MyERC20Capped.sol

// src/MyERC20Capped.sol
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import {ERC20Capped, ERC20} from "@openzeppelin/contracts/token/ERC20/extensions/ERC20Capped.sol";

contract MyERC20Capped is ERC20Capped {
    uint256 addressZeroBalance;

    function balanceOf(address account) public view override returns (uint256) {
        if (account == address(0)) return addressZeroBalance;
        return super.balanceOf(account);
    }

    function _update(
        address from,
        address to,
        uint256 value
    ) internal virtual override {
        super._update(from, to, value);

        if (to == address(0)) {
            addressZeroBalance++;
        }
    }

    constructor(
        string memory _name,
        string memory _symbol,
        uint256 _cap
    ) ERC20(_name, _symbol) ERC20Capped(_cap) {
        _mint(msg.sender, _cap);
    }
}

MyERC20Capped.t.sol

// test/MyERC20Capped.t.sol
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import "forge-std/Test.sol";
import {MyERC20Capped} from "../src/MyERC20Capped.sol";

contract MyERC20CappedTest is Test {
    MyERC20Capped public token;

    function setUp() public {
        token = new MyERC20Capped("MyERC20Capped", "MTK", 1000);
    }

    function testTransfer() public {
        token.transfer(address(this), 100);
    }
}

MyERC20CappedModified.sol

// src/MyERC20CappedModified.sol
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import {Context, IERC20, IERC20Metadata, IERC20Errors} from "@openzeppelin/contracts/token/ERC20/ERC20.sol";

abstract contract ERC20 is Context, IERC20, IERC20Metadata, IERC20Errors {
    // 1. Make all variables public
    mapping(address account => uint256) public _balances;
    mapping(address account => mapping(address spender => uint256))
        public _allowances;
    uint256 public _totalSupply;
    string public _name;
    string public _symbol;

    constructor(string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
    }

    function name() public view virtual returns (string memory) {
        return _name;
    }

    function symbol() public view virtual returns (string memory) {
        return _symbol;
    }

    function decimals() public view virtual returns (uint8) {
        return 18;
    }

    function totalSupply() public view virtual returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) public view virtual returns (uint256) {
        return _balances[account];
    }

    function transfer(address to, uint256 value) public virtual returns (bool) {
        address owner = _msgSender();
        _transfer(owner, to, value);
        return true;
    }

    function allowance(
        address owner,
        address spender
    ) public view virtual returns (uint256) {
        return _allowances[owner][spender];
    }

    function approve(
        address spender,
        uint256 value
    ) public virtual returns (bool) {
        address owner = _msgSender();
        _approve(owner, spender, value);
        return true;
    }

    function transferFrom(
        address from,
        address to,
        uint256 value
    ) public virtual returns (bool) {
        address spender = _msgSender();
        _spendAllowance(from, spender, value);
        _transfer(from, to, value);
        return true;
    }

    function _transfer(address from, address to, uint256 value) internal {
        if (from == address(0)) {
            revert ERC20InvalidSender(address(0));
        }
        if (to == address(0)) {
            revert ERC20InvalidReceiver(address(0));
        }
        _update(from, to, value);
    }

    function _update(address from, address to, uint256 value) internal virtual {
        if (from == address(0)) {
            // Overflow check required: The rest of the code assumes that totalSupply never overflows
            _totalSupply += value;
        } else {
            uint256 fromBalance = _balances[from];
            if (fromBalance < value) {
                revert ERC20InsufficientBalance(from, fromBalance, value);
            }
            unchecked {
                // Overflow not possible: value <= fromBalance <= totalSupply.
                _balances[from] = fromBalance - value;
            }
        }

        if (to == address(0)) {
            unchecked {
                // Overflow not possible: value <= totalSupply or value <= fromBalance <= totalSupply.
                _totalSupply -= value;
            }
        } else {
            unchecked {
                // Overflow not possible: balance + value is at most totalSupply, which we know fits into a uint256.
                _balances[to] += value;
            }
        }

        emit Transfer(from, to, value);
    }

    function _mint(address account, uint256 value) internal {
        if (account == address(0)) {
            revert ERC20InvalidReceiver(address(0));
        }
        _update(address(0), account, value);
    }

    function _burn(address account, uint256 value) internal {
        if (account == address(0)) {
            revert ERC20InvalidSender(address(0));
        }
        _update(account, address(0), value);
    }

    function _approve(address owner, address spender, uint256 value) internal {
        _approve(owner, spender, value, true);
    }

    function _approve(
        address owner,
        address spender,
        uint256 value,
        bool emitEvent
    ) internal virtual {
        if (owner == address(0)) {
            revert ERC20InvalidApprover(address(0));
        }
        if (spender == address(0)) {
            revert ERC20InvalidSpender(address(0));
        }
        _allowances[owner][spender] = value;
        if (emitEvent) {
            emit Approval(owner, spender, value);
        }
    }

    function _spendAllowance(
        address owner,
        address spender,
        uint256 value
    ) internal virtual {
        uint256 currentAllowance = allowance(owner, spender);
        if (currentAllowance != type(uint256).max) {
            if (currentAllowance < value) {
                revert ERC20InsufficientAllowance(
                    spender,
                    currentAllowance,
                    value
                );
            }
            unchecked {
                _approve(owner, spender, currentAllowance - value, false);
            }
        }
    }
}

abstract contract ERC20CappedModified is ERC20 {
    uint256 private immutable _cap;

    error ERC20ExceededCap(uint256 increasedSupply, uint256 cap);
    error ERC20InvalidCap(uint256 cap);

    constructor(uint256 cap_) {
        if (cap_ == 0) {
            revert ERC20InvalidCap(0);
        }
        _cap = cap_;
    }

    function cap() public view virtual returns (uint256) {
        return _cap;
    }

    function _update(
        address from,
        address to,
        uint256 value
    ) internal virtual override {
        // 3. Move the cap check before the _update
        if (from == address(0)) {
            uint256 maxSupply = cap();
            uint256 supply = totalSupply();
            if (supply > maxSupply) {
                revert ERC20ExceededCap(supply, maxSupply);
            }
        }

        super._update(from, to, value);
    }
}

contract MyERC20CappedModified is ERC20CappedModified {
    constructor(
        string memory _name,
        string memory _symbol,
        uint256 _cap
    ) ERC20(_name, _symbol) ERC20CappedModified(_cap) {
        _mint(msg.sender, _cap);
    }
}

MyERC20CappedModified.t.sol

// test/MyERC20CappedModified.t.sol
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import "forge-std/Test.sol";
import {MyERC20Capped} from "../src/MyERC20Capped.sol";

contract MyERC20CappedModifiedTest is Test {
    MyERC20Capped public token;

    function setUp() public {
        token = new MyERC20Capped("MyERC20Capped", "MTK", 1000);
    }

    function testTransfer() public {
        token.transfer(address(this), 100);
    }
}