Upgrade contracts to solc-6

contracts/EtherToken.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 import "./GnosisStandardToken.sol";
 
 /// @title Token contract - Token exchanging Ether 1:1

contracts/GnosisStandardToken.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 import "./Token.sol";
 import "./Math.sol";
 import "./Proxy.sol";
@@ -29,7 +30,7 @@ contract GnosisStandardToken is Token, StandardTokenData {
     /// @param to Address of token receiver
     /// @param value Number of tokens to transfer
     /// @return Was transfer successful?
-    function transfer(address to, uint value) public returns (bool) {
+    function transfer(address to, uint value) public override returns (bool) {
         if (!balances[msg.sender].safeToSub(value) || !balances[to].safeToAdd(value)) {
             return false;
         }
@@ -45,7 +46,7 @@ contract GnosisStandardToken is Token, StandardTokenData {
     /// @param to Address to where tokens are sent
     /// @param value Number of tokens to transfer
     /// @return Was transfer successful?
-    function transferFrom(address from, address to, uint value) public returns (bool) {
+    function transferFrom(address from, address to, uint value) public override returns (bool) {
         if (!balances[from].safeToSub(value) || !allowances[from][msg.sender].safeToSub(
             value
         ) || !balances[to].safeToAdd(value)) {
@@ -62,7 +63,7 @@ contract GnosisStandardToken is Token, StandardTokenData {
     /// @param spender Address of allowed account
     /// @param value Number of approved tokens
     /// @return Was approval successful?
-    function approve(address spender, uint value) public returns (bool) {
+    function approve(address spender, uint value) public override returns (bool) {
         allowances[msg.sender][spender] = value;
         emit Approval(msg.sender, spender, value);
         return true;
@@ -72,20 +73,20 @@ contract GnosisStandardToken is Token, StandardTokenData {
     /// @param owner Address of token owner
     /// @param spender Address of token spender
     /// @return Remaining allowance for spender
-    function allowance(address owner, address spender) public view returns (uint) {
+    function allowance(address owner, address spender) public override view returns (uint) {
         return allowances[owner][spender];
     }
 
     /// @dev Returns number of tokens owned by given address
     /// @param owner Address of token owner
     /// @return Balance of owner
-    function balanceOf(address owner) public view returns (uint) {
+    function balanceOf(address owner) public override view returns (uint) {
         return balances[owner];
     }
 
     /// @dev Returns total supply of tokens
     /// @return Total supply
-    function totalSupply() public view returns (uint) {
+    function totalSupply() public override view returns (uint) {
         return totalTokens;
     }
 }

contracts/HumanFriendlyToken.sol

@@ -1,14 +1,16 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+
 /// Implements ERC 20 Token standard including extra accessors for human readability.
 /// See: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md
-pragma solidity ^0.5.2;
+pragma solidity ^0.6.0;
 import "./Token.sol";
 
 /// @title Abstract human-friendly token contract - Functions to be implemented by token contracts
-contract HumanFriendlyToken is Token {
+abstract contract HumanFriendlyToken is Token {
     /*
      *  Public functions
      */
-    function name() public view returns (string memory);
-    function symbol() public view returns (string memory);
-    function decimals() public view returns (uint8);
+    function name() public virtual view returns (string memory);
+    function symbol() public virtual view returns (string memory);
+    function decimals() public virtual view returns (uint8);
 }

contracts/Math.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 
 /// @title Math library - Allows calculation of logarithmic and exponential functions
 /// @author Alan Lu - <[email protected]>
@@ -137,7 +138,7 @@ library GnosisMath {
 
     /// @dev Returns base 2 logarithm value of given x
     /// @param x x
-    /// @return logarithmic value
+    /// @return lo - logarithmic value
     function floorLog2(uint x) public pure returns (int lo) {
         lo = -64;
         int hi = 193;
@@ -152,7 +153,7 @@ library GnosisMath {
 
     /// @dev Returns maximum of an array
     /// @param nums Numbers to look through
-    /// @return Maximum number
+    /// @return maxNum - Maximum number
     function max(int[] memory nums) public pure returns (int maxNum) {
         require(nums.length > 0);
         maxNum = -2 ** 255;

contracts/Migrations.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 
 contract Migrations {
     address public owner;

contracts/Proxy.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 
 /// @title Proxied - indicates that a contract will be proxied. Also defines storage requirements for Proxy.
 /// @author Alan Lu - <[email protected]>
@@ -17,19 +18,35 @@ contract Proxy is Proxied {
     }
 
     /// @dev Fallback function forwards all transactions and returns all received return data.
-    function() external payable {
+    function _fallback() internal {
         address _masterCopy = masterCopy;
         assembly {
-            calldatacopy(0, 0, calldatasize)
-            let success := delegatecall(not(0), _masterCopy, 0, calldatasize, 0, 0)
-            returndatacopy(0, 0, returndatasize)
+            calldatacopy(0, 0, calldatasize())
+            let success := delegatecall(not(0), _masterCopy, 0, calldatasize(), 0, 0)
+            returndatacopy(0, 0, returndatasize())
             switch success
                 case 0 {
-                    revert(0, returndatasize)
+                    revert(0, returndatasize())
                 }
                 default {
-                    return(0, returndatasize)
+                    return(0, returndatasize())
                 }
         }
     }
+
+    /**
+     * @dev Fallback function that delegates calls to the masterCopy. 
+     * Runs when no other function in the contract matches the call data.
+     */
+    fallback () external payable {
+        _fallback();
+    }
+
+    /**
+     * @dev Fallback function that receives ether and delegates calls to masterCopy. 
+     * Runs when call data is empty.
+     */
+    receive () external payable {
+        _fallback();
+    }
 }

contracts/StorageAccessible.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 
 /// @title StorageAccessible - generic base contract that allows callers to access all internal storage.
 contract StorageAccessible {

contracts/Token.sol

@@ -1,8 +1,9 @@
+// SPDX-License-Identifier: LGPL-3.0-only
 /// Implements ERC 20 Token standard: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md
-pragma solidity ^0.5.2;
+pragma solidity ^0.6.0;
 
 /// @title Abstract token contract - Functions to be implemented by token contracts
-contract Token {
+abstract contract Token {
     /*
      *  Events
      */
@@ -12,10 +13,10 @@ contract Token {
     /*
      *  Public functions
      */
-    function transfer(address to, uint value) public returns (bool);
-    function transferFrom(address from, address to, uint value) public returns (bool);
-    function approve(address spender, uint value) public returns (bool);
-    function balanceOf(address owner) public view returns (uint);
-    function allowance(address owner, address spender) public view returns (uint);
-    function totalSupply() public view returns (uint);
+    function transfer(address to, uint value) public virtual returns (bool);
+    function transferFrom(address from, address to, uint value) public virtual returns (bool);
+    function approve(address spender, uint value) public virtual returns (bool);
+    function balanceOf(address owner) public virtual view returns (uint);
+    function allowance(address owner, address spender) public virtual view returns (uint);
+    function totalSupply() public virtual view returns (uint);
 }

contracts/test/StorageAccessibleWrapper.sol

@@ -1,4 +1,5 @@
-pragma solidity ^0.5.2;
+// SPDX-License-Identifier: LGPL-3.0-only
+pragma solidity ^0.6.0;
 import "../StorageAccessible.sol";
 
 contract StorageAccessibleWrapper is StorageAccessible {

package.json

@@ -45,11 +45,11 @@
     "eslint-plugin-standard": "^4.0.0",
     "prettier": "^1.15.3",
     "prettier-plugin-solidity-refactor": "^1.0.0-alpha.12",
-    "solc": "0.5.8",
-    "truffle": "^5.0.2",
-    "@truffle/contract": "^4.1.2"
+    "solc": "0.6.12",
+    "truffle": "^5.1.50"
   },
   "dependencies": {
-    "@truffle/hdwallet-provider": "^1.0.42"
+    "@truffle/hdwallet-provider": "^1.0.42",
+    "truffle-assertions": "^0.9.2"
   }
 }

truffle.js

@@ -16,7 +16,7 @@ if (!privateKey && !mnemonic) {
 
 // Solc
 const solcUseDocker = process.env.SOLC_USE_DOCKER === 'true' || false
-const solcVersion = '0.5.2'
+const solcVersion = '0.6.12'
 
 // Gas price
 const gasPriceGWei = process.env.GAS_PRICE_GWEI || DEFAULT_GAS_PRICE_GWEI