ERC20 extension for governance tokens (vote delegation and snapshots) (#2632)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/token/ERC20/README.adoc

@@ -21,6 +21,7 @@ Additionally there are multiple custom extensions, including:
 * {ERC20Snapshot}: efficient storage of past token balances to be later queried at any point in time.
 * {ERC20Permit}: gasless approval of tokens (standardized as ERC2612).
 * {ERC20FlashMint}: token level support for flash loans through the minting and burning of ephemeral tokens (standardized as ERC3156).
+* {ERC20Votes}: support for voting and vote delegation (compatible with Compound's token).
 
 Finally, there are some utilities to interact with ERC20 contracts in various ways.
 
@@ -31,6 +32,7 @@ The following related EIPs are in draft status.
 
 - {ERC20Permit}
 - {ERC20FlashMint}
+- {ERC20Votes}
 
 NOTE: This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC20 (such as <<ERC20-_mint-address-uint256-,`_mint`>>) and expose them as external functions in the way they prefer. On the other hand, xref:ROOT:erc20.adoc#Presets[ERC20 Presets] (such as {ERC20PresetMinterPauser}) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.
 
@@ -60,6 +62,8 @@ The following EIPs are still in Draft status. Due to their nature as drafts, the
 
 {{ERC20FlashMint}}
 
+{{ERC20Votes}}
+
 == 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.

contracts/token/ERC20/extensions/ERC20Snapshot.sol

@@ -20,6 +20,13 @@ import "../../../utils/Counters.sol";
  * id. To get the balance of an account at the time of a snapshot, call the {balanceOfAt} function with the snapshot id
  * and the account address.
  *
+ * NOTE: Snapshot policy can be customized by overriding the {_getCurrentSnapshotId} method. For example, having it
+ * return `block.number` will trigger the creation of snapshot at the begining of each new block. When overridding this
+ * function, be careful about the monotonicity of its result. Non-monotonic snapshot ids will break the contract.
+ *
+ * Implementing snapshots for every block using this method will incur significant gas costs. For a gas-efficient
+ * alternative consider {ERC20Votes}.
+ *
  * ==== Gas Costs
  *
  * Snapshots are efficient. Snapshot creation is _O(1)_. Retrieval of balances or total supply from a snapshot is _O(log
@@ -30,6 +37,7 @@ import "../../../utils/Counters.sol";
  * only significant for the first transfer that immediately follows a snapshot for a particular account. Subsequent
  * transfers will have normal cost until the next snapshot, and so on.
  */
+
 abstract contract ERC20Snapshot is ERC20 {
     // Inspired by Jordi Baylina's MiniMeToken to record historical balances:
     // https://github.com/Giveth/minimd/blob/ea04d950eea153a04c51fa510b068b9dded390cb/contracts/MiniMeToken.sol
@@ -79,11 +87,18 @@ abstract contract ERC20Snapshot is ERC20 {
     function _snapshot() internal virtual returns (uint256) {
         _currentSnapshotId.increment();
 
-        uint256 currentId = _currentSnapshotId.current();
+        uint256 currentId = _getCurrentSnapshotId();
         emit Snapshot(currentId);
         return currentId;
     }
 
+    /**
+     * @dev Get the current snapshotId
+     */
+    function _getCurrentSnapshotId() internal view virtual returns (uint256) {
+        return _currentSnapshotId.current();
+    }
+
     /**
      * @dev Retrieves the balance of `account` at the time `snapshotId` was created.
      */
@@ -102,7 +117,6 @@ abstract contract ERC20Snapshot is ERC20 {
         return snapshotted ? value : totalSupply();
     }
 
-
     // Update balance and/or total supply snapshots before the values are modified. This is implemented
     // in the _beforeTokenTransfer hook, which is executed for _mint, _burn, and _transfer operations.
     function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual override {
@@ -127,8 +141,7 @@ abstract contract ERC20Snapshot is ERC20 {
         private view returns (bool, uint256)
     {
         require(snapshotId > 0, "ERC20Snapshot: id is 0");
-        // solhint-disable-next-line max-line-length
-        require(snapshotId <= _currentSnapshotId.current(), "ERC20Snapshot: nonexistent id");
+        require(snapshotId <= _getCurrentSnapshotId(), "ERC20Snapshot: nonexistent id");
 
         // When a valid snapshot is queried, there are three possibilities:
         //  a) The queried value was not modified after the snapshot was taken. Therefore, a snapshot entry was never
@@ -162,7 +175,7 @@ abstract contract ERC20Snapshot is ERC20 {
     }
 
     function _updateSnapshot(Snapshots storage snapshots, uint256 currentValue) private {
-        uint256 currentId = _currentSnapshotId.current();
+        uint256 currentId = _getCurrentSnapshotId();
         if (_lastSnapshotId(snapshots.ids) < currentId) {
             snapshots.ids.push(currentId);
             snapshots.values.push(currentValue);

contracts/token/ERC20/extensions/draft-ERC20Votes.sol

@@ -0,0 +1,172 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "./draft-ERC20Permit.sol";
+import "./draft-IERC20Votes.sol";
+import "../../../utils/math/Math.sol";
+import "../../../utils/math/SafeCast.sol";
+import "../../../utils/cryptography/ECDSA.sol";
+
+/**
+ * @dev Extension of the ERC20 token contract to support Compound's voting and delegation.
+ *
+ * This extensions keeps a history (checkpoints) of each account's vote power. Vote power can be delegated either
+ * by calling the {delegate} function directly, or by providing a signature to be used with {delegateBySig}. Voting
+ * power can be queried through the public accessors {getCurrentVotes} and {getPriorVotes}.
+ *
+ * By default, token balance does not account for voting power. This makes transfers cheaper. The downside is that it
+ * requires users to delegate to themselves in order to activate checkpoints and have their voting power tracked.
+ * Enabling self-delegation can easily be done by overriding the {delegates} function. Keep in mind however that this
+ * will significantly increase the base gas cost of transfers.
+ *
+ * _Available since v4.2._
+ */
+abstract contract ERC20Votes is IERC20Votes, ERC20Permit {
+    bytes32 private constant _DELEGATION_TYPEHASH = keccak256("Delegation(address delegatee,uint256 nonce,uint256 expiry)");
+
+    mapping (address => address) private _delegates;
+    mapping (address => Checkpoint[]) private _checkpoints;
+
+    /**
+     * @dev Get the `pos`-th checkpoint for `account`.
+     */
+    function checkpoints(address account, uint32 pos) external view virtual override returns (Checkpoint memory) {
+        return _checkpoints[account][pos];
+    }
+
+    /**
+     * @dev Get number of checkpoints for `account`.
+     */
+    function numCheckpoints(address account) external view virtual override returns (uint32) {
+        return SafeCast.toUint32(_checkpoints[account].length);
+    }
+
+    /**
+     * @dev Get the address `account` is currently delegating to.
+     */
+    function delegates(address account) public view virtual override returns (address) {
+        return _delegates[account];
+    }
+
+    /**
+     * @dev Gets the current votes balance for `account`
+     */
+    function getCurrentVotes(address account) external view override returns (uint256) {
+        uint256 pos = _checkpoints[account].length;
+        return pos == 0 ? 0 : _checkpoints[account][pos - 1].votes;
+    }
+
+    /**
+     * @dev Determine the number of votes for `account` at the begining of `blockNumber`.
+     */
+    function getPriorVotes(address account, uint256 blockNumber) external view override returns (uint256) {
+        require(blockNumber < block.number, "ERC20Votes::getPriorVotes: not yet determined");
+
+        Checkpoint[] storage ckpts = _checkpoints[account];
+
+        // We run a binary search to look for the earliest checkpoint taken after `blockNumber`.
+        //
+        // During the loop, the index of the wanted checkpoint remains in the range [low, high).
+        // With each iteration, either `low` or `high` is moved towards the middle of the range to maintain the invariant.
+        // - If the middle checkpoint is after `blockNumber`, we look in [low, mid)
+        // - If the middle checkpoint is before `blockNumber`, we look in [mid+1, high)
+        // Once we reach a single value (when low == high), we've found the right checkpoint at the index high-1, if not
+        // out of bounds (in which case we're looking too far in the past and the result is 0).
+        // Note that if the latest checkpoint available is exactly for `blockNumber`, we end up with an index that is
+        // past the end of the array, so we technically don't find a checkpoint after `blockNumber`, but it works out
+        // the same.
+        uint256 high = ckpts.length;
+        uint256 low = 0;
+        while (low < high) {
+            uint256 mid = Math.average(low, high);
+            if (ckpts[mid].fromBlock > blockNumber) {
+                high = mid;
+            } else {
+                low = mid + 1;
+            }
+        }
+
+        return high == 0 ? 0 : ckpts[high - 1].votes;
+    }
+
+    /**
+     * @dev Delegate votes from the sender to `delegatee`.
+     */
+    function delegate(address delegatee) public virtual override {
+        return _delegate(_msgSender(), delegatee);
+    }
+
+    /**
+     * @dev Delegates votes from signer to `delegatee`
+     */
+    function delegateBySig(address delegatee, uint256 nonce, uint256 expiry, uint8 v, bytes32 r, bytes32 s)
+        public virtual override
+    {
+        require(block.timestamp <= expiry, "ERC20Votes::delegateBySig: signature expired");
+        address signer = ECDSA.recover(
+            _hashTypedDataV4(keccak256(abi.encode(
+                _DELEGATION_TYPEHASH,
+                delegatee,
+                nonce,
+                expiry
+            ))),
+            v, r, s
+        );
+        require(nonce == _useNonce(signer), "ERC20Votes::delegateBySig: invalid nonce");
+        return _delegate(signer, delegatee);
+    }
+
+    /**
+     * @dev Change delegation for `delegator` to `delegatee`.
+     */
+    function _delegate(address delegator, address delegatee) internal virtual {
+        address currentDelegate = delegates(delegator);
+        uint256 delegatorBalance = balanceOf(delegator);
+        _delegates[delegator] = delegatee;
+
+        emit DelegateChanged(delegator, currentDelegate, delegatee);
+
+        _moveVotingPower(currentDelegate, delegatee, delegatorBalance);
+    }
+
+    function _moveVotingPower(address src, address dst, uint256 amount) private {
+        if (src != dst && amount > 0) {
+            if (src != address(0)) {
+                uint256 srcCkptLen = _checkpoints[src].length;
+                uint256 srcCkptOld = srcCkptLen == 0 ? 0 : _checkpoints[src][srcCkptLen - 1].votes;
+                uint256 srcCkptNew = srcCkptOld - amount;
+                _writeCheckpoint(src, srcCkptLen, srcCkptOld, srcCkptNew);
+            }
+
+            if (dst != address(0)) {
+                uint256 dstCkptLen = _checkpoints[dst].length;
+                uint256 dstCkptOld = dstCkptLen == 0 ? 0 : _checkpoints[dst][dstCkptLen - 1].votes;
+                uint256 dstCkptNew = dstCkptOld + amount;
+                _writeCheckpoint(dst, dstCkptLen, dstCkptOld, dstCkptNew);
+            }
+        }
+    }
+
+    function _writeCheckpoint(address delegatee, uint256 pos, uint256 oldWeight, uint256 newWeight) private {
+      if (pos > 0 && _checkpoints[delegatee][pos - 1].fromBlock == block.number) {
+          _checkpoints[delegatee][pos - 1].votes = SafeCast.toUint224(newWeight);
+      } else {
+          _checkpoints[delegatee].push(Checkpoint({
+              fromBlock: SafeCast.toUint32(block.number),
+              votes: SafeCast.toUint224(newWeight)
+          }));
+      }
+
+      emit DelegateVotesChanged(delegatee, oldWeight, newWeight);
+    }
+
+    function _mint(address account, uint256 amount) internal virtual override {
+        super._mint(account, amount);
+        require(totalSupply() <= type(uint224).max, "ERC20Votes: total supply exceeds 2**224");
+    }
+
+    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual override {
+        _moveVotingPower(delegates(from), delegates(to), amount);
+    }
+}

contracts/token/ERC20/extensions/draft-IERC20Votes.sol

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../IERC20.sol";
+
+interface IERC20Votes is IERC20 {
+    struct Checkpoint {
+        uint32  fromBlock;
+        uint224 votes;
+    }
+
+    event DelegateChanged(address indexed delegator, address indexed fromDelegate, address indexed toDelegate);
+    event DelegateVotesChanged(address indexed delegate, uint256 previousBalance, uint256 newBalance);
+
+    function delegates(address owner) external view returns (address);
+    function checkpoints(address account, uint32 pos) external view returns (Checkpoint memory);
+    function numCheckpoints(address account) external view returns (uint32);
+    function getCurrentVotes(address account) external view returns (uint256);
+    function getPriorVotes(address account, uint256 blockNumber) external view returns (uint256);
+    function delegate(address delegatee) external;
+    function delegateBySig(address delegatee, uint nonce, uint expiry, uint8 v, bytes32 r, bytes32 s) external;
+}