Simple ImmutableBeaconProxy implementation

src/contracts/immutable-beacon-proxy/ImmutableBeaconProxy.sol

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import {IBeacon} from './interfaces/IBeacon.sol';
+import {Proxy} from '../oz-common/Proxy.sol';
+import {Address} from '../oz-common/Address.sol';
+
+/**
+ * @dev This contract implements a proxy that gets the implementation address for each call from an {UpgradeableBeacon}.
+ * The beacon address is immutable. The purpose of it, is to be able to access this proxy via delegatecall
+
+ * !!! IMPORTANT CONSIDERATION !!!
+ * We expect that the implementation will not have any storage associated,
+ * because it when accessed via delegatecall, will not work as expected creating dangerous side-effects. Preferable, the implementation should be declared as a library
+ */
+contract ImmutableBeaconProxy is Proxy {
+  address internal immutable _beacon;
+
+  event ImmutableBeaconSet(address indexed beacon);
+
+  constructor(address beacon) {
+    require(Address.isContract(beacon), 'INVALID_BEACON');
+    require(Address.isContract(IBeacon(beacon).implementation()), 'INVALID_IMPLEMENTATION');
+
+    // there is no initialization call, because we expect that implementation should have no storage
+    _beacon = beacon;
+    emit ImmutableBeaconSet(beacon);
+  }
+
+  /**
+   * @dev Returns the current implementation address of the associated beacon.
+   */
+  function _implementation() internal view virtual override returns (address) {
+    return IBeacon(_beacon).implementation();
+  }
+}

src/contracts/immutable-beacon-proxy/UpgradeableBeacon.sol

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (proxy/beacon/UpgradeableBeacon.sol)
+
+pragma solidity ^0.8.0;
+
+import './interfaces/IBeacon.sol';
+import {Ownable} from '../oz-common/Ownable.sol';
+import {Address} from '../oz-common/Address.sol';
+
+/**
+ * @dev This contract is used in conjunction with one or more instances of {BeaconProxy} to determine their
+ * implementation contract, which is where they will delegate all function calls.
+ *
+ * An owner is able to change the implementation the beacon points to, thus upgrading the proxies that use this beacon.
+ */
+contract UpgradeableBeacon is IBeacon, Ownable {
+  address private _implementation;
+
+  /**
+   * @dev Emitted when the implementation returned by the beacon is changed.
+   */
+  event Upgraded(address indexed implementation);
+
+  /**
+   * @dev Sets the address of the initial implementation, and the deployer account as the owner who can upgrade the
+   * beacon.
+   */
+  constructor(address implementation_) {
+    _setImplementation(implementation_);
+  }
+
+  /**
+   * @dev Returns the current implementation address.
+   */
+  function implementation() public view virtual override returns (address) {
+    return _implementation;
+  }
+
+  /**
+   * @dev Upgrades the beacon to a new implementation.
+   *
+   * Emits an {Upgraded} event.
+   *
+   * Requirements:
+   *
+   * - msg.sender must be the owner of the contract.
+   * - `newImplementation` must be a contract.
+   */
+  function upgradeTo(address newImplementation) public virtual onlyOwner {
+    _setImplementation(newImplementation);
+    emit Upgraded(newImplementation);
+  }
+
+  /**
+   * @dev Sets the implementation contract address for this beacon
+   *
+   * Requirements:
+   *
+   * - `newImplementation` must be a contract.
+   */
+  function _setImplementation(address newImplementation) private {
+    require(
+      Address.isContract(newImplementation),
+      'UpgradeableBeacon: implementation is not a contract'
+    );
+    _implementation = newImplementation;
+  }
+}

src/contracts/immutable-beacon-proxy/interfaces/IBeacon.sol

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (proxy/beacon/IBeacon.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev This is the interface that {BeaconProxy} expects of its beacon.
+ */
+interface IBeacon {
+  /**
+   * @dev Must return an address that can be used as a delegate call target.
+   *
+   * {BeaconProxy} will check that this address is a contract.
+   */
+  function implementation() external view returns (address);
+}

src/contracts/transparent-proxy/ERC1967Proxy.sol

@@ -9,7 +9,7 @@
 
 pragma solidity ^0.8.0;
 
-import './Proxy.sol';
+import '../oz-common/Proxy.sol';
 import './ERC1967Upgrade.sol';
 
 /**

test/ImmutableBeaconProxy.t.sol

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+import 'forge-std/Test.sol';
+import {ImmutableBeaconProxy, IBeacon} from '../src/contracts/immutable-beacon-proxy/ImmutableBeaconProxy.sol';
+
+contract ImplementationMock {}
+
+contract BeaconMock is IBeacon {
+  address public implementation;
+
+  constructor(address newImplementation) {
+    implementation = newImplementation;
+  }
+}
+
+contract ImmutableBeaconProxyMock is ImmutableBeaconProxy {
+  constructor(address beacon) ImmutableBeaconProxy(beacon) {}
+
+  function implementation() public view returns (address) {
+    return _implementation();
+  }
+}
+
+contract ImmutableBeaconProxyTest is Test {
+  event ImmutableBeaconSet(address indexed beacon);
+
+  function testResolvesImplementationCorrectly() public {
+    address implementation = address(new ImplementationMock());
+    address beacon = address(new BeaconMock(implementation));
+
+    vm.expectEmit(true, false, false, true);
+    emit ImmutableBeaconSet(beacon);
+    assertEq(implementation, (new ImmutableBeaconProxyMock(beacon)).implementation());
+  }
+
+  function testBeaconNotAContract() public {
+    vm.expectRevert(bytes('INVALID_BEACON'));
+    new ImmutableBeaconProxy(address(1));
+  }
+
+  function testImplementationNotAContract() public {
+    address beacon = address(new BeaconMock(address(1)));
+
+    vm.expectRevert(bytes('INVALID_IMPLEMENTATION'));
+    new ImmutableBeaconProxy(beacon);
+  }
+}