CHANGELOG.md

Changelog

Unreleased

• crosschain: Add a new set of contracts for cross-chain applications. CrossChainEnabled is a base contract with instantiations for several chains and bridges, and AccessControlCrossChain is an extension of access control that allows cross-chain operation. (#3183)
• AccessControl: add a virtual _checkRole(bytes32) function that can be overridden to alter the onlyRole modifier behavior. (#3137)
• EnumerableMap: add new AddressToUintMap map type. (#3150)
• EnumerableMap: add new Bytes32ToBytes32Map map type. (#3192)
• ERC20FlashMint: support infinite allowance when paying back a flash loan. (#3226)
• ERC20Wrapper: the decimals() function now tries to fetch the value from the underlying token instance. If that calls revert, then the default value is used. (#3259)
• draft-ERC20Permit: replace immutable with constant for _PERMIT_TYPEHASH since the keccak256 of string literals is treated specially and the hash is evaluated at compile time. (#3196)
• ERC1155: Add a _afterTokenTransfer hook for improved extensibility. (#3166)
• ERC1155URIStorage: add a new extension that implements a _setURI behavior similar to ERC721's _setTokenURI. (#3210)
• DoubleEndedQueue: a new data structure that supports efficient push and pop to both front and back, useful for FIFO and LIFO queues. (#3153)
• Governor: improved security of onlyGovernance modifier when using an external executor contract (e.g. a timelock) that can operate without necessarily going through the governance protocol. (#3147)
• Governor: Add a way to parameterize votes. This can be used to implement voting systems such as fractionalized voting, ERC721 based voting, or any number of other systems. The params argument added to _countVote method, and included in the newly added _getVotes method, can be used by counting and voting modules respectively for such purposes. (#3043)
• Governor: rewording of revert reason for consistency. (#3275)
• Governor: fix an inconsistency in data locations that could lead to invalid bytecode being produced. (#3295)
• Governor: Implement IERC721Receiver and IERC1155Receiver to improve token custody by governors. (#3230)
• TimelockController: Implement IERC721Receiver and IERC1155Receiver to improve token custody by timelocks. (#3230)
• TimelockController: Add a separate canceller role for the ability to cancel. (#3165)
• Initializable: add a reinitializer modifier that enables the initialization of new modules, added to already initialized contracts through upgradeability. (#3232)
• Initializable: add an Initialized event that tracks initialized version numbers. (#3294)

Breaking changes

• Governor: Adds internal virtual _getVotes method that must be implemented; this is a breaking change for existing concrete extensions to Governor. To fix this on an existing voting module extension, rename getVotes to _getVotes and add a bytes memory argument. (#3043)
• Governor: Adds params parameter to internal virtual _countVote method; this is a breaking change for existing concrete extensions to Governor. To fix this on an existing counting module extension, add a bytes memory argument to _countVote. (#3043)
• Governor: Does not emit VoteCast event when params data is non-empty; instead emits VoteCastWithParams event. To fix this on an integration that consumes the VoteCast event, also fetch/monitor VoteCastWithParams events. (#3043)
• Votes: The internal virtual function _getVotingUnits was made view (which was accidentally missing). Any overrides should now be updated so they are view as well.

4.5.0 (2022-02-09)

• ERC2981: add implementation of the royalty standard, and the respective extensions for ERC721 and ERC1155. (#3012)
• GovernorTimelockControl: improve the state() function to have it reflect cases where a proposal has been canceled directly on the timelock. (#2977)
• Preset contracts are now deprecated in favor of Contracts Wizard. (#2986)
• Governor: add a relay function to help recover assets sent to a governor that is not its own executor (e.g. when using a timelock). (#2926)
• GovernorPreventLateQuorum: add new module to ensure a minimum voting duration is available after the quorum is reached. (#2973)
• ERC721: improved revert reason when transferring from wrong owner. (#2975)
• Votes: Added a base contract for vote tracking with delegation. (#2944)
• ERC721Votes: Added an extension of ERC721 enabled with vote tracking and delegation. (#2944)
• ERC2771Context: use immutable storage to store the forwarder address, no longer an issue since Solidity >=0.8.8 allows reading immutable variables in the constructor. (#2917)
• Base64: add a library to parse bytes into base64 strings using encode(bytes memory) function, and provide examples to show how to use to build URL-safe tokenURIs. (#2884)
• ERC20: reduce allowance before triggering transfer. (#3056)
• ERC20: do not update allowance on transferFrom when allowance is type(uint256).max. (#3085)
• ERC20: add a _spendAllowance internal function. (#3170)
• ERC20Burnable: do not update allowance on burnFrom when allowance is type(uint256).max. (#3170)
• ERC777: do not update allowance on transferFrom when allowance is type(uint256).max. (#3085)
• ERC777: add a _spendAllowance internal function. (#3170)
• SignedMath: a new signed version of the Math library with max, min, and average. (#2686)
• SignedMath: add a abs(int256) method that returns the unsigned absolute value of a signed value. (#2984)
• ERC1967Upgrade: Refactor the secure upgrade to use ERC1822 instead of the previous rollback mechanism. This reduces code complexity and attack surface with similar security guarantees. (#3021)
• UUPSUpgradeable: Add ERC1822 compliance to support the updated secure upgrade mechanism. (#3021)
• Some more functions have been made virtual to customize them via overrides. In many cases this will not imply that other functions in the contract will automatically adapt to the overridden definitions. People who wish to override should consult the source code to understand the impact and if they need to override any additional functions to achieve the desired behavior.

Breaking changes

• ERC1967Upgrade: The function _upgradeToAndCallSecure was renamed to _upgradeToAndCallUUPS, along with the change in security mechanism described above.
• Address: The Solidity pragma is increased from ^0.8.0 to ^0.8.1. This is required by the account.code.length syntax that replaces inline assembly. This may require users to bump their compiler version from 0.8.0 to 0.8.1 or later. Note that other parts of the code already include stricter requirements.

4.4.2 (2022-01-11)

Bugfixes

• GovernorCompatibilityBravo: Fix error in the encoding of calldata for proposals submitted through the compatibility interface with explicit signatures. (#3100)

4.4.1 (2021-12-14)

• Initializable: change the existing initializer modifier and add a new onlyInitializing modifier to prevent reentrancy risk. (#3006)

Breaking change

It is no longer possible to call an initializer-protected function from within another initializer function outside the context of a constructor. Projects using OpenZeppelin upgradeable proxies should continue to work as is, since in the common case the initializer is invoked in the constructor directly. If this is not the case for you, the suggested change is to use the new onlyInitializing modifier in the following way:

 contract A {
-    function initialize() public   initializer { ... }
+    function initialize() internal onlyInitializing { ... }
 }
 contract B is A {
     function initialize() public initializer {
         A.initialize();
     }
 }

4.4.0 (2021-11-25)

• Ownable: add an internal _transferOwnership(address). (#2568)
• AccessControl: add internal _grantRole(bytes32,address) and _revokeRole(bytes32,address). (#2568)
• AccessControl: mark _setupRole(bytes32,address) as deprecated in favor of _grantRole(bytes32,address). (#2568)
• AccessControlEnumerable: hook into _grantRole(bytes32,address) and _revokeRole(bytes32,address). (#2946)
• EIP712: cache address(this) to immutable storage to avoid potential issues if a vanilla contract is used in a delegatecall context. (#2852)
• Add internal _setApprovalForAll to ERC721 and ERC1155. (#2834)
• Governor: shift vote start and end by one block to better match Compound's GovernorBravo and prevent voting at the Governor level if the voting snapshot is not ready. (#2892)
• GovernorCompatibilityBravo: consider quorum an inclusive rather than exclusive minimum to match Compound's GovernorBravo. (#2974)
• GovernorSettings: a new governor module that manages voting settings updatable through governance actions. (#2904)
• PaymentSplitter: now supports ERC20 assets in addition to Ether. (#2858)
• ECDSA: add a variant of toEthSignedMessageHash for arbitrary length message hashing. (#2865)
• MerkleProof: add a processProof function that returns the rebuilt root hash given a leaf and a proof. (#2841)
• VestingWallet: new contract that handles the vesting of Ether and ERC20 tokens following a customizable vesting schedule. (#2748)
• Governor: enable receiving Ether when a Timelock contract is not used. (#2748)
• GovernorTimelockCompound: fix ability to use Ether stored in the Timelock contract. (#2748)

4.3.3

• ERC1155Supply: Handle totalSupply changes by hooking into _beforeTokenTransfer to ensure consistency of balances and supply during IERC1155Receiver.onERC1155Received calls.

4.3.2 (2021-09-14)

• UUPSUpgradeable: Add modifiers to prevent upgradeTo and upgradeToAndCall being executed on any contract that is not the active ERC1967 proxy. This prevents these functions being called on implementation contracts or minimal ERC1167 clones, in particular.

4.3.1 (2021-08-26)

• TimelockController: Add additional isOperationReady check.

4.3.0 (2021-08-17)

• ERC2771Context: use private variable from storage to store the forwarder address. Fixes issues where _msgSender() was not callable from constructors. (#2754)
• EnumerableSet: add values() functions that returns an array containing all values in a single call. (#2768)
• Governor: added a modular system of Governor contracts based on GovernorAlpha and GovernorBravo. (#2672)
• Add an interfaces folder containing solidity interfaces to final ERCs. (#2517)
• ECDSA: add tryRecover functions that will not throw if the signature is invalid, and will return an error flag instead. (#2661)
• SignatureChecker: Reduce gas usage of the isValidSignatureNow function for the "signature by EOA" case. (#2661)

4.2.0 (2021-06-30)

• ERC20Votes: add a new extension of the ERC20 token with support for voting snapshots and delegation. (#2632)
• ERC20VotesComp: Variant of ERC20Votes that is compatible with Compound's Comp token interface but restricts supply to uint96. (#2706)
• ERC20Wrapper: add a new extension of the ERC20 token which wraps an underlying token. Deposit and withdraw guarantee that the total supply is backed by a corresponding amount of underlying token. (#2633)
• Enumerables: Improve gas cost of removal in EnumerableSet and EnumerableMap.
• Enumerables: Improve gas cost of lookup in EnumerableSet and EnumerableMap.
• Counter: add a reset method. (#2678)
• Tokens: Wrap definitely safe subtractions in unchecked blocks.
• Math: Add a ceilDiv method for performing ceiling division.
• ERC1155Supply: add a new ERC1155 extension that keeps track of the totalSupply of each tokenId. (#2593)
• BitMaps: add a new BitMaps library that provides a storage efficient datastructure for uint256 to bool mapping with contiguous keys. (#2710)

Breaking Changes

• ERC20FlashMint is no longer a Draft ERC. (#2673))

How to update: Change your import paths by removing the draft- prefix from @openzeppelin/contracts/token/ERC20/extensions/draft-ERC20FlashMint.sol.

See Releases and Stability: Drafts.

4.1.0 (2021-04-29)

• IERC20Metadata: add a new extended interface that includes the optional name(), symbol() and decimals() functions. (#2561)
• ERC777: make reception acquirement optional in _mint. (#2552)
• ERC20Permit: add a _useNonce to enable further usage of ERC712 signatures. (#2565)
• ERC20FlashMint: add an implementation of the ERC3156 extension for flash-minting ERC20 tokens. (#2543)
• SignatureChecker: add a signature verification library that supports both EOA and ERC1271 compliant contracts as signers. (#2532)
• Multicall: add abstract contract with multicall(bytes[] calldata data) function to bundle multiple calls together (#2608)
• ECDSA: add support for ERC2098 short-signatures. (#2582)
• AccessControl: add a onlyRole modifier to restrict specific function to callers bearing a specific role. (#2609)
• StorageSlot: add a library for reading and writing primitive types to specific storage slots. (#2542)
• UUPS Proxies: add UUPSUpgradeable to implement the UUPS proxy pattern together with EIP1967Proxy. (#2542)

Breaking changes

This release includes two small breaking changes in TimelockController.

1. The onlyRole modifier in this contract was designed to let anyone through if the role was granted to address(0), allowing the possibility to to make a role "open", which can be used for EXECUTOR_ROLE. This modifier is now replaced by AccessControl.onlyRole, which does not have this ability. The previous behavior was moved to the modifier TimelockController.onlyRoleOrOpenRole.
2. It was possible to make PROPOSER_ROLE an open role (as described in the previous item) if it was granted to address(0). This would affect the schedule, scheduleBatch, and cancel operations in TimelockController. This ability was removed as it does not make sense to open up the PROPOSER_ROLE in the same way that it does for EXECUTOR_ROLE.

4.0.0 (2021-03-23)

• Now targeting the 0.8.x line of Solidity compilers. For 0.6.x (resp 0.7.x) support, use version 3.4.0 (resp 3.4.0-solc-0.7) of OpenZeppelin.
• Context: making _msgData return bytes calldata instead of bytes memory (#2492)
• ERC20: removed the _setDecimals function and the storage slot associated to decimals. (#2502)
• Strings: addition of a toHexString function. (#2504)
• EnumerableMap: change implementation to optimize for key → value lookups instead of enumeration. (#2518)
• GSN: deprecate GSNv1 support in favor of upcoming support for GSNv2. (#2521)
• ERC165: remove uses of storage in the base ERC165 implementation. ERC165 based contracts now use storage-less virtual functions. Old behavior remains available in the ERC165Storage extension. (#2505)
• Initializable: make initializer check stricter during construction. (#2531)
• ERC721: remove enumerability of tokens from the base implementation. This feature is now provided separately through the ERC721Enumerable extension. (#2511)
• AccessControl: removed enumerability by default for a more lightweight contract. It is now opt-in through AccessControlEnumerable. (#2512)
• Meta Transactions: add ERC2771Context and a MinimalForwarder for meta-transactions. (#2508)
• Overall reorganization of the contract folder to improve clarity and discoverability. (#2503)
• ERC20Capped: optimize gas usage by enforcing the check directly in _mint. (#2524)
• Rename UpgradeableProxy to ERC1967Proxy. (#2547)
• ERC777: optimize the gas costs of the constructor. (#2551)
• ERC721URIStorage: add a new extension that implements the _setTokenURI behavior as it was available in 3.4.0. (#2555)
• AccessControl: added ERC165 interface detection. (#2562)
• ERC1155: make uri public so overloading function can call it using super. (#2576)

Bug fixes for beta releases

• AccessControlEnumerable: Fixed renounceRole not updating enumerable set of addresses for a role. (#2572)

How to upgrade from 3.x

Since this version has moved a few contracts to different directories, users upgrading from a previous version will need to adjust their import statements. To make this easier, the package includes a script that will migrate import statements automatically. After upgrading to the latest version of the package, run:

npx openzeppelin-contracts-migrate-imports

Make sure you're using git or another version control system to be able to recover from any potential error in our script.

How to upgrade from 4.0-beta.x

Some further changes have been done between the different beta iterations. Transitions made during this period are configured in the migrate-imports script. Consequently, you can upgrade from any previous 4.0-beta.x version using the same script as described in the How to upgrade from 3.x section.

3.4.2

• TimelockController: Add additional isOperationReady check.

3.4.1 (2021-03-03)

• ERC721: made _approve an internal function (was private).

3.4.0 (2021-02-02)

• BeaconProxy: added new kind of proxy that allows simultaneous atomic upgrades. (#2411)
• EIP712: added helpers to verify EIP712 typed data signatures on chain. (#2418)
• ERC20Permit: added an implementation of the ERC20 permit extension for gasless token approvals. (#2237)
• Presets: added token presets with preminted fixed supply ERC20PresetFixedSupply and ERC777PresetFixedSupply. (#2399)
• Address: added functionDelegateCall, similar to the existing functionCall. (#2333)
• Clones: added a library for deploying EIP 1167 minimal proxies. (#2449)
• Context: moved from contracts/GSN to contracts/utils. (#2453)
• PaymentSplitter: replace usage of .transfer() with Address.sendValue for improved compatibility with smart wallets. (#2455)
• UpgradeableProxy: bubble revert reasons from initialization calls. (#2454)
• SafeMath: fix a memory allocation issue by adding new SafeMath.tryOp(uint,uint)→(bool,uint) functions. SafeMath.op(uint,uint,string)→uint are now deprecated. (#2462)
• EnumerableMap: fix a memory allocation issue by adding new EnumerableMap.tryGet(uint)→(bool,address) functions. EnumerableMap.get(uint)→string is now deprecated. (#2462)
• ERC165Checker: added batch getSupportedInterfaces. (#2469)
• RefundEscrow: beneficiaryWithdraw will forward all available gas to the beneficiary. (#2480)
• Many view and pure functions have been made virtual to customize them via overrides. In many cases this will not imply that other functions in the contract will automatically adapt to the overridden definitions. People who wish to override should consult the source code to understand the impact and if they need to override any additional functions to achieve the desired behavior.

Security Fixes

• ERC777: fix potential reentrancy issues for custom extensions to ERC777. (#2483)

If you're using our implementation of ERC777 from version 3.3.0 or earlier, and you define a custom _beforeTokenTransfer function that writes to a storage variable, you may be vulnerable to a reentrancy attack. If you're affected and would like assistance please write to security@openzeppelin.com. Read more in the pull request.

3.3.0 (2020-11-26)

• Now supports both Solidity 0.6 and 0.7. Compiling with solc 0.7 will result in warnings. Install the solc-0.7 tag to compile without warnings.
• Address: added functionStaticCall, similar to the existing functionCall. (#2333)
• TimelockController: added a contract to augment access control schemes with a delay. (#2354)
• EnumerableSet: added Bytes32Set, for sets of bytes32. (#2395)

3.2.2-solc-0.7 (2020-10-28)

• Resolve warnings introduced by Solidity 0.7.4. (#2396)

3.2.1-solc-0.7 (2020-09-15)

• ERC777: Remove a warning about function state visibility in Solidity 0.7. (#2327)

3.2.0 (2020-09-10)

New features

• Proxies: added the proxy contracts from OpenZeppelin SDK. (#2335)

Proxy changes with respect to OpenZeppelin SDK

Aside from upgrading them from Solidity 0.5 to 0.6, we've changed a few minor things from the proxy contracts as they were found in OpenZeppelin SDK.

• UpgradeabilityProxy was renamed to UpgradeableProxy.
• AdminUpgradeabilityProxy was renamed to TransparentUpgradeableProxy.
• Proxy._willFallback was renamed to Proxy._beforeFallback.
• UpgradeabilityProxy._setImplementation and AdminUpgradeabilityProxy._setAdmin were made private.

Improvements

• Address.isContract: switched from extcodehash to extcodesize for less gas usage. (#2311)

Breaking changes

• ERC20Snapshot: switched to using _beforeTokenTransfer hook instead of overriding ERC20 operations. (#2312)

This small change in the way we implemented ERC20Snapshot may affect users who are combining this contract with other ERC20 flavors, since it no longer overrides _transfer, _mint, and _burn. This can result in having to remove Solidity override(...) specifiers in derived contracts for these functions, and to instead have to add it for _beforeTokenTransfer. See Using Hooks in the documentation.

3.1.0 (2020-06-23)

New features

• SafeCast: added functions to downcast signed integers (e.g. toInt32), improving usability of SignedSafeMath. (#2243)
• functionCall: new helpers that replicate Solidity's function call semantics, reducing the need to rely on call. (#2264)
• ERC1155: added support for a base implementation, non-standard extensions and a preset contract. (#2014, #2230)

Improvements

• ReentrancyGuard: reduced overhead of using the nonReentrant modifier. (#2171)
• AccessControl: added a RoleAdminChanged event to _setAdminRole. (#2214)
• Made all public functions in the token preset contracts virtual. (#2257)

Deprecations

• SafeERC20: deprecated safeApprove. (#2268)

3.0.2 (2020-06-08)

Improvements

• Added SPX license identifier to all contracts. (#2235)

3.0.1 (2020-04-27)

Bugfixes

• ERC777: fixed the _approve internal function not validating some of their arguments for non-zero addresses. (#2213)

3.0.0 (2020-04-20)

New features

• AccessControl: new contract for managing permissions in a system, replacement for Ownable and Roles. (#2112)
• SafeCast: new functions to convert to and from signed and unsigned values: toUint256 and toInt256. (#2123)
• EnumerableMap: a new data structure for key-value pairs (like mapping) that can be iterated over. (#2160)

Breaking changes

• ERC721: burn(owner, tokenId) was removed, use burn(tokenId) instead. (#2125)
• ERC721: _checkOnERC721Received was removed. (#2125)
• ERC721: _transferFrom and _safeTransferFrom were renamed to _transfer and _safeTransfer. (#2162)
• Ownable: removed _transferOwnership. (#2162)
• PullPayment, Escrow: withdrawWithGas was removed. The old withdraw function now forwards all gas. (#2125)
• Roles was removed, use AccessControl as a replacement. (#2112)
• ECDSA: when receiving an invalid signature, recover now reverts instead of returning the zero address. (#2114)
• Create2: added an amount argument to deploy for contracts with payable constructors. (#2117)
• Pausable: moved to the utils directory. (#2122)
• Strings: moved to the utils directory. (#2122)
• Counters: moved to the utils directory. (#2122)
• SignedSafeMath: moved to the math directory. (#2122)
• ERC20Snapshot: moved to the token/ERC20 directory. snapshot was changed into an internal function. (#2122)
• Ownable: moved to the access directory. (#2120)
• Ownable: removed isOwner. (#2120)
• Secondary: removed from the library, use Ownable instead. (#2120)
• Escrow, ConditionalEscrow, RefundEscrow: these now use Ownable instead of Secondary, their external API changed accordingly. (#2120)
• ERC20: removed _burnFrom. (#2119)
• Address: removed toPayable, use payable(address) instead. (#2133)
• ERC777: _send, _mint and _burn now use the caller as the operator. (#2134)
• ERC777: removed _callsTokensToSend and _callTokensReceived. (#2134)
• EnumerableSet: renamed get to at. (#2151)
• ERC165Checker: functions no longer have a leading underscore. (#2150)
• ERC721Metadata, ERC721Enumerable: these contracts were removed, and their functionality merged into ERC721. (#2160)
• ERC721: added a constructor for name and symbol. (#2160)
• ERC20Detailed: this contract was removed and its functionality merged into ERC20. (#2161)
• ERC20: added a constructor for name and symbol. decimals now defaults to 18. (#2161)
• Strings: renamed fromUint256 to toString (#2188)

2.5.1 (2020-04-24)

Bugfixes

• ERC777: fixed the _send and _approve internal functions not validating some of their arguments for non-zero addresses. (#2212)

2.5.0 (2020-02-04)

New features

• SafeCast.toUintXX: new library for integer downcasting, which allows for safe operation on smaller types (e.g. uint32) when combined with SafeMath. (#1926)
• ERC721Metadata: added baseURI, which can be used for dramatic gas savings when all token URIs share a prefix (e.g. http://api.myapp.com/tokens/<id>). (#1970)
• EnumerableSet: new library for storing enumerable sets of values. Only AddressSet is supported in this release. (#2061)
• Create2: simple library to make usage of the CREATE2 opcode easier. (#1744)

Improvements

• ERC777: _burn is now internal, providing more flexibility and making it easier to create tokens that deflate. (#1908)
• ReentrancyGuard: greatly improved gas efficiency by using the net gas metering mechanism introduced in the Istanbul hardfork. (#1992, #1996)
• ERC777: improve extensibility by making _send and related functions internal. (#2027)
• ERC721: improved revert reason when transferring tokens to a non-recipient contract. (#2018)

Breaking changes

• ERC165Checker now requires a minimum Solidity compiler version of 0.5.10. (#1829)

2.4.0 (2019-10-29)

New features

• Address.toPayable: added a helper to convert between address types without having to resort to low-level casting. (#1773)
• Facilities to make metatransaction-enabled contracts through the Gas Station Network. (#1844)
• Address.sendValue: added a replacement to Solidity's transfer, removing the fixed gas stipend. (#1962)
• Added replacement for functions that don't forward all gas (which have been deprecated): (#1976)
  • PullPayment.withdrawPaymentsWithGas(address payable payee)
  • Escrow.withdrawWithGas(address payable payee)
• SafeMath: added support for custom error messages to sub, div and mod functions. (#1828)

Improvements

• Address.isContract: switched from extcodesize to extcodehash for less gas usage. (#1802)
• ERC20 and ERC777 updated to throw custom errors on subtraction overflows. (#1828)

Deprecations

• Deprecated functions that don't forward all gas: (#1976)
  • PullPayment.withdrawPayments(address payable payee)
  • Escrow.withdraw(address payable payee)

Breaking changes

• Address now requires a minimum Solidity compiler version of 0.5.5. (#1802)
• SignatureBouncer has been removed from drafts, both to avoid confusions with the GSN and GSNRecipientSignature (previously called GSNBouncerSignature) and because the API was not very clear. (#1879)

How to upgrade from 2.4.0-beta

The final 2.4.0 release includes a refactor of the GSN contracts that will be a breaking change for 2.4.0-beta users.

• The default empty implementations of _preRelayedCall and _postRelayedCall were removed and must now be explicitly implemented always in custom recipients. If your custom recipient didn't include an implementation, you can provide an empty one.
• GSNRecipient, GSNBouncerBase, and GSNContext were all merged into GSNRecipient.
• GSNBouncerSignature and GSNBouncerERC20Fee were renamed to GSNRecipientSignature and GSNRecipientERC20Fee.
• It is no longer necessary to inherit from GSNRecipient when using GSNRecipientSignature and GSNRecipientERC20Fee.

For example, a contract using GSNBouncerSignature would have to be changed in the following way.

-contract MyDapp is GSNRecipient, GSNBouncerSignature {
+contract MyDapp is GSNRecipientSignature {

Refer to the table below to adjust your inheritance list.

2.4.0-beta	2.4.0
GSNRecipient, GSNBouncerSignature	GSNRecipientSignature
GSNRecipient, GSNBouncerERC20Fee	GSNRecipientERC20Fee
GSNBouncerBase	GSNRecipient

2.3.0 (2019-05-27)

New features

• ERC1820: added support for interacting with the ERC1820 registry contract (IERC1820Registry), as well as base contracts that can be registered as implementers there. (#1677)
• ERC777: support for the ERC777 token, which has multiple improvements over ERC20 (but is backwards compatible with it) such as built-in burning, a more straightforward permission system, and optional sender and receiver hooks on transfer (mandatory for contracts!). (#1684)
• All contracts now have revert reason strings, which give insight into error conditions, and help debug failing transactions. (#1704)

Improvements

• Reverted the Solidity version bump done in v2.2.0, setting the minimum compiler version to v0.5.0, to prevent unexpected build breakage. Users are encouraged however to stay on top of new compiler releases, which usually include bugfixes. (#1729)

Bugfixes

• PostDeliveryCrowdsale: some validations where skipped when paired with other crowdsale flavors, such as AllowanceCrowdsale, or MintableCrowdsale and ERC20Capped, which could cause buyers to not be able to claim their purchased tokens. (#1721)
• ERC20._transfer: the from argument was allowed to be the zero address, so it was possible to internally trigger a transfer of 0 tokens from the zero address. This address is not a valid destinatary of transfers, nor can it give or receive allowance, so this behavior was inconsistent. It now reverts. (#1752)

2.2.0 (2019-03-14)

New features

• ERC20Snapshot: create snapshots on demand of the token balances and total supply, to later retrieve and e.g. calculate dividends at a past time. (#1617)
• SafeERC20: ERC20 contracts with no return value (i.e. that revert on failure) are now supported. (#1655)
• ERC20: added internal _approve(address owner, address spender, uint256 value), allowing derived contracts to set the allowance of arbitrary accounts. (#1609)
• ERC20Metadata: added internal _setTokenURI(string memory tokenURI). (#1618)
• TimedCrowdsale: added internal _extendTime(uint256 newClosingTime) as well as TimedCrowdsaleExtended(uint256 prevClosingTime, uint256 newClosingTime) event allowing to extend the crowdsale, as long as it hasn't already closed.

Improvements

• Upgraded the minimum compiler version to v0.5.2: this removes many Solidity warnings that were false positives. (#1606)
• ECDSA: recover no longer accepts malleable signatures (those using upper-range values for s, or 0/1 for v). (#1622)
• ERC721's transfers are now more gas efficient due to removal of unnecessary SafeMath calls. (#1610)
• Fixed variable shadowing issues. (#1606)

Bugfixes

• (minor) SafeERC20: safeApprove wasn't properly checking for a zero allowance when attempting to set a non-zero allowance. (#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)

2.1.3 (2019-02-26)

• Backported SafeERC20.safeApprove bugfix. (#1647)

2.1.2 (2019-01-17)

• Removed most of the test suite from the npm package, except PublicRole.behavior.js, which may be useful to users testing their own Roles.

2.1.1 (2019-01-04)

• Version bump to avoid conflict in the npm registry.

2.1.0 (2019-01-04)

New features

• Now targeting the 0.5.x line of Solidity compilers. For 0.4.24 support, use version 2.0 of OpenZeppelin.
• WhitelistCrowdsale: a crowdsale where only whitelisted accounts (WhitelistedRole) can purchase tokens. Adding or removing accounts from the whitelist is done by whitelist admins (WhitelistAdminRole). Similar to the pre-2.0 WhitelistedCrowdsale. (#1525, #1589)
• RefundablePostDeliveryCrowdsale: replacement for RefundableCrowdsale (deprecated, see below) where tokens are only granted once the crowdsale ends (if it meets its goal). (#1543)
• PausableCrowdsale: allows for pausers (PauserRole) to pause token purchases. Other crowdsale operations (e.g. withdrawals and refunds, if applicable) are not affected. (#832)
• ERC20: transferFrom and _burnFrom now emit Approval events, to represent the token's state comprehensively through events. (#1524)
• ERC721: added _burn(uint256 tokenId), replacing the similar deprecated function (see below). (#1550)
• ERC721: added _tokensOfOwner(address owner), allowing to internally retrieve the array of an account's owned tokens. (#1522)
• Crowdsales: all constructors are now public, meaning it is not necessary to extend these contracts in order to deploy them. The exception is FinalizableCrowdsale, since it is meaningless unless extended. (#1564)
• SignedSafeMath: added overflow-safe operations for signed integers (int256). (#1559, #1588)

Improvements

• The compiler version required by Array was behind the rest of the library so it was updated to v0.4.24. (#1553)
• Now conforming to a 4-space indentation code style. (1508)
• ERC20: more gas efficient due to removed redundant requires. (#1409)
• ERC721: fixed a bug that prevented internal data structures from being properly cleaned, missing potential gas refunds. (#1539 and #1549)
• ERC721: general gas savings on transferFrom, _mint and _burn, due to redudant requires and SSTOREs. (#1549)

Bugfixes

Breaking changes

Deprecations

• ERC721._burn(address owner, uint256 tokenId): due to the owner parameter being unnecessary. (#1550)
• RefundableCrowdsale: due to trading abuse potential on crowdsales that miss their goal. (#1543)