Remove EIP712 immutable cache

contracts/drafts/EIP712.sol

@@ -21,14 +21,9 @@ pragma solidity >=0.6.0 <0.8.0;
  */
 abstract contract EIP712 {
     /* solhint-disable var-name-mixedcase */
-    // Cache the domain separator as an immutable value, but also store the chain id that it corresponds to, in order to
-    // invalidate the cached domain separator if the chain id changes.
-    bytes32 private immutable _CACHED_DOMAIN_SEPARATOR;
-    uint256 private immutable _CACHED_CHAIN_ID;
-
     bytes32 private immutable _HASHED_NAME;
     bytes32 private immutable _HASHED_VERSION;
-    bytes32 private immutable _TYPE_HASH;
+    bytes32 private constant _TYPE_HASH = keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)");
     /* solhint-enable var-name-mixedcase */
 
     /**
@@ -46,23 +41,15 @@ abstract contract EIP712 {
     constructor(string memory name, string memory version) internal {
         bytes32 hashedName = keccak256(bytes(name));
         bytes32 hashedVersion = keccak256(bytes(version));
-        bytes32 typeHash = keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"); 
         _HASHED_NAME = hashedName;
         _HASHED_VERSION = hashedVersion;
-        _CACHED_CHAIN_ID = _getChainId();
-        _CACHED_DOMAIN_SEPARATOR = _buildDomainSeparator(typeHash, hashedName, hashedVersion);
-        _TYPE_HASH = typeHash;
     }
 
     /**
      * @dev Returns the domain separator for the current chain.
      */
     function _domainSeparatorV4() internal view returns (bytes32) {
-        if (_getChainId() == _CACHED_CHAIN_ID) {
-            return _CACHED_DOMAIN_SEPARATOR;
-        } else {
-            return _buildDomainSeparator(_TYPE_HASH, _HASHED_NAME, _HASHED_VERSION);
-        }
+        return _buildDomainSeparator(_TYPE_HASH, _EIP712NameHash(), _EIP712VersionHash());
     }
 
     function _buildDomainSeparator(bytes32 typeHash, bytes32 name, bytes32 version) private view returns (bytes32) {
@@ -102,4 +89,24 @@ abstract contract EIP712 {
             chainId := chainid()
         }
     }
+
+    /**
+     * @dev The hash of the name parameter for the EIP712 domain.
+     *
+     * NOTE: This function reads from storage by default, but can be redefined to return a constant value if gas costs
+     * are a concern.
+     */
+    function _EIP712NameHash() internal virtual view returns (bytes32) {
+        return _HASHED_NAME;
+    }
+
+    /**
+     * @dev The hash of the version parameter for the EIP712 domain.
+     *
+     * NOTE: This function reads from storage by default, but can be redefined to return a constant value if gas costs
+     * are a concern.
+     */
+    function _EIP712VersionHash() internal virtual view returns (bytes32) {
+        return _HASHED_VERSION;
+    }
 }