Upgraded solidity version from 0.5.16 to 0.8.13.

contracts/Governance/GovernorAlpha2.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 pragma experimental ABIEncoderV2;
 
 contract GovernorAlpha2 {

contracts/Governance/Timelock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 import "../Utils/SafeMath.sol";
 
@@ -51,7 +51,7 @@ contract Timelock {
         delay = delay_;
     }
 
-    function() external payable {}
+    receive() external payable {}
 
     function setDelay(uint delay_) public {
         require(msg.sender == address(this), "Timelock::setDelay: Call must come from Timelock.");
@@ -137,7 +137,7 @@ contract Timelock {
         }
 
         // solium-disable-next-line security/no-call-value
-        (bool success, bytes memory returnData) = target.call.value(value)(callData);
+        (bool success, bytes memory returnData) = target.call{ value: value }(callData);
         require(success, "Timelock::executeTransaction: Transaction execution reverted.");
 
         emit ExecuteTransaction(txHash, target, value, signature, data, eta);

contracts/Governance/VTreasury.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 import "../Utils/IBEP20.sol";
 import "../Utils/SafeBEP20.sol";
@@ -20,7 +20,7 @@ contract VTreasury is Ownable {
     /**
      * @notice To receive BNB
      */
-    function() external payable {}
+    receive() external payable {}
 
     /**
      * @notice Withdraw Treasury BEP20 Tokens, Only owner call it

contracts/InterestRateModels/InterestRateModel.sol

@@ -1,10 +1,10 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 /**
  * @title Venus's InterestRateModel Interface
  * @author Venus
  */
-contract InterestRateModel {
+abstract contract InterestRateModel {
     /// @notice Indicator that this is an InterestRateModel contract (for inspection)
     bool public constant isInterestRateModel = true;
 
@@ -15,7 +15,7 @@ contract InterestRateModel {
      * @param reserves The total amnount of reserves the market has
      * @return The borrow rate per block (as a percentage, and scaled by 1e18)
      */
-    function getBorrowRate(uint cash, uint borrows, uint reserves) external view returns (uint);
+    function getBorrowRate(uint cash, uint borrows, uint reserves) external view virtual returns (uint);
 
     /**
      * @notice Calculates the current supply interest rate per block
@@ -30,5 +30,5 @@ contract InterestRateModel {
         uint borrows,
         uint reserves,
         uint reserveFactorMantissa
-    ) external view returns (uint);
+    ) external view virtual returns (uint);
 }

contracts/InterestRateModels/JumpRateModel.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 import "../Utils/SafeMath.sol";
 import "./InterestRateModel.sol";
@@ -76,7 +76,7 @@ contract JumpRateModel is InterestRateModel {
      * @param reserves The amount of reserves in the market
      * @return The borrow rate percentage per block as a mantissa (scaled by 1e18)
      */
-    function getBorrowRate(uint cash, uint borrows, uint reserves) public view returns (uint) {
+    function getBorrowRate(uint cash, uint borrows, uint reserves) public view override returns (uint) {
         uint util = utilizationRate(cash, borrows, reserves);
 
         if (util <= kink) {
@@ -101,7 +101,7 @@ contract JumpRateModel is InterestRateModel {
         uint borrows,
         uint reserves,
         uint reserveFactorMantissa
-    ) public view returns (uint) {
+    ) public view override returns (uint) {
         uint oneMinusReserveFactor = uint(1e18).sub(reserveFactorMantissa);
         uint borrowRate = getBorrowRate(cash, borrows, reserves);
         uint rateToPool = borrowRate.mul(oneMinusReserveFactor).div(1e18);

contracts/InterestRateModels/WhitePaperInterestRateModel.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 import "../Utils/SafeMath.sol";
 import "./InterestRateModel.sol";
@@ -63,7 +63,7 @@ contract WhitePaperInterestRateModel is InterestRateModel {
      * @param reserves The amount of reserves in the market
      * @return The borrow rate percentage per block as a mantissa (scaled by 1e18)
      */
-    function getBorrowRate(uint cash, uint borrows, uint reserves) public view returns (uint) {
+    function getBorrowRate(uint cash, uint borrows, uint reserves) public view override returns (uint) {
         uint ur = utilizationRate(cash, borrows, reserves);
         return ur.mul(multiplierPerBlock).div(1e18).add(baseRatePerBlock);
     }
@@ -81,7 +81,7 @@ contract WhitePaperInterestRateModel is InterestRateModel {
         uint borrows,
         uint reserves,
         uint reserveFactorMantissa
-    ) public view returns (uint) {
+    ) public view override returns (uint) {
         uint oneMinusReserveFactor = uint(1e18).sub(reserveFactorMantissa);
         uint borrowRate = getBorrowRate(cash, borrows, reserves);
         uint rateToPool = borrowRate.mul(oneMinusReserveFactor).div(1e18);

contracts/Lens/ComptrollerLens.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 pragma experimental ABIEncoderV2;
 
 import "../Tokens/VTokens/VBep20.sol";

contracts/Oracle/PriceOracle.sol

@@ -1,8 +1,8 @@
-pragma solidity ^0.5.16;
+pragma solidity ^0.8.13;
 
 import "../Tokens/VTokens/VToken.sol";
 
-contract PriceOracle {
+abstract contract PriceOracle {
     /// @notice Indicator that this is a PriceOracle contract (for inspection)
     bool public constant isPriceOracle = true;
 
@@ -12,5 +12,5 @@ contract PriceOracle {
      * @return The underlying asset price mantissa (scaled by 1e18).
      *  Zero means the price is unavailable.
      */
-    function getUnderlyingPrice(VToken vToken) external view returns (uint);
+    function getUnderlyingPrice(VToken vToken) external view virtual returns (uint);
 }

contracts/Utils/Context.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 /*
  * @dev Provides information about the current execution context, including the
@@ -10,13 +10,13 @@ pragma solidity ^0.5.16;
  *
  * This contract is only required for intermediate, library-like contracts.
  */
-contract Context {
+abstract contract Context {
     // Empty internal constructor, to prevent people from mistakenly deploying
     // an instance of this contract, which should be used via inheritance.
     constructor() internal {}
 
     function _msgSender() internal view returns (address payable) {
-        return msg.sender;
+        return payable(msg.sender);
     }
 
     function _msgData() internal view returns (bytes memory) {

contracts/VRTVault/VRTVaultProxy.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 import "./VRTVaultStorage.sol";
 
@@ -62,7 +62,7 @@ contract VRTVaultProxy is VRTVaultAdminStorage {
         (bool success, bytes memory returnData) = callee.delegatecall(data);
         assembly {
             if eq(success, 0) {
-                revert(add(returnData, 0x20), returndatasize)
+                revert(add(returnData, 0x20), returndatasize())
             }
         }
         return returnData;
@@ -149,20 +149,20 @@ contract VRTVaultProxy is VRTVaultAdminStorage {
      * It returns to the external caller whatever the implementation returns
      * or forwards reverts.
      */
-    function() external payable {
+    fallback() external payable {
         // delegate all other functions to current implementation
         (bool success, ) = implementation.delegatecall(msg.data);
 
         assembly {
             let free_mem_ptr := mload(0x40)
-            returndatacopy(free_mem_ptr, 0, returndatasize)
+            returndatacopy(free_mem_ptr, 0, returndatasize())
 
             switch success
             case 0 {
-                revert(free_mem_ptr, returndatasize)
+                revert(free_mem_ptr, returndatasize())
             }
             default {
-                return(free_mem_ptr, returndatasize)
+                return(free_mem_ptr, returndatasize())
             }
         }
     }

contracts/XVSVault/XVSVaultProxy.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.16;
+pragma solidity 0.8.13;
 
 import "./XVSVaultStorage.sol";
 import "./XVSVaultErrorReporter.sol";
@@ -125,20 +125,20 @@ contract XVSVaultProxy is XVSVaultAdminStorage, XVSVaultErrorReporter {
      * It returns to the external caller whatever the implementation returns
      * or forwards reverts.
      */
-    function() external payable {
+    fallback() external payable {
         // delegate all other functions to current implementation
         (bool success, ) = implementation.delegatecall(msg.data);
 
         assembly {
             let free_mem_ptr := mload(0x40)
-            returndatacopy(free_mem_ptr, 0, returndatasize)
+            returndatacopy(free_mem_ptr, 0, returndatasize())
 
             switch success
             case 0 {
-                revert(free_mem_ptr, returndatasize)
+                revert(free_mem_ptr, returndatasize())
             }
             default {
-                return(free_mem_ptr, returndatasize)
+                return(free_mem_ptr, returndatasize())
             }
         }
     }