Add ERC721Votes for NFT-based governance

contracts/governance/IGovernor.sol

@@ -193,7 +193,7 @@ abstract contract IGovernor is IERC165 {
     function castVote(uint256 proposalId, uint8 support) public virtual returns (uint256 balance);
 
     /**
-     * @dev Cast a with a reason
+     * @dev Cast a vote with a reason
      *
      * Emits a {VoteCast} event.
      */

contracts/governance/README.adoc

@@ -22,6 +22,8 @@ Votes modules determine the source of voting power, and sometimes quorum number.
 
 * {GovernorVotes}: Extracts voting weight from an {ERC20Votes} token.
 
+* {GovernorVotesERC721}: Extracts voting weight from an {ERC721Votes} token.
+
 * {GovernorVotesComp}: Extracts voting weight from a COMP-like or {ERC20VotesComp} token.
 
 * {GovernorVotesQuorumFraction}: Combines with `GovernorVotes` to set the quorum as a fraction of the total token supply.
@@ -64,6 +66,8 @@ NOTE: Functions of the `Governor` contract do not include access control. If you
 
 {{GovernorVotesQuorumFraction}}
 
+{{GovernorVotesERC721}}
+
 {{GovernorVotesComp}}
 
 === Extensions

contracts/governance/extensions/GovernorVotesERC721.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.3.2 (governance/extensions/ERC721GovernorVotesERC721.sol)
+
+pragma solidity ^0.8.0;
+
+import "../Governor.sol";
+import "../../token/ERC721/extensions/draft-ERC721Votes.sol";
+import "../../utils/math/Math.sol";
+
+/**
+ * @dev Extension of {Governor} for voting weight extraction from an {ERC721Votes} token.
+ *
+ * _Available since v4.5._
+ */
+abstract contract GovernorVotesERC721 is Governor {
+    ERC721Votes public immutable token;
+
+    /**
+     * @dev Need the ERC721Votes address to be initialized
+     */
+    constructor(ERC721Votes tokenAddress) {
+        token = tokenAddress;
+    }
+
+    /**
+     * @dev Read the voting weight from the token's built in snapshot mechanism (see {IGovernor-getVotes}).
+     */
+    function getVotes(address account, uint256 blockNumber) public view virtual override returns (uint256) {
+        return token.getPastVotes(account, blockNumber);
+    }
+}

contracts/mocks/ERC721VotesMock.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../token/ERC721/extensions/draft-ERC721Votes.sol";
+
+contract ERC721VotesMock is ERC721Votes {
+    constructor(string memory name, string memory symbol) ERC721Votes(name, symbol) {}
+
+    function mint(address account, uint256 tokenId) public {
+        _mint(account, tokenId);
+    }
+
+    function burn(uint256 tokenId) public {
+        _burn(tokenId);
+    }
+
+    function getChainId() external view returns (uint256) {
+        return block.chainid;
+    }
+
+    function _maxSupply() internal pure override returns (uint224) {
+        return uint224(5);
+    }
+}

contracts/mocks/GovernorERC721Mock.sol

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../governance/extensions/GovernorCountingSimple.sol";
+import "../governance/extensions/GovernorVotesERC721.sol";
+
+contract GovernorERC721Mock is GovernorVotesERC721, GovernorCountingSimple {
+    constructor(string memory name_, ERC721Votes token_) Governor(name_) GovernorVotesERC721(token_) {}
+
+    function quorum(uint256) public pure override returns (uint256) {
+        return 0;
+    }
+
+    function votingDelay() public pure override returns (uint256) {
+        return 4;
+    }
+
+    function votingPeriod() public pure override returns (uint256) {
+        return 16;
+    }
+
+    function cancel(
+        address[] memory targets,
+        uint256[] memory values,
+        bytes[] memory calldatas,
+        bytes32 salt
+    ) public returns (uint256 proposalId) {
+        return _cancel(targets, values, calldatas, salt);
+    }
+
+    function getVotes(address account, uint256 blockNumber)
+        public
+        view
+        virtual
+        override(IGovernor, GovernorVotesERC721)
+        returns (uint256)
+    {
+        return super.getVotes(account, blockNumber);
+    }
+}

contracts/token/ERC721/ERC721.sol

@@ -342,6 +342,8 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
         _owners[tokenId] = to;
 
         emit Transfer(from, to, tokenId);
+
+        _afterTokenTransfer(from, to);
     }
 
     /**
@@ -421,4 +423,17 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
         address to,
         uint256 tokenId
     ) internal virtual {}
+
+    /**
+     * @dev Hook that is called after any transfer of tokens. This includes
+     * minting and burning.
+     *
+     * Calling conditions:
+     *
+     * - when `from` and `to` are both non-zero.
+     * - `from` and `to` are never both zero.
+     *
+     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
+     */
+    function _afterTokenTransfer(address from, address to) internal virtual {}
 }

contracts/token/ERC721/README.adoc

@@ -15,6 +15,7 @@ Additionally there are multiple custom extensions, including:
 
 * designation of addresses that can pause token transfers for all users ({ERC721Pausable}).
 * destruction of own tokens ({ERC721Burnable}).
+* support for voting and vote delegation ({ERC721Votes})
 
 NOTE: This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC721 (such as <<ERC721-_mint-address-uint256-,`_mint`>>) and expose them as external functions in the way they prefer. On the other hand, xref:ROOT:erc721.adoc#Presets[ERC721 Presets] (such as {ERC721PresetMinterPauserAutoId}) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.
 
@@ -41,6 +42,8 @@ NOTE: This core set of contracts is designed to be unopinionated, allowing devel
 
 {{ERC721URIStorage}}
 
+{{ERC721Votes}}
+
 == Presets
 
 These contracts are preconfigured combinations of the above features. They can be used through inheritance or as models to copy and paste their source code.