note approval to zero reverts

Author: d-xo

readme.md

@@ -155,6 +155,15 @@ in the wild problems caused by this issue.
 
 *example*: [Approval.sol](./src/Approval.sol)
 
+## Revert on Approval To Zero Address
+
+Some tokens (e.g. OpenZeppelin) will revert if trying to approve the zero address to spend tokens
+(i.e. a call to `approve(address(0), amt)`).
+
+Integrators may need to add special cases to handle this logic if working with such a token.
+
+*example*: [ApprovalToZero.sol](./src/ApprovalToZero.sol)
+
 ## Revert on Zero Value Transfers
 
 Some tokens (e.g. `LEND`) revert when transfering a zero value amount.

src/ApprovalToZero.sol

@@ -0,0 +1,17 @@
+// Copyright (C) 2020 d-xo
+// SPDX-License-Identifier: AGPL-3.0-only
+
+pragma solidity >=0.6.12;
+
+import {ERC20} from "./ERC20.sol";
+
+contract ApprovalToZeroToken is ERC20 {
+    // --- Init ---
+    constructor(uint _totalSupply) ERC20(_totalSupply) public {}
+
+    // --- Token ---
+    function approve(address usr, uint wad) override public returns (bool) {
+        require(usr != address(0), "no approval for the zero address");
+        return super.approve(usr, wad);
+    }
+}