No changes from the release candidate for this one, we're ironing out the kinks in the release process! 🚫 🐛

This minor release includes a way to store token balances and supply so that they can be later queried in a gas-efficient manner 🔖, allows safe interaction with some old, non-compliant tokens 🔒, prevents user errors when using ECDSA signatures 📝 (the magic behind metatransactions! ✨), and provides multiple minor additions and improvements to the API.

To install the release run npm install openzeppelin-solidity@latest.

We would love your help by reviewing newly added contracts, their interface and documentation so that we can make names clearer, features easier to use, and the library better as a whole! Your feedback is extremely useful to us :)

Highlights

New features

• ERC20Snapshot: this variant allows for snapshots to be created on demand, storing the current token balances and total supply so that they can be later retrieved in a gas-efficient manner and e.g. calculate dividends at a past time. (#1617)
• SafeERC20: the ERC20 standard requires that all function calls (e.g. transfer, approve, etc.) return a boolean value indicating success. However, they are multiple widely used tokens out there that return no such value: they simply revert when encountering an error condition. Since Solidity v0.4.22, special code was needed to interact with this non-compliant tokens: now, all of SafeERC20 can be used to safely call both compliant and non-compliant tokens, without the developer having to worry about it. (#1655)
• TimedCrowdsale: an internal _extendTime(uint256 newClosingTime) function was added (with a corresponding TimedCrowdsaleExtended(uint256 prevClosingTime, uint256 newClosingTime) event) allowing for users to safely develop mechanisms to extend the durations of unclosed crowdsales. Note that due to it being internal, there's no out-of-the-box way to do it: this feature is opt-in and must be explicitly invoked by users.

Improvements

• ECDSA: recover no longer accepts malleable signatures (those using upper-range values for s, or 0/1 for v). This helps prevent multiple issues when using signatures as unique identifiers. Read more about common ECDSA issues here. (#1622)
• ERC721's transfers are now more gas efficient due to removal of unnecessary SafeMath calls. (#1610)

Bugfixes:

• (minor) SafeERC20: safeApprove wasn't properly checking for a zero allowance when attempting to set a non-zero allowance. This bug was reported independently by @nikeshnazareth. Thanks a lot! (#1647)

Breaking changes in drafts:

• TokenMetadata has been renamed to ERC20Metadata. (#1618)
• The library Counter has been renamed to Counters and its API has been improved. See an example in ERC721, lines 17 and 204. (#1610)

You can also see all details of this release in our changelog.