use _validateCancel instead of overriding cancel

contracts/governance/Governor.sol

@@ -470,6 +470,8 @@ abstract contract Governor is Context, ERC165, EIP712, Nonces, IGovernor, IERC72
         }
     }
 
+    error UnableToCancel();
+
     /**
      * @dev See {IGovernor-cancel}.
      */
@@ -484,13 +486,19 @@ abstract contract Governor is Context, ERC165, EIP712, Nonces, IGovernor, IERC72
         // changes it. The `getProposalId` duplication has a cost that is limited, and that we accept.
         uint256 proposalId = getProposalId(targets, values, calldatas, descriptionHash);
 
+        if (!_validateCancel(_msgSender(), proposalId)) revert UnableToCancel();
+
+        return _cancel(targets, values, calldatas, descriptionHash);
+    }
+
+    function _validateCancel(address caller, uint256 proposalId) internal view virtual returns (bool) {
         // public cancel restrictions (on top of existing _cancel restrictions).
         _validateStateBitmap(proposalId, _encodeStateBitmap(ProposalState.Pending));
-        if (_msgSender() != proposalProposer(proposalId)) {
-            revert GovernorOnlyProposer(_msgSender());
+        if (caller != proposalProposer(proposalId)) {
+            return false;
         }
 
-        return _cancel(targets, values, calldatas, descriptionHash);
+        return true;
     }
 
     /**

contracts/governance/extensions/GovernorProposalGuardian.sol

@@ -14,37 +14,32 @@ abstract contract GovernorProposalGuardian is Governor {
     event ProposalGuardianSet(address oldProposalGuardian, address newProposalGuardian);
 
     /**
-     * @dev Override {IGovernor-cancel} that implements the extended cancellation logic.
+     * @dev Override `_validateCancel` that implements the extended cancellation logic.
      *
      * * The {proposalGuardian} can cancel any proposal at any point in the lifecycle.
      * * if no proposal guardian is set, the {proposalProposer} can cancel their proposals at any point in the lifecycle.
      * * if the proposal guardian is set, the {proposalProposer} keeps their default rights defined in {IGovernor-cancel} (calling `super`).
      */
-    function cancel(
-        address[] memory targets,
-        uint256[] memory values,
-        bytes[] memory calldatas,
-        bytes32 descriptionHash
-    ) public virtual override returns (uint256) {
-        address caller = _msgSender();
+    function _validateCancel(address caller, uint256 proposalId) internal view virtual override returns (bool) {
+        // no additional validation is required
+
         address guardian = proposalGuardian();
 
         if (guardian == address(0)) {
             // if there is no proposal guardian
             // ... only the proposer can cancel
             // ... no restriction on when the proposer can cancel
-            uint256 proposalId = getProposalId(targets, values, calldatas, descriptionHash);
             address proposer = proposalProposer(proposalId);
             if (caller != proposer) revert GovernorOnlyProposer(caller);
-            return _cancel(targets, values, calldatas, descriptionHash);
+            return true;
         } else if (guardian == caller) {
             // if there is a proposal guardian, and the caller is the proposal guardian
             // ... just cancel
-            return _cancel(targets, values, calldatas, descriptionHash);
+            return true;
         } else {
             // if there is a proposal guardian, and the caller is not the proposal guardian
             // ... apply default behavior
-            return super.cancel(targets, values, calldatas, descriptionHash);
+            return super._validateCancel(caller, proposalId);
         }
     }
 

contracts/mocks/governance/GovernorProposalGuardianMock.sol

@@ -18,12 +18,10 @@ abstract contract GovernorProposalGuardianMock is
         return super.proposalThreshold();
     }
 
-    function cancel(
-        address[] memory targets,
-        uint256[] memory values,
-        bytes[] memory calldatas,
-        bytes32 descriptionHash
-    ) public override(Governor, GovernorProposalGuardian) returns (uint256) {
-        return super.cancel(targets, values, calldatas, descriptionHash);
+    function _validateCancel(
+        address caller,
+        uint256 proposalId
+    ) internal view override(Governor, GovernorProposalGuardian) returns (bool) {
+        return super._validateCancel(caller, proposalId);
     }
 }