Re-enable immutable forwarder in ERC2771Context (#2917)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

CHANGELOG.md

@@ -10,6 +10,7 @@
  * `ERC721`: improved revert reason when transferring from wrong owner. ([#2975](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2975))
  * `Votes`: Added a base contract for vote tracking with delegation. ([#2944](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2944))
  * `ERC721Votes`: Added an extension of ERC721 enabled with vote tracking and delegation. ([#2944](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2944))
+ * `ERC2771Context`: use immutable storage to store the forwarder address, no longer an issue since Solidity >=0.8.8 allows reading immutable variables in the constructor. ([#2917](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2917))
 
 ## 4.4.1 (2021-12-14)
 

contracts/metatx/ERC2771Context.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: MIT
 // OpenZeppelin Contracts v4.4.1 (metatx/ERC2771Context.sol)
 
-pragma solidity ^0.8.0;
+pragma solidity ^0.8.9;
 
 import "../utils/Context.sol";
 
@@ -9,8 +9,10 @@ import "../utils/Context.sol";
  * @dev Context variant with ERC2771 support.
  */
 abstract contract ERC2771Context is Context {
-    address private _trustedForwarder;
+    /// @custom:oz-upgrades-unsafe-allow state-variable-immutable
+    address private immutable _trustedForwarder;
 
+    /// @custom:oz-upgrades-unsafe-allow constructor
     constructor(address trustedForwarder) {
         _trustedForwarder = trustedForwarder;
     }

contracts/mocks/ERC2771ContextMock.sol

@@ -1,13 +1,15 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity ^0.8.9;
 
 import "./ContextMock.sol";
 import "../metatx/ERC2771Context.sol";
 
 // By inheriting from ERC2771Context, Context's internal functions are overridden automatically
 contract ERC2771ContextMock is ContextMock, ERC2771Context {
-    constructor(address trustedForwarder) ERC2771Context(trustedForwarder) {}
+    constructor(address trustedForwarder) ERC2771Context(trustedForwarder) {
+        emit Sender(_msgSender()); // _msgSender() should be accessible during construction
+    }
 
     function _msgSender() internal view virtual override(Context, ERC2771Context) returns (address) {
         return ERC2771Context._msgSender();

hardhat.config.js

@@ -3,7 +3,7 @@
 // - COVERAGE:          enable coverage report
 // - ENABLE_GAS_REPORT: enable gas report
 // - COMPILE_MODE:      production modes enables optimizations (default: development)
-// - COMPILE_VERSION:   compiler version (default: 0.8.3)
+// - COMPILE_VERSION:   compiler version (default: 0.8.9)
 // - COINMARKETCAP:     coinmarkercat api key for USD value in gas report
 
 const fs = require('fs');
@@ -33,7 +33,7 @@ const argv = require('yargs/yargs')()
     compiler: {
       alias: 'compileVersion',
       type: 'string',
-      default: '0.8.3',
+      default: '0.8.9',
     },
     coinmarketcap: {
       alias: 'coinmarketcapApiKey',