MockERC20 token that implements fee-on transfer.

MockERC20Fee.sol

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;
import "solmate/tokens/ERC20.sol";

contract MockERC20Fee is ERC20 {
    // 2% fee for all transfers
    uint256 internal feeRate = (2 / 100) * 1 ether; 

    constructor(
        string memory name_,
        string memory symbol_,
        uint8 decimals_
    ) ERC20(name_, symbol_, decimals_) {}

    // Expose external mint function
    function mint(address to, uint256 amount) external {
        _mint(to, amount);
    }

    function transfer(address to, uint256 amount) public override returns (bool) {
        balanceOf[msg.sender] -= amount;
        uint256 fee = (amount * feeRate) / 1 ether;

        // Cannot overflow because the sum of all user
        // balances can't exceed the max uint256 value.
        unchecked {
            balanceOf[to] += amount - fee;
        }

        emit Transfer(msg.sender, to, amount);

        return true;
    }

    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) public override returns (bool) {
        uint256 allowed = allowance[from][msg.sender]; // Saves gas for limited approvals.

        if (allowed != type(uint256).max) allowance[from][msg.sender] = allowed - amount;

        balanceOf[from] -= amount;

        // Cannot overflow because the sum of all user
        // balances can't exceed the max uint256 value.
        uint256 fee = (amount * feeRate) / 1 ether;
        unchecked {
            balanceOf[to] += amount - fee;
        }

        emit Transfer(from, to, amount);

        return true;
    }
}