Fix EIP712 for delegatecalls (#2852)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

CHANGELOG.md

@@ -5,6 +5,7 @@
  * `Ownable`: add an internal `_transferOwnership(address)`. ([#2568](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/#2568))
  * `AccessControl`: add internal `_grantRole(bytes32,address)` and `_revokeRole(bytes32,address)`. ([#2568](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/#2568))
  * `AccessControl`: mark `_setupRole(bytes32,address)` as deprecated in favor of `_grantRole(bytes32,address)`. ([#2568](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/#2568))
+ * `EIP712`: cache `address(this)` to immutable storage to avoid potential issues if a vanilla contract is used in a delegatecall context. ([#2852](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/#2852))
 
 ## 4.3.2 (2021-09-14)
 

contracts/utils/cryptography/draft-EIP712.sol

@@ -29,6 +29,7 @@ abstract contract EIP712 {
     // invalidate the cached domain separator if the chain id changes.
     bytes32 private immutable _CACHED_DOMAIN_SEPARATOR;
     uint256 private immutable _CACHED_CHAIN_ID;
+    address private immutable _CACHED_THIS;
 
     bytes32 private immutable _HASHED_NAME;
     bytes32 private immutable _HASHED_VERSION;
@@ -58,6 +59,7 @@ abstract contract EIP712 {
         _HASHED_VERSION = hashedVersion;
         _CACHED_CHAIN_ID = block.chainid;
         _CACHED_DOMAIN_SEPARATOR = _buildDomainSeparator(typeHash, hashedName, hashedVersion);
+        _CACHED_THIS = address(this);
         _TYPE_HASH = typeHash;
     }
 
@@ -65,7 +67,7 @@ abstract contract EIP712 {
      * @dev Returns the domain separator for the current chain.
      */
     function _domainSeparatorV4() internal view returns (bytes32) {
-        if (block.chainid == _CACHED_CHAIN_ID) {
+        if (address(this) == _CACHED_THIS && block.chainid == _CACHED_CHAIN_ID) {
             return _CACHED_DOMAIN_SEPARATOR;
         } else {
             return _buildDomainSeparator(_TYPE_HASH, _HASHED_NAME, _HASHED_VERSION);