fixes #1404

contracts/mocks/SafeERC20Helper.sol

@@ -3,24 +3,38 @@ pragma solidity ^0.4.24;
 import "../token/ERC20/IERC20.sol";
 import "../token/ERC20/SafeERC20.sol";
 
-contract ERC20FailingMock {
-    uint256 private _allowance;
+contract ERC20FailingMock is IERC20 {
+  function totalSupply() public view returns (uint256) {
+    return 0;
+  }
 
-    function transfer(address, uint256) public returns (bool) {
-        return false;
-    }
+  function transfer(address, uint256) public returns (bool) {
+    return false;
+  }
 
-    function transferFrom(address, 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 approve(address, uint256) public returns (bool) {
+    return false;
+  }
 
-    function allowance(address, address) public view returns (uint256) {
-        return 0;
-    }
+  function increaseAllowance(address, uint256) public returns (bool){
+    return false;
+  }
+
+  function decreaseAllowance(address, uint256) public returns (bool){
+    return false;
+  }
+
+  function balanceOf(address) public view returns (uint256) {
+    return 0;
+  }
+
+  function allowance(address, address) public view returns (uint256) {
+    return 0;
+  }
 }
 
 contract ERC20SucceedingMock {
@@ -38,9 +52,17 @@ contract ERC20SucceedingMock {
         return true;
     }
 
-    function setAllowance(uint256 allowance_) public {
-        _allowance = allowance_;
-    }
+  function increaseAllowance(address, uint256) public returns (bool){
+    return true;
+  }
+
+  function decreaseAllowance(address, uint256) public returns (bool){
+    return true;
+  }
+
+  function balanceOf(address) public view returns (uint256) {
+    return 0;
+  }
 
     function allowance(address, address) public view returns (uint256) {
         return _allowance;
@@ -98,15 +120,31 @@ contract SafeERC20Helper {
         _succeeding.safeIncreaseAllowance(address(0), amount);
     }
 
-    function doSucceedingDecreaseAllowance(uint256 amount) public {
-        _succeeding.safeDecreaseAllowance(address(0), amount);
-    }
+  function doFailingIncreaseAllowance() public {
+    _failing.safeIncreaseAllowance(address(0), 0);
+  }
+
+  function doFailingDecreaseAllowance() public {
+    _failing.safeDecreaseAllowance(address(0), 0);
+  }
+
+  function doSucceedingTransfer() public {
+    _succeeding.safeTransfer(address(0), 0);
+  }
 
     function setAllowance(uint256 allowance_) public {
         ERC20SucceedingMock(_succeeding).setAllowance(allowance_);
     }
 
-    function allowance() public view returns (uint256) {
-        return _succeeding.allowance(address(0), address(0));
-    }
+  function doSucceedingApprove() public {
+    _succeeding.safeApprove(address(0), 0);
+  }
+
+  function doSucceedingIncreaseAllowance() public {
+    _succeeding.safeIncreaseAllowance(address(0), 0);
+  }
+
+  function doSucceedingDecreaseAllowance() public {
+    _succeeding.safeDecreaseAllowance(address(0), 0);
+  }
 }

contracts/token/ERC20/IERC20.sol

@@ -17,7 +17,17 @@ interface IERC20 {
 
     function transferFrom(address from, address to, uint256 value) external returns (bool);
 
-    event Transfer(address indexed from, address indexed to, uint256 value);
+  function increaseAllowance(address spender, uint256 addedValue) 
+    external returns (bool);
+
+  function decreaseAllowance(address spender, uint256 subtractedValue) 
+    external returns (bool);
+
+  event Transfer(
+    address indexed from,
+    address indexed to,
+    uint256 value
+  );
 
     event Approval(address indexed owner, address indexed spender, uint256 value);
 }

contracts/token/ERC20/SafeERC20.sol

@@ -16,25 +16,34 @@ library SafeERC20 {
         require(token.transfer(to, value));
     }
 
-    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
-        require(token.transferFrom(from, to, value));
-    }
+  function safeApprove(
+    IERC20 token,
+    address spender,
+    uint256 value
+  )
+    internal
+  {
+    require((value == 0) || (token.allowance(msg.sender, spender) == 0));
+    require(token.approve(spender, value));
+  }
 
-    function safeApprove(IERC20 token, address spender, uint256 value) internal {
-        // safeApprove should only be called when setting an initial allowance,
-        // or when resetting it to zero. To increase and decrease it, use
-        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
-        require((value == 0) || (token.allowance(msg.sender, spender) == 0));
-        require(token.approve(spender, value));
-    }
+  function safeIncreaseAllowance(
+    IERC20 token,
+    address spender,
+    uint256 addedValue
+  )
+    internal
+  {
+    require(token.increaseAllowance(spender, addedValue));
+  }
 
-    function safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
-        uint256 newAllowance = token.allowance(address(this), spender).add(value);
-        require(token.approve(spender, newAllowance));
-    }
-
-    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
-        uint256 newAllowance = token.allowance(address(this), spender).sub(value);
-        require(token.approve(spender, newAllowance));
-    }
+  function safeDecreaseAllowance(
+    IERC20 token,
+    address spender,
+    uint256 subtractedValue
+  )
+    internal
+  {
+    require(token.decreaseAllowance(spender, subtractedValue));
+  }
 }