Fixes #99

contracts/interfaces/ILendingPool.sol

@@ -349,11 +349,14 @@ interface ILendingPool {
 
   function getReserveData(address asset) external view returns (ReserveLogic.ReserveData memory);
 
-  function balanceDecreaseAllowed(
-    address reserve,
-    address user,
-    uint256 amount
-  ) external view returns (bool);
+  function finalizeTransfer(
+    address asset,
+    address from,
+    address to,
+    uint256 amount,
+    uint256 balanceFromAfter,
+    uint256 balanceToBefore
+  ) external;
 
   function getReservesList() external view returns (address[] memory);
 

contracts/lendingpool/LendingPool.sol

@@ -724,29 +724,46 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
   }
 
   /**
-   * @dev validate if a balance decrease for an asset is allowed
+   * @dev validates and finalizes an aToken transfer
    * @param asset the address of the reserve
-   * @param user the user related to the balance decrease
+   * @param from the user from which the aTokens are transferred
+   * @param to the user receiving the aTokens
    * @param amount the amount being transferred/redeemed
-   * @return true if the balance decrease can be allowed, false otherwise
+   * @param balanceFromBefore the balance of the from user before the transfer
+   * @param balanceToBefore the balance of the to user before the transfer
    */
-  function balanceDecreaseAllowed(
+  function finalizeTransfer(
     address asset,
-    address user,
-    uint256 amount
-  ) external override view returns (bool) {
+    address from,
+    address to,
+    uint256 amount,
+    uint256 balanceFromBefore,
+    uint256 balanceToBefore
+  ) external override {
     _whenNotPaused();
-    return
-      GenericLogic.balanceDecreaseAllowed(
-        asset,
-        user,
-        amount,
-        _reserves,
-        _usersConfig[user],
-        _reservesList,
-        _reservesCount,
-        _addressesProvider.getPriceOracle()
-      );
+
+    ValidationLogic.validateTransfer(
+      from,
+      _reserves,
+      _usersConfig[from],
+      _reservesList,
+      _reservesCount,
+      _addressesProvider.getPriceOracle()
+    );
+
+    uint256 reserveId = _reserves[asset].id;
+
+    if (from != to) {
+      if (balanceFromBefore.sub(amount) == 0) {
+        UserConfiguration.Map storage fromConfig = _usersConfig[from];
+        fromConfig.setUsingAsCollateral(reserveId, false);
+      }
+
+      if (balanceToBefore == 0) {
+        UserConfiguration.Map storage toConfig = _usersConfig[to];
+        toConfig.setUsingAsCollateral(reserveId, true);
+      }
+    }
   }
 
   /**
@@ -913,7 +930,6 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
    * @dev adds a reserve to the array of the _reserves address
    **/
   function _addReserveToList(address asset) internal {
-
     uint256 reservesCount = _reservesCount;
 
     require(reservesCount < MAX_NUMBER_RESERVES, Errors.NO_MORE_RESERVES_ALLOWED);

contracts/libraries/logic/ValidationLogic.sol

@@ -13,6 +13,8 @@ import {ReserveConfiguration} from '../configuration/ReserveConfiguration.sol';
 import {UserConfiguration} from '../configuration/UserConfiguration.sol';
 import {Errors} from '../helpers/Errors.sol';
 import {Helpers} from '../helpers/Helpers.sol';
+import "@nomiclabs/buidler/console.sol";
+
 
 /**
  * @title ReserveLogic library
@@ -46,6 +48,11 @@ library ValidationLogic {
    * @param reserveAddress the address of the reserve
    * @param amount the amount to be withdrawn
    * @param userBalance the balance of the user
+   * @param reservesData the reserves state
+   * @param userConfig the user configuration
+   * @param reserves the addresses of the reserves
+   * @param reservesCount the number of reserves
+   * @param oracle the price oracle
    */
   function validateWithdraw(
     address reserveAddress,
@@ -389,4 +396,37 @@ library ValidationLogic {
 
     return (uint256(Errors.CollateralManagerErrors.NO_ERROR), Errors.NO_ERRORS);
   }
+
+  /**
+   * @dev validates an aToken transfer.
+   * @param from the user from which the aTokens are being transferred
+   * @param reservesData the state of all the reserves
+   * @param userConfig the state of the user for the specific reserve
+   * @param reserves the addresses of all the active reserves
+   * @param oracle the price oracle
+   */
+  function validateTransfer(
+    address from,
+    mapping(address => ReserveLogic.ReserveData) storage reservesData,
+    UserConfiguration.Map storage userConfig,
+    mapping(uint256 => address) storage reserves,
+    uint256 reservesCount,
+    address oracle
+  ) internal view {
+    (, , , , uint256 healthFactor) = GenericLogic.calculateUserAccountData(
+      from,
+      reservesData,
+      userConfig,
+      reserves,
+      reservesCount,
+      oracle
+    );
+
+    console.log("Health factor after transfer %s", healthFactor);
+
+    require(
+      healthFactor >= GenericLogic.HEALTH_FACTOR_LIQUIDATION_THRESHOLD,
+      Errors.TRANSFER_NOT_ALLOWED
+    );
+  }
 }

contracts/tokenization/AToken.sol

@@ -241,16 +241,6 @@ contract AToken is VersionedInitializable, IncentivizedERC20, IAToken {
     return super.totalSupply();
   }
 
-  /**
-   * @dev Used to validate transfers before actually executing them.
-   * @param user address of the user to check
-   * @param amount the amount to check
-   * @return true if the user can transfer amount, false otherwise
-   **/
-  function isTransferAllowed(address user, uint256 amount) public override view returns (bool) {
-    return POOL.balanceDecreaseAllowed(UNDERLYING_ASSET_ADDRESS, user, amount);
-  }
-
   /**
    * @dev transfers the underlying asset to the target. Used by the lendingpool to transfer
    * assets in borrow(), redeem() and flashLoan()
@@ -317,14 +307,24 @@ contract AToken is VersionedInitializable, IncentivizedERC20, IAToken {
     uint256 amount,
     bool validate
   ) internal {
-    if (validate) {
-      require(isTransferAllowed(from, amount), Errors.TRANSFER_NOT_ALLOWED);
-    }
-
     uint256 index = POOL.getReserveNormalizedIncome(UNDERLYING_ASSET_ADDRESS);
 
+    uint256 fromBalanceBefore = super.balanceOf(from).rayMul(index);
+    uint256 toBalanceBefore = super.balanceOf(to).rayMul(index);
+
     super._transfer(from, to, amount.rayDiv(index));
 
+    if (validate) {
+      POOL.finalizeTransfer(
+        UNDERLYING_ASSET_ADDRESS,
+        from,
+        to,
+        amount,
+        fromBalanceBefore,
+        toBalanceBefore
+      );
+    }
+
     emit BalanceTransfer(from, to, amount, index);
   }
 

contracts/tokenization/interfaces/IAToken.sol

@@ -54,8 +54,6 @@ interface IAToken is IERC20, IScaledBalanceToken {
     uint256 value
   ) external;
 
-  function isTransferAllowed(address user, uint256 amount) external view returns (bool);
-
   /**
    * @dev transfer the amount of the underlying asset to the user
    * @param user address of the user

deployed-contracts.json

@@ -628,4 +628,4 @@
       "deployer": "0xc783df8a850f42e7F7e57013759C285caa701eB6"
     }
   }
-}
+}

test/atoken-transfer.spec.ts

@@ -46,8 +46,8 @@ makeSuite('AToken: Transfer', (testEnv: TestEnv) => {
     );
   });
 
-  it('User 0 deposits 1 WETH and user 1 tries to borrow, but the aTokens received as a transfer are not available as collateral (revert expected)', async () => {
-    const {users, pool, weth} = testEnv;
+  it('User 0 deposits 1 WETH and user 1 tries to borrow the WETH with the received DAI as collateral', async () => {
+    const {users, pool, weth, helpersContract} = testEnv;
     const userAddress = await pool.signer.getAddress();
 
     await weth.connect(users[0].signer).mint(await convertToCurrencyDecimals(weth.address, '1'));
@@ -57,26 +57,6 @@ makeSuite('AToken: Transfer', (testEnv: TestEnv) => {
     await pool
       .connect(users[0].signer)
       .deposit(weth.address, ethers.utils.parseEther('1.0'), userAddress, '0');
-    await expect(
-      pool
-        .connect(users[1].signer)
-        .borrow(
-          weth.address,
-          ethers.utils.parseEther('0.1'),
-          RateMode.Stable,
-          AAVE_REFERRAL,
-          users[1].address
-        ),
-      COLLATERAL_BALANCE_IS_0
-    ).to.be.revertedWith(COLLATERAL_BALANCE_IS_0);
-  });
-
-  it('User 1 sets the DAI as collateral and borrows, tries to transfer everything back to user 0 (revert expected)', async () => {
-    const {users, pool, aDai, dai, weth} = testEnv;
-    await pool.connect(users[1].signer).setUserUseReserveAsCollateral(dai.address, true);
-
-    const aDAItoTransfer = await convertToCurrencyDecimals(dai.address, '1000');
-
     await pool
       .connect(users[1].signer)
       .borrow(
@@ -87,9 +67,32 @@ makeSuite('AToken: Transfer', (testEnv: TestEnv) => {
         users[1].address
       );
 
+    const userReserveData = await helpersContract.getUserReserveData(weth.address, users[1].address);
+    
+    expect(userReserveData.currentStableDebt.toString()).to.be.eq(ethers.utils.parseEther('0.1'));
+  });
+
+  it('User 1 tries to transfer all the DAI used as collateral back to user 0 (revert expected)', async () => {
+    const {users, pool, aDai, dai, weth} = testEnv;
+
+    const aDAItoTransfer = await convertToCurrencyDecimals(dai.address, '1000');
+   
     await expect(
       aDai.connect(users[1].signer).transfer(users[0].address, aDAItoTransfer),
       TRANSFER_NOT_ALLOWED
     ).to.be.revertedWith(TRANSFER_NOT_ALLOWED);
   });
+
+  it('User 1 tries to transfer a small amount of DAI used as collateral back to user 0', async () => {
+    
+    const {users, pool, aDai, dai, weth} = testEnv;
+
+    const aDAItoTransfer = await convertToCurrencyDecimals(dai.address, '100');
+   
+    await aDai.connect(users[1].signer).transfer(users[0].address, aDAItoTransfer);
+
+    const user0Balance = await aDai.balanceOf(users[0].address);
+
+    expect(user0Balance.toString()).to.be.eq(aDAItoTransfer.toString());
+  });    
 });