ProxyERC20.sol

pragma solidity ^0.5.16;

// Inheritance
import "./Proxy.sol";
import "./interfaces/IERC20.sol";

// https://docs.synthetix.io/contracts/source/contracts/proxyerc20
contract ProxyERC20 is Proxy, IERC20 {
    constructor(address _owner) public Proxy(_owner) {}

    // ------------- ERC20 Details ------------- //

    function name() public view returns (string memory) {
        // Immutable static call from target contract
        return IERC20(address(target)).name();
    }

    function symbol() public view returns (string memory) {
        // Immutable static call from target contract
        return IERC20(address(target)).symbol();
    }

    function decimals() public view returns (uint8) {
        // Immutable static call from target contract
        return IERC20(address(target)).decimals();
    }

    // ------------- ERC20 Interface ------------- //

    /**
     * @dev Total number of tokens in existence
     */
    function totalSupply() public view returns (uint256) {
        // Immutable static call from target contract
        return IERC20(address(target)).totalSupply();
    }

    /**
     * @dev Gets the balance of the specified address.
     * @param account The address to query the balance of.
     * @return An uint256 representing the amount owned by the passed address.
     */
    function balanceOf(address account) public view returns (uint256) {
        // Immutable static call from target contract
        return IERC20(address(target)).balanceOf(account);
    }

    /**
     * @dev Function to check the amount of tokens that an owner allowed to a spender.
     * @param owner address The address which owns the funds.
     * @param spender address The address which will spend the funds.
     * @return A uint256 specifying the amount of tokens still available for the spender.
     */
    function allowance(address owner, address spender) public view returns (uint256) {
        // Immutable static call from target contract
        return IERC20(address(target)).allowance(owner, spender);
    }

    /**
     * @dev Transfer token for a specified address
     * @param to The address to transfer to.
     * @param value The amount to be transferred.
     */
    function transfer(address to, uint256 value) public returns (bool) {
        // Mutable state call requires the proxy to tell the target who the msg.sender is.
        target.setMessageSender(msg.sender);

        // Forward the ERC20 call to the target contract
        IERC20(address(target)).transfer(to, value);

        // Event emitting will occur via Synthetix.Proxy._emit()
        return true;
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
     * Beware that changing an allowance with this method brings the risk that someone may use both the old
     * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
     * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     * @param spender The address which will spend the funds.
     * @param value The amount of tokens to be spent.
     */
    function approve(address spender, uint256 value) public returns (bool) {
        // Mutable state call requires the proxy to tell the target who the msg.sender is.
        target.setMessageSender(msg.sender);

        // Forward the ERC20 call to the target contract
        IERC20(address(target)).approve(spender, value);

        // Event emitting will occur via Synthetix.Proxy._emit()
        return true;
    }

    /**
     * @dev Transfer tokens from one address to another
     * @param from address The address which you want to send tokens from
     * @param to address The address which you want to transfer to
     * @param value uint256 the amount of tokens to be transferred
     */
    function transferFrom(
        address from,
        address to,
        uint256 value
    ) public returns (bool) {
        // Mutable state call requires the proxy to tell the target who the msg.sender is.
        target.setMessageSender(msg.sender);

        // Forward the ERC20 call to the target contract
        IERC20(address(target)).transferFrom(from, to, value);

        // Event emitting will occur via Synthetix.Proxy._emit()
        return true;
    }
}