Do not emit Approval event when calling transferFrom

.changeset/heavy-drinks-fail.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': major
+---
+
+`ERC20`: Remove `Approval` event previously emitted in `transferFrom` to indicate that part of the allowance was consumed. With this change, allowances are no longer reconstructible from events. See the code for guidelines on how to re-enable this event if needed.

contracts/mocks/token/ERC20ApprovalMock.sol

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.19;
+
+import "../../token/ERC20/ERC20.sol";
+
+abstract contract ERC20ApprovalMock is ERC20 {
+    function _approve(address owner, address spender, uint256 amount, bool) internal virtual override {
+        super._approve(owner, spender, amount, true);
+    }
+}

contracts/token/ERC20/ERC20.sol

@@ -311,14 +311,37 @@ abstract contract ERC20 is Context, IERC20, IERC20Metadata, IERC20Errors {
      * - `spender` cannot be the zero address.
      */
     function _approve(address owner, address spender, uint256 amount) internal virtual {
+        _approve(owner, spender, amount, true);
+    }
+
+    /**
+     * @dev Alternative version of {_approve} with an optional flag that can enable or disable the Approval event.
+     *
+     * By default (when calling {_approve}) the flag is set to true. On the other hand, approval changes made by
+     * `_spendAllowance` during the `transferFrom` operation set the flag to false. This saves gas by not emitting any
+     * `Approval` event during `transferFrom` operations.
+     *
+     * Anyone who wishes to continue emitting `Approval` events on the`transferFrom` operation can force the flag to true
+     * using the following override:
+     * ```
+     * function _approve(address owner, address spender, uint256 amount, bool) internal virtual override {
+     *     super._approve(owner, spender, amount, true);
+     * }
+     * ```
+     *
+     * Requirements are the same as {_approve}.
+     */
+    function _approve(address owner, address spender, uint256 amount, bool emitEvent) internal virtual {
         if (owner == address(0)) {
             revert ERC20InvalidApprover(address(0));
         }
         if (spender == address(0)) {
             revert ERC20InvalidSpender(address(0));
         }
         _allowances[owner][spender] = amount;
-        emit Approval(owner, spender, amount);
+        if (emitEvent) {
+            emit Approval(owner, spender, amount);
+        }
     }
 
     /**
@@ -336,7 +359,7 @@ abstract contract ERC20 is Context, IERC20, IERC20Metadata, IERC20Errors {
                 revert ERC20InsufficientAllowance(spender, currentAllowance, amount);
             }
             unchecked {
-                _approve(owner, spender, currentAllowance - amount);
+                _approve(owner, spender, currentAllowance - amount, false);
             }
         }
     }

test/token/ERC20/ERC20.behavior.js

@@ -4,7 +4,10 @@ const { ZERO_ADDRESS, MAX_UINT256 } = constants;
 
 const { expectRevertCustomError } = require('../../helpers/customError');
 
-function shouldBehaveLikeERC20(initialSupply, initialHolder, recipient, anotherAccount) {
+function shouldBehaveLikeERC20(initialSupply, accounts, opts = {}) {
+  const [initialHolder, recipient, anotherAccount] = accounts;
+  const { forcedApproval } = opts;
+
   describe('total supply', function () {
     it('returns the total amount of tokens', async function () {
       expect(await this.token.totalSupply()).to.be.bignumber.equal(initialSupply);
@@ -70,13 +73,22 @@ function shouldBehaveLikeERC20(initialSupply, initialHolder, recipient, anotherA
               });
             });
 
-            it('emits an approval event', async function () {
-              expectEvent(await this.token.transferFrom(tokenOwner, to, amount, { from: spender }), 'Approval', {
-                owner: tokenOwner,
-                spender: spender,
-                value: await this.token.allowance(tokenOwner, spender),
+            if (forcedApproval) {
+              it('emits an approval event', async function () {
+                expectEvent(await this.token.transferFrom(tokenOwner, to, amount, { from: spender }), 'Approval', {
+                  owner: tokenOwner,
+                  spender: spender,
+                  value: await this.token.allowance(tokenOwner, spender),
+                });
               });
-            });
+            } else {
+              it('does not emit an approval event', async function () {
+                expectEvent.notEmitted(
+                  await this.token.transferFrom(tokenOwner, to, amount, { from: spender }),
+                  'Approval',
+                );
+              });
+            }
           });
 
           describe('when the token owner does not have enough balance', function () {