address(0), 0);
  }

  function doFailingTransferFrom() public {
    _failing.safeTransferFrom(address(0), address(0), 0);
  }

  function doFailingApprove() public {
    _failing.safeApprove(address(0), 0);
  }

  function doFailingIncreaseAllowance() public {
    _failing.safeIncreaseAllowance(address(0), 0);
  }

  function doFailingDecreaseAllowance() public {
    _failing.safeDecreaseAllowance(address(0), 0);
  }

  // Using _succeeding

  function doSucceedingTransfer() public {
    _succeeding.safeTransfer(address(0), 0);
  }

  function doSucceedingTransferFrom() public {
    _succeeding.safeTransferFrom(address(0), address(0), 0);
  }

  function doSucceedingApprove(uint256 amount) public {
    _succeeding.safeApprove(address(0), amount);
  }

  function doSucceedingIncreaseAllowance(uint256 amount) public {
    _succeeding.safeIncreaseAllowance(address(0), amount);
  }

  function doSucceedingDecreaseAllowance(uint256 amount) public {
    _succeeding.safeDecreaseAllowance(address(0), amount);
  }

  function setAllowance(uint256 allowance_) public {
    ERC20SucceedingMock(_succeeding).setAllowance(allowance_);
  }

  function allowance() public view returns (uint256) {
    return _succeeding.allowance(address(0), address(0));
  }
}