add onERC721Received support

contracts/prize-pool/PrizePool.sol

@@ -6,6 +6,7 @@ import "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/utils/SafeCastUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/utils/ReentrancyGuardUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/token/ERC721/IERC721Upgradeable.sol";
+import "@openzeppelin/contracts-upgradeable/token/ERC721/IERC721ReceiverUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/introspection/ERC165CheckerUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/token/ERC20/SafeERC20Upgradeable.sol";
 import "@pooltogether/fixed-point/contracts/FixedPoint.sol";
@@ -23,7 +24,7 @@ import "./PrizePoolInterface.sol";
 /// @title Escrows assets and deposits them into a yield source.  Exposes interest to Prize Strategy.  Users deposit and withdraw from this contract to participate in Prize Pool.
 /// @notice Accounting is managed using Controlled Tokens, whose mint and burn functions can only be called by this contract.
 /// @dev Must be inherited to provide specific yield-bearing asset control, such as Compound cTokens
-abstract contract PrizePool is PrizePoolInterface, OwnableUpgradeable, ReentrancyGuardUpgradeable, TokenControllerInterface {
+abstract contract PrizePool is PrizePoolInterface, OwnableUpgradeable, ReentrancyGuardUpgradeable, TokenControllerInterface, IERC721ReceiverUpgradeable {
   using SafeMathUpgradeable for uint256;
   using SafeCastUpgradeable for uint256;
   using SafeERC20Upgradeable for IERC20Upgradeable;
@@ -1050,6 +1051,15 @@ abstract contract PrizePool is PrizePoolInterface, OwnableUpgradeable, Reentranc
       compLike.delegate(to);
     }
   }
+
+  /// @notice Required for ERC721 safe token transfers from smart contracts.
+  /// @param operator The address that acts on behalf of the owner
+  /// @param from The current owner of the NFT
+  /// @param tokenId The NFT to transfer
+  /// @param data Additional data with no specified format, sent in call to `_to`.
+  function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external override returns (bytes4){
+    return IERC721ReceiverUpgradeable.onERC721Received.selector;
+  }
 
   /// @notice The total of all controlled tokens and timelock.
   /// @return The current total of all tokens and timelock.

contracts/test/NFT.sol

@@ -0,0 +1,15 @@
+pragma solidity 0.6.12;
+import "@openzeppelin/contracts-upgradeable/token/ERC721/ERC721Upgradeable.sol";
+
+contract NFT is ERC721Upgradeable {
+  function initialize (
+    string memory name_, string memory symbol_
+  ) external initializer {
+    __ERC721_init(name_, symbol_);
+    _safeMint(msg.sender, 0);
+  }
+
+  function simulateSafeTransferFrom(address from, address to, uint256 tokenId) public {
+    ERC721Upgradeable.safeTransferFrom(from, to, tokenId);
+  }
+}

test/PrizePool.test.js

@@ -24,7 +24,7 @@ describe('PrizePool', function() {
   let poolMaxExitFee = toWei('0.5')
   let poolMaxTimelockDuration = 10000
 
-  let ticket, sponsorship
+  let ticket, sponsorship, nft
 
   let compLike
 
@@ -963,4 +963,31 @@ describe('PrizePool', function() {
         .withArgs(wallet.address, erc721token.address, [NFT_TOKEN_ID])
     })
   })
+
+  describe('onERC721Received()', () => {
+    beforeEach(async () => {
+      await prizePool.initializeAll(
+        prizeStrategy.address,
+        [ticket.address],
+        poolMaxExitFee,
+        poolMaxTimelockDuration,
+        yieldSourceStub.address
+      )
+      await prizePool.setPrizeStrategy(prizeStrategy.address)
+      await prizePool.setCreditPlanOf(ticket.address, toWei('0.01'), toWei('0.1'))
+      debug('deploying NFT test contract...')
+      const NFTFactory = await hre.ethers.getContractFactory("NFT", wallet, overrides)
+      NFT = await NFTFactory.deploy()
+      await NFT.initialize('NFT Token', 'NFT')
+    })
+
+    it('should receive an ERC721 token when using safeTransferFrom', async () => {
+      expect(await NFT.balanceOf(prizePool.address)).to.equal('0')
+      expect(await NFT.simulateSafeTransferFrom(wallet.address, prizePool.address, 0))
+        .to.emit(NFT, 'Transfer')
+        .withArgs(wallet.address, prizePool.address, 0);
+      expect(await NFT.balanceOf(prizePool.address)).to.equal('1')
+    })
+
+  })
 });