feat: Add increase/decrease allowance functionality

.gitmodules

@@ -1,3 +1,3 @@
-[submodule "modules/ds-test"]
-	path = modules/ds-test
-	url = https://github.com/dapphub/ds-test
+[submodule "modules/contract-test-utils"]
+	path = modules/contract-test-utils
+	url = https://github.com/maple-labs/contract-test-utils

contracts/ERC20.sol

@@ -40,6 +40,16 @@ contract ERC20 is IERC20 {
         return true;
     }
 
+    function decreaseAllowance(address spender_, uint256 subtractedAmount_) external override returns (bool success_) {
+        _approve(msg.sender, spender_, allowance[msg.sender][spender_] - subtractedAmount_);
+        return true;
+    }
+
+    function increaseAllowance(address spender_, uint256 addedAmount_) external override returns (bool success_) {
+        _approve(msg.sender, spender_, allowance[msg.sender][spender_] + addedAmount_);
+        return true;
+    }
+
     function transfer(address recipient_, uint256 amount_) external override returns (bool success_) {
         _transfer(msg.sender, recipient_, amount_);
         return true;

contracts/ERC20Permit.sol

@@ -69,6 +69,16 @@ contract ERC20Permit is IERC20Permit {
         return true;
     }
 
+    function decreaseAllowance(address spender_, uint256 subtractedAmount_) external override returns (bool success_) {
+        _approve(msg.sender, spender_, allowance[msg.sender][spender_] - subtractedAmount_);
+        return true;
+    }
+
+    function increaseAllowance(address spender_, uint256 addedAmount_) external override returns (bool success_) {
+        _approve(msg.sender, spender_, allowance[msg.sender][spender_] + addedAmount_);
+        return true;
+    }
+
     function permit(address owner, address spender, uint256 amount, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external override {
         require(deadline >= block.timestamp, "ERC20Permit:EXPIRED");
         bytes32 digest = keccak256(

contracts/interfaces/IERC20.sol

@@ -62,6 +62,24 @@ interface IERC20 {
      */
     function approve(address spender_, uint256 amount_) external returns (bool success_);
 
+    /**
+     * @dev    Function that allows one account to decrease the allowance of another account over their tokens.
+     *         Emits an {Approval} event.
+     * @param  spender_          Account that tokens are approved for.
+     * @param  subtractedAmount_ Amount to decrease approval by.
+     * @return success_          Boolean indicating whether the operation succeeded.
+     */
+    function decreaseAllowance(address spender_, uint256 subtractedAmount_) external returns (bool success_);
+
+    /**
+     * @dev    Function that allows one account to increase the allowance of another account over their tokens.
+     *         Emits an {Approval} event.
+     * @param  spender_     Account that tokens are approved for.
+     * @param  addedAmount_ Amount to increase approval by.
+     * @return success_     Boolean indicating whether the operation succeeded.
+     */
+    function increaseAllowance(address spender_, uint256 addedAmount_) external returns (bool success_);
+
     /**
      * @dev    Moves an amount of tokens from `msg.sender` to a specified account.
      *         Emits a {Transfer} event.

contracts/test/ERC20.t.sol

@@ -1,17 +1,12 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 pragma solidity ^0.8.7;
 
-import { DSTest } from "../../modules/ds-test/src/test.sol";
+import { InvariantTest, TestUtils } from "../../modules/contract-test-utils/contracts/test.sol";
 
-import { ERC20User }     from "./accounts/ERC20User.sol";
-import { MockERC20 }     from "./mocks/MockERC20.sol";
+import { ERC20User } from "./accounts/ERC20User.sol";
+import { MockERC20 } from "./mocks/MockERC20.sol";
 
-import { InvariantTest } from "./utils/InvariantTest.sol";
-import { Vm }            from "./utils/Vm.sol";
-
-contract ERC20Test is DSTest {
-
-    Vm vm = Vm(0x7109709ECfa91a80626fF3989D68f67F5b1DD12D);
+contract ERC20Test is TestUtils {
 
     bytes constant ARITHMETIC_ERROR = abi.encodeWithSignature("Panic(uint256)", 0x11);
 
@@ -60,6 +55,32 @@ contract ERC20Test is DSTest {
         assertEq(token.allowance(self, account), amount);
     }
 
+    function test_increaseAllowance(address account, uint256 initialAmount, uint256 addedAmount) public {
+        initialAmount = constrictToRange(initialAmount, 0, type(uint256).max / 2);
+        addedAmount   = constrictToRange(addedAmount,   0, type(uint256).max / 2);
+
+        token.approve(account, initialAmount);
+
+        assertEq(token.allowance(self, account), initialAmount);
+
+        assertTrue(token.increaseAllowance(account, addedAmount));
+
+        assertEq(token.allowance(self, account), initialAmount + addedAmount);
+    }
+
+    function test_decreaseAllowance(address account, uint256 initialAmount, uint256 subtractedAmount) public {
+        initialAmount    = constrictToRange(initialAmount,    0, type(uint256).max);
+        subtractedAmount = constrictToRange(subtractedAmount, 0, initialAmount);
+
+        token.approve(account, initialAmount);
+
+        assertEq(token.allowance(self, account), initialAmount);
+
+        assertTrue(token.decreaseAllowance(account, subtractedAmount));
+
+        assertEq(token.allowance(self, account), initialAmount - subtractedAmount);
+    }
+
     function test_transfer(address account, uint256 amount) public {
         token.mint(self, amount);
 
@@ -152,7 +173,7 @@ contract ERC20Test is DSTest {
 
 }
 
-contract ERC20Invariants is DSTest, InvariantTest {
+contract ERC20Invariants is TestUtils, InvariantTest {
 
     BalanceSum balanceSum;
 

contracts/test/ERC20Permit.t.sol

@@ -1,17 +1,12 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 pragma solidity ^0.8.7;
 
-import { DSTest } from "../../modules/ds-test/src/test.sol";
+import { InvariantTest, TestUtils } from "../../modules/contract-test-utils/contracts/test.sol";
 
 import { ERC20Permit } from "../ERC20Permit.sol";
 
-import { ERC20PermitUser } from "./accounts/ERC20User.sol";
-
-import { MockERC20Permit } from "./mocks/MockERC20.sol";
-
-import { Vm }            from "./utils/Vm.sol";
-import { InvariantTest } from "./utils/InvariantTest.sol";
-
+import { ERC20PermitUser }      from "./accounts/ERC20User.sol";
+import { MockERC20Permit }      from "./mocks/MockERC20.sol";
 import { ERC20Test, MockERC20 } from "./ERC20.t.sol";
 
 contract ERC20PermitBaseTest is ERC20Test {
@@ -22,9 +17,7 @@ contract ERC20PermitBaseTest is ERC20Test {
 
 }
 
-contract ERC20PermitTest is DSTest {
-
-    Vm vm = Vm(0x7109709ECfa91a80626fF3989D68f67F5b1DD12D);
+contract ERC20PermitTest is TestUtils {
 
     bytes constant ARITHMETIC_ERROR = abi.encodeWithSignature("Panic(uint256)", 0x11);
 
@@ -155,7 +148,7 @@ contract ERC20PermitTest is DSTest {
 
 }
 
-contract ERC20Invariants is DSTest, InvariantTest {
+contract ERC20Invariants is TestUtils, InvariantTest {
 
     BalanceSum balanceSum;