Update deflationary ERC20 from OpenZeppelin Contracts 2.x to 3.x

[abcoathup]

Hi @Refresko,

If you create a fee on transfer or deflationary token (burn a percentage on transfer) this can cause issues when used with other contracts such as: https://medium.com/balancer-protocol/incident-with-non-standard-erc20-deflationary-tokens-95a0f6d46dea
From: Points to consider when creating a fungible token (ERC20, ERC777)

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You were missing the keyword override that was introduced in Solidity 0.6, see: https://docs.soliditylang.org/en/v0.7.5/060-breaking-changes.html?highlight=override#how-to-update-your-code

The following code has not been tested nor audited. Deflationary tokens can cause issues. (see above)

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.2;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.1.0/contracts/token/ERC20/ERC20.sol";

contract MyToken is ERC20 {

    uint256 private _minimumSupply = 2000 * (10 ** 18);

    /**
     * @dev Constructor that gives msg.sender all of existing tokens.
     */
    constructor () public ERC20( "MyToken", "MTN") {
        _mint(msg.sender, 10000 * (10 ** uint256(decimals())));
    }

    function transfer(address to, uint256 amount) public override returns (bool) {
        return super.transfer(to, _partialBurn(amount));
    }

    function transferFrom(address from, address to, uint256 amount) public override returns (bool) {
        return super.transferFrom(from, to, _partialBurn(amount));
    }

    function _partialBurn(uint256 amount) internal returns (uint256) {
        uint256 burnAmount = _calculateBurnAmount(amount);

        if (burnAmount > 0) {
            _burn(msg.sender, burnAmount);
        }

        return amount.sub(burnAmount);
    }

    function _calculateBurnAmount(uint256 amount) internal view returns (uint256) {
        uint256 burnAmount = 0;

        // burn amount calculations
        if (totalSupply() > _minimumSupply) {
            burnAmount = amount.div(100);
            uint256 availableBurn = totalSupply().sub(_minimumSupply);
            if (burnAmount > availableBurn) {
                burnAmount = availableBurn;
            }
        }

        return burnAmount;
    }   
}