update solidity to 0.7.6

contracts/ERC20Tornado.sol

@@ -10,7 +10,7 @@
  */
 
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "./Tornado.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
@@ -26,7 +26,7 @@ contract ERC20Tornado is Tornado {
     uint256 _denomination,
     uint32 _merkleTreeHeight,
     IERC20 _token
-  ) public Tornado(_verifier, _hasher, _denomination, _merkleTreeHeight) {
+  ) Tornado(_verifier, _hasher, _denomination, _merkleTreeHeight) {
     token = _token;
   }
 

contracts/ETHTornado.sol

@@ -10,7 +10,7 @@
  */
 
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "./Tornado.sol";
 
@@ -20,7 +20,7 @@ contract ETHTornado is Tornado {
     IHasher _hasher,
     uint256 _denomination,
     uint32 _merkleTreeHeight
-  ) public Tornado(_verifier, _hasher, _denomination, _merkleTreeHeight) {}
+  ) Tornado(_verifier, _hasher, _denomination, _merkleTreeHeight) {}
 
   function _processDeposit() internal override {
     require(msg.value == denomination, "Please send `mixDenomination` ETH along with transaction");

contracts/MerkleTreeWithHistory.sol

@@ -10,7 +10,7 @@
  */
 
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 interface IHasher {
   function MiMCSponge(uint256 in_xL, uint256 in_xR) external pure returns (uint256 xL, uint256 xR);
@@ -35,7 +35,7 @@ contract MerkleTreeWithHistory {
   uint32 public currentRootIndex = 0;
   uint32 public nextIndex = 0;
 
-  constructor(uint32 _levels, IHasher _hasher) public {
+  constructor(uint32 _levels, IHasher _hasher) {
     require(_levels > 0, "_levels should be greater than zero");
     require(_levels < 32, "_levels should be less than 32");
     levels = _levels;

contracts/Mocks/BadRecipient.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 contract BadRecipient {
   fallback() external {

contracts/Mocks/ERC20Mock.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 

contracts/Mocks/IDeployer.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 interface IDeployer {
   function deploy(bytes memory _initCode, bytes32 _salt) external returns (address payable createdContract);

contracts/Mocks/IUSDT.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 interface ERC20Basic {
   function _totalSupply() external returns (uint256);

contracts/Mocks/MerkleTreeWithHistoryMock.sol

@@ -1,10 +1,10 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../MerkleTreeWithHistory.sol";
 
 contract MerkleTreeWithHistoryMock is MerkleTreeWithHistory {
-  constructor(uint32 _treeLevels, IHasher _hasher) public MerkleTreeWithHistory(_treeLevels, _hasher) {}
+  constructor(uint32 _treeLevels, IHasher _hasher) MerkleTreeWithHistory(_treeLevels, _hasher) {}
 
   function insert(bytes32 _leaf) public {
     _insert(_leaf);

contracts/Tornado.sol

@@ -10,7 +10,7 @@
  */
 
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "./MerkleTreeWithHistory.sol";
 import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
@@ -42,7 +42,7 @@ abstract contract Tornado is MerkleTreeWithHistory, ReentrancyGuard {
     IHasher _hasher,
     uint256 _denomination,
     uint32 _merkleTreeHeight
-  ) public MerkleTreeWithHistory(_merkleTreeHeight, _hasher) {
+  ) MerkleTreeWithHistory(_merkleTreeHeight, _hasher) {
     require(_denomination > 0, "denomination should be greater than 0");
     verifier = _verifier;
     denomination = _denomination;

contracts/Verifier.sol

@@ -32,7 +32,7 @@
 
 // 2019 OKIMS
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 library Pairing {
   uint256 constant PRIME_Q = 21888242871839275222246405745257275088696311157297823662689037894645226208583;

contracts/cTornado.sol

@@ -10,7 +10,7 @@
  */
 
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "./ERC20Tornado.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
@@ -26,7 +26,7 @@ contract cTornado is ERC20Tornado {
     uint256 _denomination,
     uint32 _merkleTreeHeight,
     IERC20 _token
-  ) public ERC20Tornado(_verifier, _hasher, _denomination, _merkleTreeHeight, _token) {
+  ) ERC20Tornado(_verifier, _hasher, _denomination, _merkleTreeHeight, _token) {
     require(address(_comp) != address(0), "Invalid COMP token address");
     comp = _comp;
   }

package.json

@@ -30,7 +30,7 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
-    "@openzeppelin/contracts": "^3.4.0",
+    "@openzeppelin/contracts": "^3.4.1",
     "@truffle/contract": "^4.0.39",
     "@truffle/hdwallet-provider": "^1.0.24",
     "axios": "^0.19.0",

truffle-config.js

@@ -107,7 +107,7 @@ module.exports = {
   // Configure your compilers
   compilers: {
     solc: {
-      version: '0.6.12', // Fetch exact version from solc-bin (default: truffle's version)
+      version: '0.7.6', // Fetch exact version from solc-bin (default: truffle's version)
       // docker: true,        // Use "0.5.1" you've installed locally with docker (default: false)
       settings: {
         // See the solidity docs for advice about optimization and evmVersion

yarn.lock

@@ -1127,10 +1127,10 @@
     "@nodelib/fs.scandir" "2.1.4"
     fastq "^1.6.0"
 
-"@openzeppelin/contracts@^3.4.0":
-  version "3.4.0"
-  resolved "https://registry.yarnpkg.com/@openzeppelin/contracts/-/contracts-3.4.0.tgz#9a1669ad5f9fdfb6e273bb5a4fed10cb4cc35eb0"
-  integrity sha512-qh+EiHWzfY/9CORr+eRUkeEUP1WiFUcq3974bLHwyYzLBUtK6HPaMkIUHi74S1rDTZ0sNz42DwPc5A4IJvN3rg==
+"@openzeppelin/contracts@^3.4.1":
+  version "3.4.1"
+  resolved "https://registry.yarnpkg.com/@openzeppelin/contracts/-/contracts-3.4.1.tgz#03c891fec7f93be0ae44ed74e57a122a38732ce7"
+  integrity sha512-cUriqMauq1ylzP2TxePNdPqkwI7Le3Annh4K9rrpvKfSBB/bdW+Iu1ihBaTIABTAAJ85LmKL5SSPPL9ry8d1gQ==
 
 "@protobufjs/aspromise@^1.1.1", "@protobufjs/aspromise@^1.1.2":
   version "1.1.2"