Add extension that emits erc20 events for etherscan visibility

.changeset/stupid-dogs-bow.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-confidential-contracts': minor
+---
+
+`ConfidentialFungibleTokenERC20Events`: Extension of `ConfidentialFungibleToken` that emits ERC20 events on transfers.

contracts/mocks/ConfidentialFungibleTokenERC20EventsMock.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.24;
+
+import {SepoliaConfig} from "@fhevm/solidity/config/ZamaConfig.sol";
+import {FHE, euint64, externalEuint64} from "@fhevm/solidity/lib/FHE.sol";
+import {ConfidentialFungibleTokenERC20Events} from "./../token/extensions/ConfidentialFungibleTokenERC20Events.sol";
+
+// solhint-disable func-name-mixedcase
+abstract contract ConfidentialFungibleTokenERC20EventsMock is ConfidentialFungibleTokenERC20Events, SepoliaConfig {
+    function $_mint(
+        address to,
+        externalEuint64 encryptedAmount,
+        bytes calldata inputProof
+    ) public returns (euint64 transferred) {
+        return _mint(to, FHE.fromExternal(encryptedAmount, inputProof));
+    }
+}

contracts/token/README.adoc

@@ -7,13 +7,15 @@ This set of interfaces, contracts, and utilities are all related to the evolving
 
 - {ConfidentialFungibleToken}: Implementation of {IConfidentialFungibleToken}.
 - {ConfidentialFungibleTokenERC20Wrapper}: Extension of {ConfidentialFungibleToken} which wraps an `ERC20` into a confidential token. The wrapper allows for free conversion in both directions at a fixed rate.
+- {ConfidentialFungibleTokenERC20Events}: Extension of {ConfidentialFungibleToken} that emits ERC20 events on transfers.
 - {ConfidentialFungibleTokenUtils}: A library that provides the on-transfer callback check used by {ConfidentialFungibleToken}. 
 
 == Core
 {{ConfidentialFungibleToken}}
 
 == Extensions
 {{ConfidentialFungibleTokenERC20Wrapper}}
+{{ConfidentialFungibleTokenERC20Events}}
 
 == Utilities
 {{ConfidentialFungibleTokenUtils}}

contracts/token/extensions/ConfidentialFungibleTokenERC20Events.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.26;
+
+import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+import {ConfidentialFungibleToken, euint64} from "../ConfidentialFungibleToken.sol";
+
+/**
+ * @dev Extension of `ConfidentialFungibleToken` that emits ERC20 events on transfers. This
+ * can be useful for surfacing confidential transfers on applications that support ERC20 events such as Etherscan.
+ *
+ * NOTE: The ERC20 events emitted only have meaningful data for the `to` and `from` fields. The `amount` field
+ * is fixed to 1.
+ */
+abstract contract ConfidentialFungibleTokenERC20Events is ConfidentialFungibleToken {
+    function _update(address from, address to, euint64 amount) internal virtual override returns (euint64) {
+        emit IERC20.Transfer(from, to, 1);
+        return super._update(from, to, amount);
+    }
+}

test/token/extensions/ConfidentialFungibleTokenERC20Events.test.ts

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+import { expect } from 'chai';
+import { ethers, fhevm } from 'hardhat';
+
+const name = 'ConfidentialFungibleToken';
+const symbol = 'CFT';
+const uri = 'https://example.com/metadata';
+
+describe('ConfidentialFungibleTokenERC20Events', function () {
+  beforeEach(async function () {
+    const [holder] = await ethers.getSigners();
+
+    const token = await ethers.deployContract('$ConfidentialFungibleTokenERC20EventsMock', [name, symbol, uri]);
+    this.token = token;
+    this.holder = holder;
+  });
+
+  it('should emit ERC20 transfer event', async function () {
+    const encryptedInput = await fhevm
+      .createEncryptedInput(this.token.target, this.holder.address)
+      .add64(100)
+      .encrypt();
+
+    await expect(
+      this.token
+        .connect(this.holder)
+        ['$_mint(address,bytes32,bytes)'](this.holder, encryptedInput.handles[0], encryptedInput.inputProof),
+    )
+      .to.emit(this.token, 'Transfer')
+      .withArgs(ethers.ZeroAddress, this.holder.address, 1);
+  });
+});