This is copied from their ERC20 contract from github (https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol):

function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
    _approve(_msgSender(), spender, _allowances[_msgSender()] + addedValue);
    return true;

function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
    uint256 currentAllowance = _allowances[_msgSender()][spender];
    require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
    unchecked {
        _approve(_msgSender(), spender, currentAllowance - subtractedValue);
return true;

Why would they not be implemented the same way?

1 Answer 1


In PR #2669 where those changes were made they mention Issue #2665. In that issue a comment by vladyan18 points that using unchecked reduces gas usage at deployment by 1723.

  • Thanks for the info. I think the other ideas that switching the position of a condition check conditions reduces gas cost are interesting. May 28, 2021 at 19:05
  • @LauriPeltonen It is a little odd but is isn't surprising because solidity developers are usually cautious about optimizations.
    – Ismael
    May 28, 2021 at 20:03

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