install Multicall3 using forge install

.gitmodules

 [submodule "tests"]
 	path = l2geth/tests/testdata
 	url = https://github.com/ethereum/tests
+[submodule "packages/contracts-periphery/lib/multicall"]
+	path = packages/contracts-periphery/lib/multicall
+	url = https://github.com/mds1/multicall
+[submodule "lib/multicall"]
+	branch = v3.1.0

packages/contracts-periphery/contracts/foundry-tests/MulticallContractCompiler.t.sol

+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import { Multicall3 } from "multicall/src/Multicall3.sol";
+
+/**
+ * Just exists so we can compile this contract.
+ */
+contract MulticallContractCompiler {
+
+}

packages/contracts-periphery/contracts/foundry-tests/Optimist.t.sol

 //SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity >=0.6.2 <0.9.0;
 
 /* Testing utilities */
 import { Test } from "forge-std/Test.sol";
@@ -8,10 +8,27 @@ import { Optimist } from "../universal/op-nft/Optimist.sol";
 import { OptimistAllowlist } from "../universal/op-nft/OptimistAllowlist.sol";
 import { OptimistInviter } from "../universal/op-nft/OptimistInviter.sol";
 import { OptimistInviterHelper } from "../testing/helpers/OptimistInviterHelper.sol";
-import { Multicall3 } from "../testing/helpers/Multicall3.sol";
 import { Strings } from "@openzeppelin/contracts/utils/Strings.sol";
 import { IERC721 } from "@openzeppelin/contracts/token/ERC721/IERC721.sol";
 
+interface IMulticall3 {
+    struct Call3 {
+        address target;
+        bool allowFailure;
+        bytes callData;
+    }
+
+    struct Result {
+        bool success;
+        bytes returnData;
+    }
+
+    function aggregate3(Call3[] calldata calls)
+        external
+        payable
+        returns (Result[] memory returnData);
+}
+
 contract Optimist_Initializer is Test {
     event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
     event Initialized(uint8);
@@ -35,7 +52,7 @@ contract Optimist_Initializer is Test {
     OptimistInviterHelper optimistInviterHelper;
 
     // To test multicall for claiming and minting in one call
-    Multicall3 multicall3;
+    IMulticall3 multicall3;
 
     address internal carol_baseURIAttestor;
     address internal alice_allowlistAttestor;
@@ -216,7 +233,8 @@ contract Optimist_Initializer is Test {
             _optimistAllowlist: optimistAllowlist
         });
 
-        multicall3 = new Multicall3();
+        // address test = deployCode("Multicall3.sol");
+        multicall3 = IMulticall3(deployCode("Multicall3.sol"));
     }
 }
 
@@ -599,10 +617,10 @@ contract OptimistTest is Optimist_Initializer {
         // wait minimum commitment period
         vm.warp(optimistInviter.MIN_COMMITMENT_PERIOD() + block.timestamp);
 
-        Multicall3.Call3[] memory calls = new Multicall3.Call3[](2);
+        IMulticall3.Call3[] memory calls = new IMulticall3.Call3[](2);
 
         // First call is to claim the invite, receiving the attestation
-        calls[0] = Multicall3.Call3({
+        calls[0] = IMulticall3.Call3({
             target: address(optimistInviter),
             callData: abi.encodeWithSelector(
                 optimistInviter.claimInvite.selector,
@@ -614,7 +632,7 @@ contract OptimistTest is Optimist_Initializer {
         });
 
         // Second call is to mint the Optimist NFT
-        calls[1] = Multicall3.Call3({
+        calls[1] = IMulticall3.Call3({
             target: address(optimist),
             callData: abi.encodeWithSelector(optimist.mint.selector, bob),
             allowFailure: false

packages/contracts-periphery/contracts/testing/helpers/ExternalContractCompiler.sol

@@ -3,6 +3,7 @@ pragma solidity ^0.8.0;
 
 import { ProxyAdmin } from "@eth-optimism/contracts-bedrock/contracts/universal/ProxyAdmin.sol";
 import { Proxy } from "@eth-optimism/contracts-bedrock/contracts/universal/Proxy.sol";
+import { Multicall3 } from "multicall/src/Multicall3.sol";
 
 /**
  * Just exists so we can compile external contracts.

packages/contracts-periphery/foundry.toml

@@ -16,9 +16,13 @@ remappings = [
 	'@rari-capital/solmate/=node_modules/@rari-capital/solmate',
 	'forge-std/=node_modules/forge-std/src',
 	'ds-test/=node_modules/ds-test/src',
+  'multicall/=lib/multicall',
   '@openzeppelin/contracts/=node_modules/@openzeppelin/contracts/',
   '@openzeppelin/contracts-upgradeable/=node_modules/@openzeppelin/contracts-upgradeable/',
   '@eth-optimism/contracts-bedrock/=../../node_modules/@eth-optimism/contracts-bedrock',
 ]
 # The metadata hash can be removed from the bytecode by setting "none"
 bytecode_hash = "none"
+libs = ["node_modules", "lib"]
+# Required to use `deployCode` to deploy the multicall contract which has incompatible version
+fs_permissions = [{ access = "read", path = "./forge-artifacts/Multicall3.sol/Multicall3.json"}]