Add @openzeppelin/contracts as alternative package name (#1840)

* Rename package and repository name from docs and scripts

* undo root package rename

* add @openzeppelin/contracts as subpackage with release automation

* synchronize @openzeppelin/contracts version

* remove private field from package.json

* make file patterns absolute

* change wording of a comment

* use a saner version script

(cherry picked from commit b8c8308d77)

.github/ISSUE_TEMPLATE/bug_report.md

@@ -6,7 +6,7 @@ about: Report a bug in OpenZeppelin
 
 <!-- Briefly describe the issue you're experiencing. Tell us what you were trying to do and what happened instead. -->
 
-<!-- Remember, this is not a place to ask for help debugging code. For that, we welcome you in the OpenZeppelin Slack channel: https://slack.openzeppelin.org/. -->
+<!-- Remember, this is not a place to ask for help debugging code. For that, we welcome you in the Zeppelin Forum: https://forum.zeppelin.solutions/. -->
 
 **💻 Environment**
 

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,6 +1,6 @@
 ---
 name: Feature request
-about: Suggest an idea for OpenZeppelin
+about: Suggest an idea for OpenZeppelin Contracts
 
 ---
 
@@ -11,4 +11,4 @@ about: Suggest an idea for OpenZeppelin
 <!-- Please describe your feature request in detail. -->
 
 <!-- Make sure that you have reviewed the OpenZeppelin Contributor Guidelines. -->
-<!-- https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/CONTRIBUTING.md -->
+<!-- https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/CONTRIBUTING.md -->

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,9 +13,10 @@ Fixes #
 
 <!-- 3. Before submitting, please make sure that you have:
   - reviewed the OpenZeppelin Contributor Guidelines
-    (https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/CONTRIBUTING.md),
+    (https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/CONTRIBUTING.md),
   - added tests where applicable to test new functionality,
   - made sure that your contracts are well-documented,
-  - run the JS/Solidity linters and fixed any issues (`npm run lint:fix`), and
+  - run the Solidity linter (`npm run lint:sol`) and fixed any issues,
+  - run the JS linter and fixed any issues (`npm run lint:fix`), and
   - updated the changelog, if applicable.
 -->

.github/stale.yml

@@ -0,0 +1,66 @@
+# Configuration for probot-stale - https://github.com/probot/stale
+
+# Number of days of inactivity before an Issue or Pull Request becomes stale
+daysUntilStale: 15
+
+# Number of days of inactivity before an Issue or Pull Request with the stale label is closed.
+# Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+daysUntilClose: 15
+
+# Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
+exemptLabels:
+  - on hold
+
+# Set to true to ignore issues in a project (defaults to false)
+exemptProjects: false
+
+# Set to true to ignore issues in a milestone (defaults to false)
+exemptMilestones: false
+
+# Set to true to ignore issues with an assignee (defaults to false)
+exemptAssignees: false
+
+# Label to use when marking as stale
+staleLabel: stale
+
+# Comment to post when marking as stale. Set to `false` to disable
+markComment: >
+  Hi all!
+
+  This Pull Request has not had any recent activity, is it still relevant? If so, what is blocking it?
+  Is there anything we can do to help move it forward?
+
+  Thanks!
+
+
+# Comment to post when removing the stale label.
+# unmarkComment: >
+#   Your comment here.
+
+# Comment to post when closing a stale Issue or Pull Request.
+closeComment: >
+  Hi folks!
+
+  This Pull Request is being closed as there was no response to the previous prompt.
+  However, please leave a comment whenever you're ready to resume, so it can be reopened.
+
+  Thanks again!
+
+
+# Limit the number of actions per hour, from 1-30. Default is 30
+limitPerRun: 30
+
+# Limit to only `issues` or `pulls`
+only: pulls
+
+# Optionally, specify configuration settings that are specific to just 'issues' or 'pulls':
+# pulls:
+#   daysUntilStale: 30
+#   markComment: >
+#     This pull request has been automatically marked as stale because it has not had
+#     recent activity. It will be closed if no further activity occurs. Thank you
+#     for your contributions.
+
+# issues:
+#   exemptLabels:
+#     - confirmed

.gitignore

@@ -32,8 +32,19 @@ npm-debug.log
 # truffle build directory
 build/
 
-# lol macs
-.DS_Store/
+# macOS
+.DS_Store
 
 # truffle
 .node-xmlhttprequest-*
+
+# IntelliJ IDE
+.idea
+
+# docsite artifacts
+docsite-build
+docs/api
+
+# only used to package @openzeppelin/contracts
+contracts/build/
+contracts/README.md

.node-version

@@ -1 +0,0 @@
-v8.9.1

.solhint.json

@@ -2,10 +2,11 @@
   "extends": "default",
   "rules": {
     "indent": ["error", 4],
-
+    "func-order": false,
     "bracket-align": false,
     "compiler-fixed": false,
     "no-simple-event-func-name": false,
+    "separate-by-one-line-in-contract": false,
     "two-lines-top-level-separator": false
   }
 }

.travis.yml

@@ -25,26 +25,16 @@ jobs:
       name: "Unit tests"
       script: npm run test
 
-# solidity-coverage fails at parsing 0.5.x code
-#    - stage: tests
-#      name: "Unit tests with coverage report"
-#      script: npm run test
-#      env: SOLIDITY_COVERAGE=true
+    - stage: tests
+      name: "Unit tests with coverage report"
+      script: npm run test
+      env: SOLIDITY_COVERAGE=true
 
     - stage: tests
       name: "Unit tests using solc nightly"
       script: npm run test
       env: SOLC_NIGHTLY=true
 
-    - stage: update docs
-      if: tag =~ ^v[0-9]+\.[0-9]+\.[0-9]+$
-      addons:
-        apt:
-          packages:
-            - curl
-      script:
-        - ./scripts/ci/trigger_docs_update "${TRAVIS_TAG}"
-
 notifications:
   slack:
     rooms:

CHANGELOG.md

@@ -1,14 +1,51 @@
 # Changelog
 
-## 2.2.0 (unreleased)
+## 2.3.0 (2019-05-27)
 
-## 2.1.2 (2019-17-01)
+### New features:
+ * `ERC1820`: added support for interacting with the [ERC1820](https://eips.ethereum.org/EIPS/eip-1820) registry contract (`IERC1820Registry`), as well as base contracts that can be registered as implementers there. ([#1677](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1677))
+ * `ERC777`: support for the [ERC777 token](https://eips.ethereum.org/EIPS/eip-777), 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](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1684))
+ * All contracts now have revert reason strings, which give insight into error conditions, and help debug failing transactions. ([#1704](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/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](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/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](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/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](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/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](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1617))
+ * `SafeERC20`: `ERC20` contracts with no return value (i.e. that revert on failure) are now supported. ([#1655](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1655))
+ * `ERC20`: added internal `_approve(address owner, address spender, uint256 value)`, allowing derived contracts to set the allowance of arbitrary accounts. ([#1609](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1609))
+ * `ERC20Metadata`: added internal `_setTokenURI(string memory tokenURI)`. ([#1618](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/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](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1606))
+ * `ECDSA`: `recover` no longer accepts malleable signatures (those using upper-range values for `s`, or 0/1 for `v`). ([#1622](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1622))
+ * `ERC721`'s transfers are now more gas efficient due to removal of unnecessary `SafeMath` calls. ([#1610](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1610))
+ * Fixed variable shadowing issues. ([#1606](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1606))
+
+### Bugfixes:
+ * (minor) `SafeERC20`: `safeApprove` wasn't properly checking for a zero allowance when attempting to set a non-zero allowance. ([#1647](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1647))
+
+### Breaking changes in drafts:
+ * `TokenMetadata` has been renamed to `ERC20Metadata`. ([#1618](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1618))
+ * The library `Counter` has been renamed to `Counters` and its API has been improved. See an example in `ERC721`, lines [17](https://github.com/OpenZeppelin/openzeppelin-solidity/blob/3cb4a00fce1da76196ac0ac3a0ae9702b99642b5/contracts/token/ERC721/ERC721.sol#L17) and [204](https://github.com/OpenZeppelin/openzeppelin-solidity/blob/3cb4a00fce1da76196ac0ac3a0ae9702b99642b5/contracts/token/ERC721/ERC721.sol#L204). ([#1610](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1610))
+
+## 2.1.3 (2019-02-26)
+ * Backported `SafeERC20.safeApprove` bugfix. ([#1647](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/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 (2019-04-01)
+## 2.1.1 (2019-01-04)
  * Version bump to avoid conflict in the npm registry.
 
-## 2.1.0 (2019-04-01)
+## 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.

CONTRIBUTING.md

@@ -1,7 +1,7 @@
-Contributing to OpenZeppelin
+Contributing to OpenZeppelin Contracts
 =======
 
-We really appreciate and value contributions to OpenZeppelin. Please take 5' to review the items listed below to make sure that your contributions are merged as soon as possible. 
+We really appreciate and value contributions to OpenZeppelin Contracts. Please take 5' to review the items listed below to make sure that your contributions are merged as soon as possible.
 
 ## Contribution guidelines
 
@@ -11,7 +11,7 @@ Smart contracts manage value and are highly vulnerable to errors and attacks. We
 
 As a contributor, you are expected to fork this repository, work on your own fork and then submit pull requests. The pull requests will be reviewed and eventually merged into the main repo. See ["Fork-a-Repo"](https://help.github.com/articles/fork-a-repo/) for how this works.
 
-*IMPORTANT* 
+*IMPORTANT*
 * Please see ["Git flow wiki entry"](https://github.com/OpenZeppelin/openzeppelin-solidity/wiki/Git-flow) for understanding how to use branches in this repository.
 
 ## A typical workflow
@@ -19,12 +19,12 @@ As a contributor, you are expected to fork this repository, work on your own for
 1) Make sure your fork is up to date with the main repository:
 
 ```
-cd openzeppelin-solidity
-git remote add upstream https://github.com/OpenZeppelin/openzeppelin-solidity.git
+cd openzeppelin-contracts
+git remote add upstream https://github.com/OpenZeppelin/openzeppelin-contracts.git
 git fetch upstream
 git pull --rebase upstream master
 ```
-NOTE: The directory `openzeppelin-solidity` represents your fork's local copy.
+NOTE: The directory `openzeppelin-contracts` represents your fork's local copy.
 
 2) Branch out from `master` into `fix/some-bug-#123`:
 (Postfixing #123 will associate your PR with the issue #123 and make everyone's life easier =D)
@@ -40,9 +40,9 @@ git commit "Fix some bug #123"
 git push origin fix/some-bug-#123
 ```
 
-4) Go to [github.com/OpenZeppelin/openzeppelin-solidity](https://github.com/OpenZeppelin/zeppelin-solidity) in your web browser and issue a new pull request.
+4) Go to [github.com/OpenZeppelin/openzeppelin-contracts](https://github.com/OpenZeppelin/openzeppelin-contracts) in your web browser and issue a new pull request.
 
-*IMPORTANT* Read the PR template very carefully and make sure to follow all the instructions. These instructions 
+*IMPORTANT* Read the PR template very carefully and make sure to follow all the instructions. These instructions
 refer to some very important conditions that your PR must meet in order to be accepted, such as making sure that all tests pass, JS linting tests pass, solidity linting tests pass, etc.
 
 5) Maintainers will review your code and possibly ask for changes before your code is pulled in to the main repository. We'll check that all tests pass, review the coding style, and check for general code correctness. If everything is OK, we'll merge your pull request and your code will be part of OpenZeppelin.
@@ -51,8 +51,8 @@ refer to some very important conditions that your PR must meet in order to be ac
 
 ## All set!
 
-If you have any questions feel free to post them to github.com/OpenZeppelin/openzeppelin-solidity/issues.
+If you have any questions feel free to post them to github.com/OpenZeppelin/openzeppelin-contracts/issues.
 
-Finally, if you're looking to collaborate and want to find easy tasks to start, look at the issues we marked as ["Good first issue"](https://github.com/OpenZeppelin/openzeppelin-solidity/labels/good%20first%20issue).
+Finally, if you're looking to collaborate and want to find easy tasks to start, look at the issues we marked as ["Good first issue"](https://github.com/OpenZeppelin/openzeppelin-contracts/labels/good%20first%20issue).
 
 Thanks for your time and code!

DOCUMENTATION.md

@@ -0,0 +1,19 @@
+We're building an improved documentation website. It's still in development and
+contributions will be really appreciated.
+
+All of the content for the site is in this repository. The guides are in the
+[docs](/docs) directory, and the API Reference is extracted from comments in
+the source code. If you want to help improve the content, this is the
+repository you should be contributing to.
+
+[`solidity-docgen`](https://github.com/OpenZeppelin/solidity-docgen/tree/next) is the
+program that extracts the API Reference from source code.
+
+The [`openzeppelin-docsite`](https://github.com/OpenZeppelin/openzeppelin-docsite/tree/next)
+repository hosts the configuration for Docusaurus, the static site generator
+that we use.
+
+To run the docsite locally you should run `npm run docsite start` on this
+repository. This will live reload as the guides are edited, but not with
+changes to the source code comments, for that you need to restart the server.
+This should be improved eventually (contributions welcome!).

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2016 Smart Contract Solutions, Inc.
+Copyright (c) 2016-2019 zOS Global Limited
 
 Permission is hereby granted, free of charge, to any person obtaining
 a copy of this software and associated documentation files (the

README.md

@@ -1,26 +1,28 @@
 # <img src="logo.png" alt="OpenZeppelin" width="400px">
 
-[![NPM Package](https://img.shields.io/npm/v/openzeppelin-solidity.svg?style=flat-square)](https://www.npmjs.org/package/openzeppelin-solidity)
-[![Build Status](https://travis-ci.com/OpenZeppelin/openzeppelin-solidity.svg?branch=master)](https://travis-ci.com/OpenZeppelin/openzeppelin-solidity)
-[![Coverage Status](https://coveralls.io/repos/github/OpenZeppelin/openzeppelin-solidity/badge.svg?branch=master)](https://coveralls.io/github/OpenZeppelin/openzeppelin-solidity?branch=master)
+[![NPM Package](https://img.shields.io/npm/v/@openzeppelin/contracts.svg)](https://www.npmjs.org/package/@openzeppelin/contracts)
+[![Build Status](https://travis-ci.com/OpenZeppelin/openzeppelin-contracts.svg?branch=master)](https://travis-ci.com/OpenZeppelin/openzeppelin-contracts)
+[![Coverage Status](https://coveralls.io/repos/github/OpenZeppelin/openzeppelin-contracts/badge.svg?branch=master)](https://coveralls.io/github/OpenZeppelin/openzeppelin-contracts?branch=master)
 
 **OpenZeppelin is a library for secure smart contract development.** It provides implementations of standards like ERC20 and ERC721 which you can deploy as-is or extend to suit your needs, as well as Solidity components to build custom contracts and more complex decentralized systems.
 
 ## Install
 
 ```
-npm install openzeppelin-solidity
+npm install @openzeppelin/contracts
 ```
 
+OpenZeppelin features a stable API, which means your contracts won't break unexpectedly when upgrading to a newer minor version. You can read ṫhe details in our [API Stability](https://forum.zeppelin.solutions/t/api-stability/138) document.
+
 ## Usage
 
 To write your custom contracts, import ours and extend them through inheritance.
 
 ```solidity
-pragma solidity ^0.4.24;
+pragma solidity ^0.5.0;
 
-import 'openzeppelin-solidity/contracts/token/ERC721/ERC721Full.sol';
-import 'openzeppelin-solidity/contracts/token/ERC721/ERC721Mintable.sol';
+import "@openzeppelin/contracts/token/ERC721/ERC721Full.sol";
+import "@openzeppelin/contracts/token/ERC721/ERC721Mintable.sol";
 
 contract MyNFT is ERC721Full, ERC721Mintable {
   constructor() ERC721Full("MyNFT", "MNFT") public {
@@ -28,9 +30,9 @@ contract MyNFT is ERC721Full, ERC721Mintable {
 }
 ```
 
-> You need an ethereum development framework for the above import statements to work! Check out these guides for [Truffle] or [Embark].
+> You need an ethereum development framework for the above import statements to work! Check out these guides for [Truffle], [Embark] or [Buidler].
 
-On our site you will find a few [guides] to learn about the diferent parts of OpenZeppelin, as well as [documentation for the API][API docs]. Keep in mind that the API docs are work in progress, and don’t hesitate to ask questions in [our Slack][Slack].
+On our site you will find a few [guides] to learn about the different parts of OpenZeppelin, as well as [documentation for the API][API docs]. Keep in mind that the API docs are work in progress, and don’t hesitate to ask questions in [our forum][forum].
 
 ## Security
 
@@ -38,6 +40,8 @@ OpenZeppelin the project is maintained by [Zeppelin] the company, and developed
 
 The core development principles and strategies that OpenZeppelin is based on include: security in depth, simple and modular code, clarity-driven naming conventions, comprehensive unit testing, pre-and-post-condition sanity checks, code consistency, and regular audits.
 
+The latest audit was done on October 2018 on version 2.0.0.
+
 Please report any security issues you find to security@openzeppelin.org.
 
 ## Contribute
@@ -51,8 +55,9 @@ OpenZeppelin is released under the [MIT License](LICENSE).
 
 [API docs]: https://openzeppelin.org/api/docs/token_ERC721_ERC721BasicToken.html
 [guides]: https://openzeppelin.org/api/docs/get-started.html
-[Slack]: https://slack.openzeppelin.org
+[forum]: https://forum.zeppelin.solutions
 [Zeppelin]: https://zeppelin.solutions
 [contribution guide]: CONTRIBUTING.md
 [Truffle]: https://truffleframework.com/docs/truffle/quickstart
 [Embark]: https://embark.status.im/docs/quick_start.html
+[Buidler]: https://buidler.dev/guides/#getting-started

RELEASING.md

@@ -2,7 +2,7 @@
 
 This document describes our release process, and contains the steps to be followed by an OpenZeppelin maintainer at the several stages of a release.
 
-We release a new version of OpenZeppelin monthly. Release cycles are tracked in the [issue milestones](https://github.com/OpenZeppelin/openzeppelin-solidity/milestones).
+We release a new version of OpenZeppelin monthly. Release cycles are tracked in the [issue milestones](https://github.com/OpenZeppelin/openzeppelin-contracts/milestones).
 
 Each release has at least one release candidate published first, intended for community review and any critical fixes that may come out of it. At the moment we leave 1 week between the first release candidate and the final release.
 
@@ -32,7 +32,7 @@ git push upstream release-vX.Y.Z
 git push upstream vX.Y.Z-rc.R
 ```
 
-Draft the release notes in our [GitHub releases](https://github.com/OpenZeppelin/openzeppelin-solidity/releases). Make sure to mark it as a pre-release! Try to be consistent with our previous release notes in the title and format of the text. Release candidates don't need a detailed changelog, but make sure to include a link to GitHub's compare page.
+Draft the release notes in our [GitHub releases](https://github.com/OpenZeppelin/openzeppelin-contracts/releases). Make sure to mark it as a pre-release! Try to be consistent with our previous release notes in the title and format of the text. Release candidates don't need a detailed changelog, but make sure to include a link to GitHub's compare page.
 
 Once the CI run for the new tag is green, publish on npm under the `next` tag. You should see the contracts compile automatically.
 
@@ -40,7 +40,7 @@ Once the CI run for the new tag is green, publish on npm under the `next` tag. Y
 npm publish --tag next
 ```
 
-Publish the release notes on GitHub and ask our community manager to announce the release candidate on at least Slack and Twitter.
+Publish the release notes on GitHub and the forum, and ask our community manager to announce the release candidate on at least Twitter.
 
 ## Creating the final release
 
@@ -51,12 +51,15 @@ git checkout release-vX.Y.Z
 git pull upstream
 ```
 
+Before starting the release process, make one final commit to CHANGELOG.md, including the date of the release.
+
 Change the version string in `package.json`, `package-lock.json` and `ethpm.json` removing the "-rc.R" suffix. Commit these changes and tag the commit as `vX.Y.Z`.
 
 ```
 git add package.json package-lock.json ethpm.json
 git commit -m "Release vX.Y.Z"
 git tag -a vX.Y.Z
+git push upstream release-vX.Y.Z
 git push upstream vX.Y.Z
 ```
 
@@ -73,7 +76,7 @@ Publish the release notes on GitHub and ask our community manager to announce th
 Delete the `next` tag in the npm package as there is no longer a release candidate.
 
 ```
-npm dist-tag rm --otp $2FA_CODE openzeppelin-solidity next
+npm dist-tag rm --otp $2FA_CODE @openzeppelin/contracts next
 ```
 
 ## Merging the release branch

contracts/.npmignore

@@ -1,2 +0,0 @@
-mocks
-examples

contracts/access/README.md

@@ -0,0 +1,9 @@
+---
+sections:
+  - title: Library
+    contracts:
+      - Roles
+  - subdirectory: roles
+---
+
+> This page is incomplete. We're working to improve it for the next release. Stay tuned!

contracts/access/Roles.sol

@@ -10,31 +10,27 @@ library Roles {
     }
 
     /**
-     * @dev give an account access to this role
+     * @dev Give an account access to this role.
      */
     function add(Role storage role, address account) internal {
-        require(account != address(0));
-        require(!has(role, account));
-
+        require(!has(role, account), "Roles: account already has role");
         role.bearer[account] = true;
     }
 
     /**
-     * @dev remove an account's access to this role
+     * @dev Remove an account's access to this role.
      */
     function remove(Role storage role, address account) internal {
-        require(account != address(0));
-        require(has(role, account));
-
+        require(has(role, account), "Roles: account does not have role");
         role.bearer[account] = false;
     }
 
     /**
-     * @dev check if an account has this role
+     * @dev Check if an account has this role.
      * @return bool
      */
     function has(Role storage role, address account) internal view returns (bool) {
-        require(account != address(0));
+        require(account != address(0), "Roles: account is the zero address");
         return role.bearer[account];
     }
 }

contracts/access/roles/CapperRole.sol

@@ -15,7 +15,7 @@ contract CapperRole {
     }
 
     modifier onlyCapper() {
-        require(isCapper(msg.sender));
+        require(isCapper(msg.sender), "CapperRole: caller does not have the Capper role");
         _;
     }
 

contracts/access/roles/MinterRole.sol

@@ -15,7 +15,7 @@ contract MinterRole {
     }
 
     modifier onlyMinter() {
-        require(isMinter(msg.sender));
+        require(isMinter(msg.sender), "MinterRole: caller does not have the Minter role");
         _;
     }
 

contracts/access/roles/PauserRole.sol

@@ -15,7 +15,7 @@ contract PauserRole {
     }
 
     modifier onlyPauser() {
-        require(isPauser(msg.sender));
+        require(isPauser(msg.sender), "PauserRole: caller does not have the Pauser role");
         _;
     }
 

contracts/access/roles/SignerRole.sol

@@ -15,7 +15,7 @@ contract SignerRole {
     }
 
     modifier onlySigner() {
-        require(isSigner(msg.sender));
+        require(isSigner(msg.sender), "SignerRole: caller does not have the Signer role");
         _;
     }
 

contracts/access/roles/WhitelistAdminRole.sol

@@ -19,7 +19,7 @@ contract WhitelistAdminRole {
     }
 
     modifier onlyWhitelistAdmin() {
-        require(isWhitelistAdmin(msg.sender));
+        require(isWhitelistAdmin(msg.sender), "WhitelistAdminRole: caller does not have the WhitelistAdmin role");
         _;
     }
 

contracts/access/roles/WhitelistedRole.sol

@@ -18,7 +18,7 @@ contract WhitelistedRole is WhitelistAdminRole {
     Roles.Role private _whitelisteds;
 
     modifier onlyWhitelisted() {
-        require(isWhitelisted(msg.sender));
+        require(isWhitelisted(msg.sender), "WhitelistedRole: caller does not have the Whitelisted role");
         _;
     }
 

contracts/crowdsale/Crowdsale.sol

@@ -11,8 +11,8 @@ import "../utils/ReentrancyGuard.sol";
  * allowing investors to purchase tokens with ether. This contract implements
  * such functionality in its most fundamental form and can be extended to provide additional
  * functionality and/or custom behavior.
- * The external interface represents the basic interface for purchasing tokens, and conform
- * the base architecture for crowdsales. They are *not* intended to be modified / overridden.
+ * The external interface represents the basic interface for purchasing tokens, and conforms
+ * the base architecture for crowdsales. It is *not* intended to be modified / overridden.
  * The internal interface conforms the extensible and modifiable surface of crowdsales. Override
  * the methods to add functionality. Consider using 'super' where appropriate to concatenate
  * behavior.
@@ -54,9 +54,9 @@ contract Crowdsale is ReentrancyGuard {
      * @param token Address of the token being sold
      */
     constructor (uint256 rate, address payable wallet, IERC20 token) public {
-        require(rate > 0);
-        require(wallet != address(0));
-        require(address(token) != address(0));
+        require(rate > 0, "Crowdsale: rate is 0");
+        require(wallet != address(0), "Crowdsale: wallet is the zero address");
+        require(address(token) != address(0), "Crowdsale: token is the zero address");
 
         _rate = rate;
         _wallet = wallet;
@@ -65,7 +65,7 @@ contract Crowdsale is ReentrancyGuard {
 
     /**
      * @dev fallback function ***DO NOT OVERRIDE***
-     * Note that other contracts will transfer fund with a base gas stipend
+     * Note that other contracts will transfer funds with a base gas stipend
      * of 2300, which is not enough to call buyTokens. Consider calling
      * buyTokens directly when purchasing tokens from a contract.
      */
@@ -136,8 +136,8 @@ contract Crowdsale is ReentrancyGuard {
      * @param weiAmount Value in wei involved in the purchase
      */
     function _preValidatePurchase(address beneficiary, uint256 weiAmount) internal view {
-        require(beneficiary != address(0));
-        require(weiAmount != 0);
+        require(beneficiary != address(0), "Crowdsale: beneficiary is the zero address");
+        require(weiAmount != 0, "Crowdsale: weiAmount is 0");
     }
 
     /**

contracts/crowdsale/README.md

@@ -0,0 +1,13 @@
+---
+title: Crowdsales
+sections:
+  - title: Core
+    contracts:
+      - Crowdsale
+  - subdirectory: emission
+  - subdirectory: price
+  - subdirectory: validation
+  - subdirectory: distribution
+---
+
+> This page is incomplete. We're working to improve it for the next release. Stay tuned!

contracts/crowdsale/distribution/FinalizableCrowdsale.sol

@@ -5,7 +5,7 @@ import "../validation/TimedCrowdsale.sol";
 
 /**
  * @title FinalizableCrowdsale
- * @dev Extension of Crowdsale with a one-off finalization action, where one
+ * @dev Extension of TimedCrowdsale with a one-off finalization action, where one
  * can do extra work after finishing.
  */
 contract FinalizableCrowdsale is TimedCrowdsale {
@@ -31,8 +31,8 @@ contract FinalizableCrowdsale is TimedCrowdsale {
      * work. Calls the contract's finalization function.
      */
     function finalize() public {
-        require(!_finalized);
-        require(hasClosed());
+        require(!_finalized, "FinalizableCrowdsale: already finalized");
+        require(hasClosed(), "FinalizableCrowdsale: not closed");
 
         _finalized = true;
 

contracts/crowdsale/distribution/PostDeliveryCrowdsale.sol

@@ -2,6 +2,8 @@ pragma solidity ^0.5.0;
 
 import "../validation/TimedCrowdsale.sol";
 import "../../math/SafeMath.sol";
+import "../../ownership/Secondary.sol";
+import "../../token/ERC20/IERC20.sol";
 
 /**
  * @title PostDeliveryCrowdsale
@@ -11,17 +13,23 @@ contract PostDeliveryCrowdsale is TimedCrowdsale {
     using SafeMath for uint256;
 
     mapping(address => uint256) private _balances;
+    __unstable__TokenVault private _vault;
+
+    constructor() public {
+        _vault = new __unstable__TokenVault();
+    }
 
     /**
      * @dev Withdraw tokens only after crowdsale ends.
      * @param beneficiary Whose tokens will be withdrawn.
      */
     function withdrawTokens(address beneficiary) public {
-        require(hasClosed());
+        require(hasClosed(), "PostDeliveryCrowdsale: not closed");
         uint256 amount = _balances[beneficiary];
-        require(amount > 0);
+        require(amount > 0, "PostDeliveryCrowdsale: beneficiary is not due any tokens");
+
         _balances[beneficiary] = 0;
-        _deliverTokens(beneficiary, amount);
+        _vault.transfer(token(), beneficiary, amount);
     }
 
     /**
@@ -32,12 +40,26 @@ contract PostDeliveryCrowdsale is TimedCrowdsale {
     }
 
     /**
-     * @dev Overrides parent by storing balances instead of issuing tokens right away.
+     * @dev Overrides parent by storing due balances, and delivering tokens to the vault instead of the end user. This
+     * ensures that the tokens will be available by the time they are withdrawn (which may not be the case if
+     * `_deliverTokens` was called later).
      * @param beneficiary Token purchaser
      * @param tokenAmount Amount of tokens purchased
      */
     function _processPurchase(address beneficiary, uint256 tokenAmount) internal {
         _balances[beneficiary] = _balances[beneficiary].add(tokenAmount);
+        _deliverTokens(address(_vault), tokenAmount);
+    }
+}
+
+/**
+ * @title __unstable__TokenVault
+ * @dev Similar to an Escrow for tokens, this contract allows its primary account to spend its tokens as it sees fit.
+ * This contract is an internal helper for PostDeliveryCrowdsale, and should not be used outside of this context.
+ */
+// solhint-disable-next-line contract-name-camelcase
+contract __unstable__TokenVault is Secondary {
+    function transfer(IERC20 token, address to, uint256 amount) public onlyPrimary {
+        token.transfer(to, amount);
     }
-
 }

contracts/crowdsale/distribution/RefundableCrowdsale.sol

@@ -6,10 +6,10 @@ import "../../payment/escrow/RefundEscrow.sol";
 
 /**
  * @title RefundableCrowdsale
- * @dev Extension of Crowdsale contract that adds a funding goal, and the possibility of users getting a refund if goal
- * is not met.
+ * @dev Extension of `FinalizableCrowdsale` contract that adds a funding goal, and the possibility of users
+ * getting a refund if goal is not met.
  *
- * Deprecated, use RefundablePostDeliveryCrowdsale instead. Note that if you allow tokens to be traded before the goal
+ * Deprecated, use `RefundablePostDeliveryCrowdsale` instead. Note that if you allow tokens to be traded before the goal
  * is met, then an attack is possible in which the attacker purchases tokens from the crowdsale and when they sees that
  * the goal is unlikely to be met, they sell their tokens (possibly at a discount). The attacker will be refunded when
  * the crowdsale is finalized, and the users that purchased from them will be left with worthless tokens.
@@ -28,7 +28,7 @@ contract RefundableCrowdsale is FinalizableCrowdsale {
      * @param goal Funding goal
      */
     constructor (uint256 goal) public {
-        require(goal > 0);
+        require(goal > 0, "RefundableCrowdsale: goal is 0");
         _escrow = new RefundEscrow(wallet());
         _goal = goal;
     }
@@ -41,12 +41,12 @@ contract RefundableCrowdsale is FinalizableCrowdsale {
     }
 
     /**
-     * @dev Investors can claim refunds here if crowdsale is unsuccessful
+     * @dev Investors can claim refunds here if crowdsale is unsuccessful.
      * @param refundee Whose refund will be claimed.
      */
     function claimRefund(address payable refundee) public {
-        require(finalized());
-        require(!goalReached());
+        require(finalized(), "RefundableCrowdsale: not finalized");
+        require(!goalReached(), "RefundableCrowdsale: goal reached");
 
         _escrow.withdraw(refundee);
     }
@@ -60,7 +60,7 @@ contract RefundableCrowdsale is FinalizableCrowdsale {
     }
 
     /**
-     * @dev escrow finalization task, called when finalize() is called
+     * @dev Escrow finalization task, called when finalize() is called.
      */
     function _finalization() internal {
         if (goalReached()) {

contracts/crowdsale/distribution/RefundablePostDeliveryCrowdsale.sol

@@ -12,8 +12,8 @@ import "./PostDeliveryCrowdsale.sol";
  */
 contract RefundablePostDeliveryCrowdsale is RefundableCrowdsale, PostDeliveryCrowdsale {
     function withdrawTokens(address beneficiary) public {
-        require(finalized());
-        require(goalReached());
+        require(finalized(), "RefundablePostDeliveryCrowdsale: not finalized");
+        require(goalReached(), "RefundablePostDeliveryCrowdsale: goal not reached");
 
         super.withdrawTokens(beneficiary);
     }

contracts/crowdsale/emission/AllowanceCrowdsale.sol

@@ -18,10 +18,10 @@ contract AllowanceCrowdsale is Crowdsale {
 
     /**
      * @dev Constructor, takes token wallet address.
-     * @param tokenWallet Address holding the tokens, which has approved allowance to the crowdsale
+     * @param tokenWallet Address holding the tokens, which has approved allowance to the crowdsale.
      */
     constructor (address tokenWallet) public {
-        require(tokenWallet != address(0));
+        require(tokenWallet != address(0), "AllowanceCrowdsale: token wallet is the zero address");
         _tokenWallet = tokenWallet;
     }
 

contracts/crowdsale/emission/MintedCrowdsale.sol

@@ -16,6 +16,9 @@ contract MintedCrowdsale is Crowdsale {
      */
     function _deliverTokens(address beneficiary, uint256 tokenAmount) internal {
         // Potentially dangerous assumption about the type of the token.
-        require(ERC20Mintable(address(token())).mint(beneficiary, tokenAmount));
+        require(
+            ERC20Mintable(address(token())).mint(beneficiary, tokenAmount),
+                "MintedCrowdsale: minting failed"
+        );
     }
 }

contracts/crowdsale/price/IncreasingPriceCrowdsale.sol

@@ -21,18 +21,19 @@ contract IncreasingPriceCrowdsale is TimedCrowdsale {
      * @param finalRate Number of tokens a buyer gets per wei at the end of the crowdsale
      */
     constructor (uint256 initialRate, uint256 finalRate) public {
-        require(finalRate > 0);
-        require(initialRate > finalRate);
+        require(finalRate > 0, "IncreasingPriceCrowdsale: final rate is 0");
+        // solhint-disable-next-line max-line-length
+        require(initialRate > finalRate, "IncreasingPriceCrowdsale: initial rate is not greater than final rate");
         _initialRate = initialRate;
         _finalRate = finalRate;
     }
 
     /**
-     * The base rate function is overridden to revert, since this crowdsale doens't use it, and
+     * The base rate function is overridden to revert, since this crowdsale doesn't use it, and
      * all calls to it are a mistake.
      */
     function rate() public view returns (uint256) {
-        revert();
+        revert("IncreasingPriceCrowdsale: rate() called");
     }
 
     /**

contracts/crowdsale/validation/CappedCrowdsale.sol

@@ -17,7 +17,7 @@ contract CappedCrowdsale is Crowdsale {
      * @param cap Max amount of wei to be contributed
      */
     constructor (uint256 cap) public {
-        require(cap > 0);
+        require(cap > 0, "CappedCrowdsale: cap is 0");
         _cap = cap;
     }
 
@@ -43,6 +43,6 @@ contract CappedCrowdsale is Crowdsale {
      */
     function _preValidatePurchase(address beneficiary, uint256 weiAmount) internal view {
         super._preValidatePurchase(beneficiary, weiAmount);
-        require(weiRaised().add(weiAmount) <= _cap);
+        require(weiRaised().add(weiAmount) <= _cap, "CappedCrowdsale: cap exceeded");
     }
 }

contracts/crowdsale/validation/IndividuallyCappedCrowdsale.sol

@@ -48,11 +48,12 @@ contract IndividuallyCappedCrowdsale is Crowdsale, CapperRole {
      */
     function _preValidatePurchase(address beneficiary, uint256 weiAmount) internal view {
         super._preValidatePurchase(beneficiary, weiAmount);
-        require(_contributions[beneficiary].add(weiAmount) <= _caps[beneficiary]);
+        // solhint-disable-next-line max-line-length
+        require(_contributions[beneficiary].add(weiAmount) <= _caps[beneficiary], "IndividuallyCappedCrowdsale: beneficiary's cap exceeded");
     }
 
     /**
-     * @dev Extend parent behavior to update beneficiary contributions
+     * @dev Extend parent behavior to update beneficiary contributions.
      * @param beneficiary Token purchaser
      * @param weiAmount Amount of wei contributed
      */

contracts/crowdsale/validation/TimedCrowdsale.sol

@@ -13,11 +13,18 @@ contract TimedCrowdsale is Crowdsale {
     uint256 private _openingTime;
     uint256 private _closingTime;
 
+    /**
+     * Event for crowdsale extending
+     * @param newClosingTime new closing time
+     * @param prevClosingTime old closing time
+     */
+    event TimedCrowdsaleExtended(uint256 prevClosingTime, uint256 newClosingTime);
+
     /**
      * @dev Reverts if not in crowdsale time range.
      */
     modifier onlyWhileOpen {
-        require(isOpen());
+        require(isOpen(), "TimedCrowdsale: not open");
         _;
     }
 
@@ -28,8 +35,9 @@ contract TimedCrowdsale is Crowdsale {
      */
     constructor (uint256 openingTime, uint256 closingTime) public {
         // solhint-disable-next-line not-rely-on-time
-        require(openingTime >= block.timestamp);
-        require(closingTime > openingTime);
+        require(openingTime >= block.timestamp, "TimedCrowdsale: opening time is before current time");
+        // solhint-disable-next-line max-line-length
+        require(closingTime > openingTime, "TimedCrowdsale: opening time is not before closing time");
 
         _openingTime = openingTime;
         _closingTime = closingTime;
@@ -67,11 +75,24 @@ contract TimedCrowdsale is Crowdsale {
     }
 
     /**
-     * @dev Extend parent behavior requiring to be within contributing period
+     * @dev Extend parent behavior requiring to be within contributing period.
      * @param beneficiary Token purchaser
      * @param weiAmount Amount of wei contributed
      */
     function _preValidatePurchase(address beneficiary, uint256 weiAmount) internal onlyWhileOpen view {
         super._preValidatePurchase(beneficiary, weiAmount);
     }
+
+    /**
+     * @dev Extend crowdsale.
+     * @param newClosingTime Crowdsale closing time
+     */
+    function _extendTime(uint256 newClosingTime) internal {
+        require(!hasClosed(), "TimedCrowdsale: already closed");
+        // solhint-disable-next-line max-line-length
+        require(newClosingTime > _closingTime, "TimedCrowdsale: new closing time is before current closing time");
+
+        emit TimedCrowdsaleExtended(_closingTime, newClosingTime);
+        _closingTime = newClosingTime;
+    }
 }

contracts/crowdsale/validation/WhitelistCrowdsale.sol

@@ -9,13 +9,13 @@ import "../../access/roles/WhitelistedRole.sol";
  */
 contract WhitelistCrowdsale is WhitelistedRole, Crowdsale {
     /**
-    * @dev Extend parent behavior requiring beneficiary to be whitelisted. Note that no
-    * restriction is imposed on the account sending the transaction.
-    * @param _beneficiary Token beneficiary
-    * @param _weiAmount Amount of wei contributed
-    */
+     * @dev Extend parent behavior requiring beneficiary to be whitelisted. Note that no
+     * restriction is imposed on the account sending the transaction.
+     * @param _beneficiary Token beneficiary
+     * @param _weiAmount Amount of wei contributed
+     */
     function _preValidatePurchase(address _beneficiary, uint256 _weiAmount) internal view {
-        require(isWhitelisted(_beneficiary));
+        require(isWhitelisted(_beneficiary), "WhitelistCrowdsale: beneficiary doesn't have the Whitelisted role");
         super._preValidatePurchase(_beneficiary, _weiAmount);
     }
 }

contracts/cryptography/ECDSA.sol

@@ -1,29 +1,41 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title Elliptic curve signature operations
- * @dev Based on https://gist.github.com/axic/5b33912c6f61ae6fd96d6c4a47afde6d
- * TODO Remove this library once solidity supports passing a signature to ecrecover.
- * See https://github.com/ethereum/solidity/issues/864
+ * @dev Elliptic Curve Digital Signature Algorithm (ECDSA) operations.
+ *
+ * These functions can be used to verify that a message was signed by the holder
+ * of the private keys of a given address.
  */
-
 library ECDSA {
     /**
-     * @dev Recover signer address from a message by using their signature
-     * @param hash bytes32 message, the hash is the signed message. What is recovered is the signer address.
-     * @param signature bytes signature, the signature is generated using web3.eth.sign()
+     * @dev Returns the address that signed a hashed message (`hash`) with
+     * `signature`. This address can then be used for verification purposes.
+     *
+     * The `ecrecover` EVM opcode allows for malleable (non-unique) signatures:
+     * this function rejects them by requiring the `s` value to be in the lower
+     * half order, and the `v` value to be either 27 or 28.
+     *
+     * (.note) This call _does not revert_ if the signature is invalid, or
+     * if the signer is otherwise unable to be retrieved. In those scenarios,
+     * the zero address is returned.
+     *
+     * (.warning) `hash` _must_ be the result of a hash operation for the
+     * verification to be secure: it is possible to craft signatures that
+     * recover to arbitrary addresses for non-hashed data. A safe way to ensure
+     * this is by receiving a hash of the original message (which may otherwise)
+     * be too long), and then calling `toEthSignedMessageHash` on it.
      */
     function recover(bytes32 hash, bytes memory signature) internal pure returns (address) {
-        bytes32 r;
-        bytes32 s;
-        uint8 v;
-
         // Check the signature length
         if (signature.length != 65) {
             return (address(0));
         }
 
         // Divide the signature in r, s and v variables
+        bytes32 r;
+        bytes32 s;
+        uint8 v;
+
         // ecrecover takes the signature parameters, and the only way to get them
         // currently is to use assembly.
         // solhint-disable-next-line no-inline-assembly
@@ -33,23 +45,34 @@ library ECDSA {
             v := byte(0, mload(add(signature, 0x60)))
         }
 
-        // Version of signature should be 27 or 28, but 0 and 1 are also possible versions
-        if (v < 27) {
-            v += 27;
+        // EIP-2 still allows signature malleability for ecrecover(). Remove this possibility and make the signature
+        // unique. Appendix F in the Ethereum Yellow paper (https://ethereum.github.io/yellowpaper/paper.pdf), defines
+        // the valid range for s in (281): 0 < s < secp256k1n ÷ 2 + 1, and for v in (282): v ∈ {27, 28}. Most
+        // signatures from current libraries generate a unique signature with an s-value in the lower half order.
+        //
+        // If your library generates malleable signatures, such as s-values in the upper range, calculate a new s-value
+        // with 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141 - s1 and flip v from 27 to 28 or
+        // vice versa. If your library also generates signatures with 0/1 for v instead 27/28, add 27 to v to accept
+        // these malleable signatures as well.
+        if (uint256(s) > 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0) {
+            return address(0);
         }
 
-        // If the version is correct return the signer address
         if (v != 27 && v != 28) {
-            return (address(0));
-        } else {
-            return ecrecover(hash, v, r, s);
+            return address(0);
         }
+
+        // If the signature is valid (and not malleable), return the signer address
+        return ecrecover(hash, v, r, s);
     }
 
     /**
-     * toEthSignedMessageHash
-     * @dev prefix a bytes32 value with "\x19Ethereum Signed Message:"
-     * and hash the result
+     * @dev Returns an Ethereum Signed Message, created from a `hash`. This
+     * replicates the behavior of the
+     * [`eth_sign`](https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign)
+     * JSON-RPC method.
+     *
+     * See `recover`.
      */
     function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32) {
         // 32 is the length in bytes of hash,

contracts/cryptography/MerkleProof.sol

@@ -1,17 +1,14 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title MerkleProof
- * @dev Merkle proof verification based on
- * https://github.com/ameensol/merkle-tree-solidity/blob/master/src/MerkleProof.sol
+ * @dev These functions deal with verification of Merkle trees (hash trees),
  */
 library MerkleProof {
     /**
-     * @dev Verifies a Merkle proof proving the existence of a leaf in a Merkle tree. Assumes that each pair of leaves
-     * and each pair of pre-images are sorted.
-     * @param proof Merkle proof containing sibling hashes on the branch from the leaf to the root of the Merkle tree
-     * @param root Merkle root
-     * @param leaf Leaf of Merkle tree
+     * @dev Returns true if a `leaf` can be proved to be a part of a Merkle tree
+     * defined by `root`. For this, a `proof` must be provided, containing
+     * sibling hashes on the branch from the leaf to the root of the tree. Each
+     * pair of leaves and each pair of pre-images are assumed to be sorted.
      */
     function verify(bytes32[] memory proof, bytes32 root, bytes32 leaf) internal pure returns (bool) {
         bytes32 computedHash = leaf;

contracts/cryptography/README.md

@@ -0,0 +1,9 @@
+---
+sections:
+  - title: Libraries
+    contracts:
+      - ECDSA
+      - MerkleProof
+---
+
+This collection of libraries provides simple and safe ways to use different cryptographic primitives.

contracts/drafts/Counter.sol

@@ -1,24 +0,0 @@
-pragma solidity ^0.5.0;
-
-/**
- * @title Counter
- * @author Matt Condon (@shrugs)
- * @dev Provides an incrementing uint256 id acquired by the `Counter#next` getter.
- * Use this for issuing ERC721 ids or keeping track of request ids, anything you want, really.
- *
- * Include with `using Counter for Counter.Counter;`
- * @notice Does not allow an Id of 0, which is popularly used to signify a null state in solidity.
- * Does not protect from overflows, but if you have 2^256 ids, you have other problems.
- * (But actually, it's generally impossible to increment a counter this many times, energy wise
- * so it's not something you have to worry about.)
- */
-library Counter {
-    struct Counter {
-        uint256 current; // default: 0
-    }
-
-    function next(Counter storage index) internal returns (uint256) {
-        index.current += 1;
-        return index.current;
-    }
-}

contracts/drafts/Counters.sol

@@ -0,0 +1,37 @@
+pragma solidity ^0.5.0;
+
+import "../math/SafeMath.sol";
+
+/**
+ * @title Counters
+ * @author Matt Condon (@shrugs)
+ * @dev Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number
+ * of elements in a mapping, issuing ERC721 ids, or counting request ids.
+ *
+ * Include with `using Counters for Counters.Counter;`
+ * Since it is not possible to overflow a 256 bit integer with increments of one, `increment` can skip the SafeMath
+ * overflow check, thereby saving gas. This does assume however correct usage, in that the underlying `_value` is never
+ * directly accessed.
+ */
+library Counters {
+    using SafeMath for uint256;
+
+    struct Counter {
+        // This variable should never be directly accessed by users of the library: interactions must be restricted to
+        // the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add
+        // this feature: see https://github.com/ethereum/solidity/issues/4637
+        uint256 _value; // default: 0
+    }
+
+    function current(Counter storage counter) internal view returns (uint256) {
+        return counter._value;
+    }
+
+    function increment(Counter storage counter) internal {
+        counter._value += 1;
+    }
+
+    function decrement(Counter storage counter) internal {
+        counter._value = counter._value.sub(1);
+    }
+}

contracts/drafts/ERC1046/ERC20Metadata.sol

@@ -7,18 +7,18 @@ import "../../token/ERC20/IERC20.sol";
  * @dev See https://eips.ethereum.org/EIPS/eip-1046
  * @dev tokenURI must respond with a URI that implements https://eips.ethereum.org/EIPS/eip-1047
  */
-contract ERC20TokenMetadata is IERC20 {
-    function tokenURI() external view returns (string memory);
-}
-
-contract ERC20WithMetadata is ERC20TokenMetadata {
+contract ERC20Metadata {
     string private _tokenURI;
 
-    constructor (string memory tokenURI) public {
-        _tokenURI = tokenURI;
+    constructor (string memory tokenURI_) public {
+        _setTokenURI(tokenURI_);
     }
 
     function tokenURI() external view returns (string memory) {
         return _tokenURI;
     }
+
+    function _setTokenURI(string memory tokenURI_) internal {
+        _tokenURI = tokenURI_;
+    }
 }

contracts/drafts/ERC20Migrator.sol

@@ -17,7 +17,7 @@ import "../math/Math.sol";
  * OpenZeppelin's ERC20Mintable, but the only functions that are needed are
  * `isMinter(address)` and `mint(address, amount)`. The migrator will check
  * that it is a minter for the token.
- * The balance from the legacy token will be transfered to the migrator, as it
+ * The balance from the legacy token will be transferred to the migrator, as it
  * is migrated, and remain there forever.
  * Although this contract can be used in many different scenarios, the main
  * motivation was to provide a way to migrate ERC20 tokens into an upgradeable
@@ -44,7 +44,7 @@ contract ERC20Migrator {
      * @param legacyToken address of the old token contract
      */
     constructor (IERC20 legacyToken) public {
-        require(address(legacyToken) != address(0));
+        require(address(legacyToken) != address(0), "ERC20Migrator: legacy token is the zero address");
         _legacyToken = legacyToken;
     }
 
@@ -65,14 +65,15 @@ contract ERC20Migrator {
     /**
      * @dev Begins the migration by setting which is the new token that will be
      * minted. This contract must be a minter for the new token.
-     * @param newToken the token that will be minted
+     * @param newToken_ the token that will be minted
      */
-    function beginMigration(ERC20Mintable newToken) public {
-        require(address(_newToken) == address(0));
-        require(address(newToken) != address(0));
-        require(newToken.isMinter(address(this)));
+    function beginMigration(ERC20Mintable newToken_) public {
+        require(address(_newToken) == address(0), "ERC20Migrator: migration already started");
+        require(address(newToken_) != address(0), "ERC20Migrator: new token is the zero address");
+        //solhint-disable-next-line max-line-length
+        require(newToken_.isMinter(address(this)), "ERC20Migrator: not a minter for new token");
 
-        _newToken = newToken;
+        _newToken = newToken_;
     }
 
     /**
@@ -82,6 +83,7 @@ contract ERC20Migrator {
      * @param amount amount of tokens to be migrated
      */
     function migrate(address account, uint256 amount) public {
+        require(address(_newToken) != address(0), "ERC20Migrator: migration not started");
         _legacyToken.safeTransferFrom(account, address(this), amount);
         _newToken.mint(account, amount);
     }

contracts/drafts/ERC20Snapshot.sol

@@ -0,0 +1,140 @@
+pragma solidity ^0.5.0;
+
+import "../math/SafeMath.sol";
+import "../utils/Arrays.sol";
+import "../drafts/Counters.sol";
+import "../token/ERC20/ERC20.sol";
+
+/**
+ * @title ERC20 token with snapshots.
+ * @dev Inspired by Jordi Baylina's MiniMeToken to record historical balances:
+ * https://github.com/Giveth/minime/blob/ea04d950eea153a04c51fa510b068b9dded390cb/contracts/MiniMeToken.sol
+ * When a snapshot is made, the balances and totalSupply at the time of the snapshot are recorded for later
+ * access.
+ *
+ * To make a snapshot, call the `snapshot` function, which will emit the `Snapshot` event and return a snapshot id.
+ * To get the total supply from a snapshot, call the function `totalSupplyAt` with the snapshot id.
+ * To get the balance of an account from a snapshot, call the `balanceOfAt` function with the snapshot id and the
+ * account address.
+ * @author Validity Labs AG <info@validitylabs.org>
+ */
+contract ERC20Snapshot is ERC20 {
+    using SafeMath for uint256;
+    using Arrays for uint256[];
+    using Counters for Counters.Counter;
+
+    // Snapshotted values have arrays of ids and the value corresponding to that id. These could be an array of a
+    // Snapshot struct, but that would impede usage of functions that work on an array.
+    struct Snapshots {
+        uint256[] ids;
+        uint256[] values;
+    }
+
+    mapping (address => Snapshots) private _accountBalanceSnapshots;
+    Snapshots private _totalSupplySnapshots;
+
+    // Snapshot ids increase monotonically, with the first value being 1. An id of 0 is invalid.
+    Counters.Counter private _currentSnapshotId;
+
+    event Snapshot(uint256 id);
+
+    // Creates a new snapshot id. Balances are only stored in snapshots on demand: unless a snapshot was taken, a
+    // balance change will not be recorded. This means the extra added cost of storing snapshotted balances is only paid
+    // when required, but is also flexible enough that it allows for e.g. daily snapshots.
+    function snapshot() public returns (uint256) {
+        _currentSnapshotId.increment();
+
+        uint256 currentId = _currentSnapshotId.current();
+        emit Snapshot(currentId);
+        return currentId;
+    }
+
+    function balanceOfAt(address account, uint256 snapshotId) public view returns (uint256) {
+        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _accountBalanceSnapshots[account]);
+
+        return snapshotted ? value : balanceOf(account);
+    }
+
+    function totalSupplyAt(uint256 snapshotId) public view returns(uint256) {
+        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _totalSupplySnapshots);
+
+        return snapshotted ? value : totalSupply();
+    }
+
+    // _transfer, _mint and _burn are the only functions where the balances are modified, so it is there that the
+    // snapshots are updated. Note that the update happens _before_ the balance change, with the pre-modified value.
+    // The same is true for the total supply and _mint and _burn.
+    function _transfer(address from, address to, uint256 value) internal {
+        _updateAccountSnapshot(from);
+        _updateAccountSnapshot(to);
+
+        super._transfer(from, to, value);
+    }
+
+    function _mint(address account, uint256 value) internal {
+        _updateAccountSnapshot(account);
+        _updateTotalSupplySnapshot();
+
+        super._mint(account, value);
+    }
+
+    function _burn(address account, uint256 value) internal {
+        _updateAccountSnapshot(account);
+        _updateTotalSupplySnapshot();
+
+        super._burn(account, value);
+    }
+
+    // When a valid snapshot is queried, there are three possibilities:
+    //  a) The queried value was not modified after the snapshot was taken. Therefore, a snapshot entry was never
+    //  created for this id, and all stored snapshot ids are smaller than the requested one. The value that corresponds
+    //  to this id is the current one.
+    //  b) The queried value was modified after the snapshot was taken. Therefore, there will be an entry with the
+    //  requested id, and its value is the one to return.
+    //  c) More snapshots were created after the requested one, and the queried value was later modified. There will be
+    //  no entry for the requested id: the value that corresponds to it is that of the smallest snapshot id that is
+    //  larger than the requested one.
+    //
+    // In summary, we need to find an element in an array, returning the index of the smallest value that is larger if
+    // it is not found, unless said value doesn't exist (e.g. when all values are smaller). Arrays.findUpperBound does
+    // exactly this.
+    function _valueAt(uint256 snapshotId, Snapshots storage snapshots)
+        private view returns (bool, uint256)
+    {
+        require(snapshotId > 0, "ERC20Snapshot: id is 0");
+        // solhint-disable-next-line max-line-length
+        require(snapshotId <= _currentSnapshotId.current(), "ERC20Snapshot: nonexistent id");
+
+        uint256 index = snapshots.ids.findUpperBound(snapshotId);
+
+        if (index == snapshots.ids.length) {
+            return (false, 0);
+        } else {
+            return (true, snapshots.values[index]);
+        }
+    }
+
+    function _updateAccountSnapshot(address account) private {
+        _updateSnapshot(_accountBalanceSnapshots[account], balanceOf(account));
+    }
+
+    function _updateTotalSupplySnapshot() private {
+        _updateSnapshot(_totalSupplySnapshots, totalSupply());
+    }
+
+    function _updateSnapshot(Snapshots storage snapshots, uint256 currentValue) private {
+        uint256 currentId = _currentSnapshotId.current();
+        if (_lastSnapshotId(snapshots.ids) < currentId) {
+            snapshots.ids.push(currentId);
+            snapshots.values.push(currentValue);
+        }
+    }
+
+    function _lastSnapshotId(uint256[] storage ids) private view returns (uint256) {
+        if (ids.length == 0) {
+            return 0;
+        } else {
+            return ids[ids.length - 1];
+        }
+    }
+}

contracts/drafts/README.md

@@ -0,0 +1,16 @@
+---
+sections:
+  - title: ERC 20
+    contracts:
+    - ERC20Migrator
+    - ERC20Snapshot
+    - TokenVesting
+  - title: Miscellenous
+    contracts:
+    - Counters
+    - SignatureBouncer
+    - SignedSafeMath
+  - subdirectory: ERC1046
+---
+
+> This page is incomplete. We're working to improve it for the next release. Stay tuned!

contracts/drafts/SignatureBouncer.sol

@@ -29,7 +29,7 @@ import "../cryptography/ECDSA.sol";
  * @notice A method that uses the `onlyValidSignatureAndData` modifier must make
  * the _signature parameter the "last" parameter. You cannot sign a message that
  * has its own signature in it so the last 128 bytes of msg.data (which
- * represents the length of the _signature data and the _signaature data itself)
+ * represents the length of the _signature data and the _signature data itself)
  * is ignored when validating. Also non fixed sized parameters make constructing
  * the data in the signature much more complex.
  * See https://ethereum.stackexchange.com/a/50616 for more details.
@@ -48,31 +48,33 @@ contract SignatureBouncer is SignerRole {
     }
 
     /**
-     * @dev requires that a valid signature of a signer was provided
+     * @dev Requires that a valid signature of a signer was provided.
      */
     modifier onlyValidSignature(bytes memory signature) {
-        require(_isValidSignature(msg.sender, signature));
+        require(_isValidSignature(msg.sender, signature), "SignatureBouncer: invalid signature for caller");
         _;
     }
 
     /**
-     * @dev requires that a valid signature with a specifed method of a signer was provided
+     * @dev Requires that a valid signature with a specified method of a signer was provided.
      */
     modifier onlyValidSignatureAndMethod(bytes memory signature) {
-        require(_isValidSignatureAndMethod(msg.sender, signature));
+        // solhint-disable-next-line max-line-length
+        require(_isValidSignatureAndMethod(msg.sender, signature), "SignatureBouncer: invalid signature for caller and method");
         _;
     }
 
     /**
-     * @dev requires that a valid signature with a specifed method and params of a signer was provided
+     * @dev Requires that a valid signature with a specified method and params of a signer was provided.
      */
     modifier onlyValidSignatureAndData(bytes memory signature) {
-        require(_isValidSignatureAndData(msg.sender, signature));
+        // solhint-disable-next-line max-line-length
+        require(_isValidSignatureAndData(msg.sender, signature), "SignatureBouncer: invalid signature for caller and data");
         _;
     }
 
     /**
-     * @dev is the signature of `this + sender` from a signer?
+     * @dev is the signature of `this + account` from a signer?
      * @return bool
      */
     function _isValidSignature(address account, bytes memory signature) internal view returns (bool) {
@@ -80,7 +82,7 @@ contract SignatureBouncer is SignerRole {
     }
 
     /**
-     * @dev is the signature of `this + sender + methodId` from a signer?
+     * @dev is the signature of `this + account + methodId` from a signer?
      * @return bool
      */
     function _isValidSignatureAndMethod(address account, bytes memory signature) internal view returns (bool) {
@@ -92,12 +94,12 @@ contract SignatureBouncer is SignerRole {
     }
 
     /**
-        * @dev is the signature of `this + sender + methodId + params(s)` from a signer?
-        * @notice the signature parameter of the method being validated must be the "last" parameter
-        * @return bool
-        */
+     * @dev is the signature of `this + account + methodId + params(s)` from a signer?
+     * @notice the signature parameter of the method being validated must be the "last" parameter
+     * @return bool
+     */
     function _isValidSignatureAndData(address account, bytes memory signature) internal view returns (bool) {
-        require(msg.data.length > _SIGNATURE_SIZE);
+        require(msg.data.length > _SIGNATURE_SIZE, "SignatureBouncer: data is too short");
 
         bytes memory data = new bytes(msg.data.length - _SIGNATURE_SIZE);
         for (uint i = 0; i < data.length; i++) {
@@ -108,8 +110,8 @@ contract SignatureBouncer is SignerRole {
     }
 
     /**
-     * @dev internal function to convert a hash to an eth signed message
-     * and then recover the signature and check it against the signer role
+     * @dev Internal function to convert a hash to an eth signed message
+     * and then recover the signature and check it against the signer role.
      * @return bool
      */
     function _isValidDataHash(bytes32 hash, bytes memory signature) internal view returns (bool) {

contracts/drafts/SignedSafeMath.sol

@@ -2,36 +2,36 @@ pragma solidity ^0.5.0;
 
 /**
  * @title SignedSafeMath
- * @dev Signed math operations with safety checks that revert on error
+ * @dev Signed math operations with safety checks that revert on error.
  */
 library SignedSafeMath {
     int256 constant private INT256_MIN = -2**255;
 
     /**
-    * @dev Multiplies two signed integers, reverts on overflow.
-    */
+     * @dev Multiplies two signed integers, reverts on overflow.
+     */
     function mul(int256 a, int256 b) internal pure returns (int256) {
         // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
         // benefit is lost if 'b' is also tested.
-        // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
+        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
         if (a == 0) {
             return 0;
         }
 
-        require(!(a == -1 && b == INT256_MIN)); // This is the only case of overflow not detected by the check below
+        require(!(a == -1 && b == INT256_MIN), "SignedSafeMath: multiplication overflow");
 
         int256 c = a * b;
-        require(c / a == b);
+        require(c / a == b, "SignedSafeMath: multiplication overflow");
 
         return c;
     }
 
     /**
-    * @dev Integer division of two signed integers truncating the quotient, reverts on division by zero.
-    */
+     * @dev Integer division of two signed integers truncating the quotient, reverts on division by zero.
+     */
     function div(int256 a, int256 b) internal pure returns (int256) {
-        require(b != 0); // Solidity only automatically asserts when dividing by 0
-        require(!(b == -1 && a == INT256_MIN)); // This is the only case of overflow
+        require(b != 0, "SignedSafeMath: division by zero");
+        require(!(b == -1 && a == INT256_MIN), "SignedSafeMath: division overflow");
 
         int256 c = a / b;
 
@@ -39,21 +39,21 @@ library SignedSafeMath {
     }
 
     /**
-    * @dev Subtracts two signed integers, reverts on overflow.
-    */
+     * @dev Subtracts two signed integers, reverts on overflow.
+     */
     function sub(int256 a, int256 b) internal pure returns (int256) {
         int256 c = a - b;
-        require((b >= 0 && c <= a) || (b < 0 && c > a));
+        require((b >= 0 && c <= a) || (b < 0 && c > a), "SignedSafeMath: subtraction overflow");
 
         return c;
     }
 
     /**
-    * @dev Adds two signed integers, reverts on overflow.
-    */
+     * @dev Adds two signed integers, reverts on overflow.
+     */
     function add(int256 a, int256 b) internal pure returns (int256) {
         int256 c = a + b;
-        require((b >= 0 && c >= a) || (b < 0 && c < a));
+        require((b >= 0 && c >= a) || (b < 0 && c < a), "SignedSafeMath: addition overflow");
 
         return c;
     }

contracts/drafts/TokenVesting.sol

@@ -13,8 +13,8 @@ import "../math/SafeMath.sol";
 contract TokenVesting is Ownable {
     // The vesting schedule is time-based (i.e. using block timestamps as opposed to e.g. block numbers), and is
     // therefore sensitive to timestamp manipulation (which is something miners can do, to a certain degree). Therefore,
-    // it is recommended to avoid using short time durations (less than a minute). Typical vesting schemes, with a cliff
-    // period of a year and a duration of four years, are safe to use.
+    // it is recommended to avoid using short time durations (less than a minute). Typical vesting schemes, with a
+    // cliff period of a year and a duration of four years, are safe to use.
     // solhint-disable not-rely-on-time
 
     using SafeMath for uint256;
@@ -47,10 +47,12 @@ contract TokenVesting is Ownable {
      * @param revocable whether the vesting is revocable or not
      */
     constructor (address beneficiary, uint256 start, uint256 cliffDuration, uint256 duration, bool revocable) public {
-        require(beneficiary != address(0));
-        require(cliffDuration <= duration);
-        require(duration > 0);
-        require(start.add(duration) > block.timestamp);
+        require(beneficiary != address(0), "TokenVesting: beneficiary is the zero address");
+        // solhint-disable-next-line max-line-length
+        require(cliffDuration <= duration, "TokenVesting: cliff is longer than duration");
+        require(duration > 0, "TokenVesting: duration is 0");
+        // solhint-disable-next-line max-line-length
+        require(start.add(duration) > block.timestamp, "TokenVesting: final time is before current time");
 
         _beneficiary = beneficiary;
         _revocable = revocable;
@@ -115,7 +117,7 @@ contract TokenVesting is Ownable {
     function release(IERC20 token) public {
         uint256 unreleased = _releasableAmount(token);
 
-        require(unreleased > 0);
+        require(unreleased > 0, "TokenVesting: no tokens are due");
 
         _released[address(token)] = _released[address(token)].add(unreleased);
 
@@ -130,8 +132,8 @@ contract TokenVesting is Ownable {
      * @param token ERC20 token which is being vested
      */
     function revoke(IERC20 token) public onlyOwner {
-        require(_revocable);
-        require(!_revoked[address(token)]);
+        require(_revocable, "TokenVesting: cannot revoke");
+        require(!_revoked[address(token)], "TokenVesting: token already revoked");
 
         uint256 balance = token.balanceOf(address(this));
 

contracts/examples/SampleCrowdsale.sol

@@ -48,6 +48,6 @@ contract SampleCrowdsale is CappedCrowdsale, RefundableCrowdsale, MintedCrowdsal
     {
         //As goal needs to be met for a successful crowdsale
         //the value needs to less or equal than a cap which is limit for accepted funds
-        require(goal <= cap);
+        require(goal <= cap, "SampleCrowdSale: goal is greater than cap");
     }
 }

contracts/introspection/ERC165.sol

@@ -3,42 +3,50 @@ pragma solidity ^0.5.0;
 import "./IERC165.sol";
 
 /**
- * @title ERC165
- * @author Matt Condon (@shrugs)
- * @dev Implements ERC165 using a lookup table.
+ * @dev Implementation of the `IERC165` interface.
+ *
+ * Contracts may inherit from this and call `_registerInterface` to declare
+ * their support of an interface.
  */
 contract ERC165 is IERC165 {
-    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
-    /**
-     * 0x01ffc9a7 ===
-     *     bytes4(keccak256('supportsInterface(bytes4)'))
+    /*
+     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
      */
+    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
 
     /**
-     * @dev a mapping of interface id to whether or not it's supported
+     * @dev Mapping of interface ids to whether or not it's supported.
      */
     mapping(bytes4 => bool) private _supportedInterfaces;
 
-    /**
-     * @dev A contract implementing SupportsInterfaceWithLookup
-     * implement ERC165 itself
-     */
     constructor () internal {
+        // Derived contracts need only register support for their own interfaces,
+        // we register support for ERC165 itself here
         _registerInterface(_INTERFACE_ID_ERC165);
     }
 
     /**
-     * @dev implement supportsInterface(bytes4) using a lookup table
+     * @dev See `IERC165.supportsInterface`.
+     *
+     * Time complexity O(1), guaranteed to always use less than 30 000 gas.
      */
     function supportsInterface(bytes4 interfaceId) external view returns (bool) {
         return _supportedInterfaces[interfaceId];
     }
 
     /**
-     * @dev internal method for registering an interface
+     * @dev Registers the contract as an implementer of the interface defined by
+     * `interfaceId`. Support of the actual ERC165 interface is automatic and
+     * registering its interface id is not required.
+     *
+     * See `IERC165.supportsInterface`.
+     *
+     * Requirements:
+     *
+     * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
      */
     function _registerInterface(bytes4 interfaceId) internal {
-        require(interfaceId != 0xffffffff);
+        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
         _supportedInterfaces[interfaceId] = true;
     }
 }

contracts/introspection/ERC165Checker.sol

@@ -1,24 +1,23 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title ERC165Checker
- * @dev Use `using ERC165Checker for address`; to include this library
- * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md
+ * @dev Library used to query support of an interface declared via `IERC165`.
+ *
+ * Note that these functions return the actual result of the query: they do not
+ * `revert` if an interface is not supported. It is up to the caller to decide
+ * what to do in these cases.
  */
 library ERC165Checker {
     // As per the EIP-165 spec, no interface should ever match 0xffffffff
     bytes4 private constant _INTERFACE_ID_INVALID = 0xffffffff;
 
-    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
-    /**
-     * 0x01ffc9a7 ===
-     *     bytes4(keccak256('supportsInterface(bytes4)'))
+    /*
+     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
      */
+    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
 
     /**
-     * @notice Query if a contract supports ERC165
-     * @param account The address of the contract to query for support of ERC165
-     * @return true if the contract at account implements ERC165
+     * @dev Returns true if `account` supports the `IERC165` interface,
      */
     function _supportsERC165(address account) internal view returns (bool) {
         // Any contract that implements ERC165 must explicitly indicate support of
@@ -28,12 +27,10 @@ library ERC165Checker {
     }
 
     /**
-     * @notice Query if a contract implements an interface, also checks support of ERC165
-     * @param account The address of the contract to query for support of an interface
-     * @param interfaceId The interface identifier, as specified in ERC-165
-     * @return true if the contract at account indicates support of the interface with
-     * identifier interfaceId, false otherwise
-     * @dev Interface identification is specified in ERC-165.
+     * @dev Returns true if `account` supports the interface defined by
+     * `interfaceId`. Support for `IERC165` itself is queried automatically.
+     *
+     * See `IERC165.supportsInterface`.
      */
     function _supportsInterface(address account, bytes4 interfaceId) internal view returns (bool) {
         // query support of both ERC165 as per the spec and support of _interfaceId
@@ -42,12 +39,13 @@ library ERC165Checker {
     }
 
     /**
-     * @notice Query if a contract implements interfaces, also checks support of ERC165
-     * @param account The address of the contract to query for support of an interface
-     * @param interfaceIds A list of interface identifiers, as specified in ERC-165
-     * @return true if the contract at account indicates support all interfaces in the
-     * interfaceIds list, false otherwise
-     * @dev Interface identification is specified in ERC-165.
+     * @dev Returns true if `account` supports all the interfaces defined in
+     * `interfaceIds`. Support for `IERC165` itself is queried automatically.
+     *
+     * Batch-querying can lead to gas savings by skipping repeated checks for
+     * `IERC165` support.
+     *
+     * See `IERC165.supportsInterface`.
      */
     function _supportsAllInterfaces(address account, bytes4[] memory interfaceIds) internal view returns (bool) {
         // query support of ERC165 itself
@@ -109,15 +107,15 @@ library ERC165Checker {
             mstore(output, 0x0)
 
             success := staticcall(
-                30000,                                 // 30k gas
-                account,                            // To addr
+                30000,                   // 30k gas
+                account,                 // To addr
                 encodedParams_data,
                 encodedParams_size,
                 output,
-                0x20                                     // Outputs are 32 bytes long
+                0x20                     // Outputs are 32 bytes long
             )
 
-            result := mload(output)    // Load the result
+            result := mload(output)      // Load the result
         }
     }
 }

contracts/introspection/ERC1820Implementer.sol

@@ -0,0 +1,35 @@
+pragma solidity ^0.5.0;
+
+import "./IERC1820Implementer.sol";
+
+/**
+ * @dev Implementation of the `IERC1820Implementer` interface.
+ *
+ * Contracts may inherit from this and call `_registerInterfaceForAddress` to
+ * declare their willingness to be implementers.
+ * `IERC1820Registry.setInterfaceImplementer` should then be called for the
+ * registration to be complete.
+ */
+contract ERC1820Implementer is IERC1820Implementer {
+    bytes32 constant private ERC1820_ACCEPT_MAGIC = keccak256(abi.encodePacked("ERC1820_ACCEPT_MAGIC"));
+
+    mapping(bytes32 => mapping(address => bool)) private _supportedInterfaces;
+
+    /**
+     * See `IERC1820Implementer.canImplementInterfaceForAddress`.
+     */
+    function canImplementInterfaceForAddress(bytes32 interfaceHash, address account) external view returns (bytes32) {
+        return _supportedInterfaces[interfaceHash][account] ? ERC1820_ACCEPT_MAGIC : bytes32(0x00);
+    }
+
+    /**
+     * @dev Declares the contract as willing to be an implementer of
+     * `interfaceHash` for `account`.
+     *
+     * See `IERC1820Registry.setInterfaceImplementer` and
+     * `IERC1820Registry.interfaceHash`.
+     */
+    function _registerInterfaceForAddress(bytes32 interfaceHash, address account) internal {
+        _supportedInterfaces[interfaceHash][account] = true;
+    }
+}

contracts/introspection/IERC165.sol

@@ -1,15 +1,22 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title IERC165
- * @dev https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md
+ * @dev Interface of the ERC165 standard, as defined in the
+ * [EIP](https://eips.ethereum.org/EIPS/eip-165).
+ *
+ * Implementers can declare support of contract interfaces, which can then be
+ * queried by others (`ERC165Checker`).
+ *
+ * For an implementation, see `ERC165`.
  */
 interface IERC165 {
     /**
-     * @notice Query if a contract implements an interface
-     * @param interfaceId The interface identifier, as specified in ERC-165
-     * @dev Interface identification is specified in ERC-165. This function
-     * uses less than 30,000 gas.
+     * @dev Returns true if this contract implements the interface defined by
+     * `interfaceId`. See the corresponding
+     * [EIP section](https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified)
+     * to learn more about how these ids are created.
+     *
+     * This function call must use less than 30 000 gas.
      */
     function supportsInterface(bytes4 interfaceId) external view returns (bool);
 }

contracts/introspection/IERC1820Implementer.sol

@@ -0,0 +1,17 @@
+pragma solidity ^0.5.0;
+
+/**
+ * @dev Interface for an ERC1820 implementer, as defined in the
+ * [EIP](https://eips.ethereum.org/EIPS/eip-1820#interface-implementation-erc1820implementerinterface).
+ * Used by contracts that will be registered as implementers in the
+ * `IERC1820Registry`.
+ */
+interface IERC1820Implementer {
+    /**
+     * @dev Returns a special value (`ERC1820_ACCEPT_MAGIC`) if this contract
+     * implements `interfaceHash` for `account`.
+     *
+     * See `IERC1820Registry.setInterfaceImplementer`.
+     */
+    function canImplementInterfaceForAddress(bytes32 interfaceHash, address account) external view returns (bytes32);
+}

contracts/introspection/IERC1820Registry.sol

@@ -0,0 +1,109 @@
+pragma solidity ^0.5.0;
+
+/**
+ * @dev Interface of the global ERC1820 Registry, as defined in the
+ * [EIP](https://eips.ethereum.org/EIPS/eip-1820). Accounts may register
+ * implementers for interfaces in this registry, as well as query support.
+ *
+ * Implementers may be shared by multiple accounts, and can also implement more
+ * than a single interface for each account. Contracts can implement interfaces
+ * for themselves, but externally-owned accounts (EOA) must delegate this to a
+ * contract.
+ *
+ * `IERC165` interfaces can also be queried via the registry.
+ *
+ * For an in-depth explanation and source code analysis, see the EIP text.
+ */
+interface IERC1820Registry {
+    /**
+     * @dev Sets `newManager` as the manager for `account`. A manager of an
+     * account is able to set interface implementers for it.
+     *
+     * By default, each account is its own manager. Passing a value of `0x0` in
+     * `newManager` will reset the manager to this initial state.
+     *
+     * Emits a `ManagerChanged` event.
+     *
+     * Requirements:
+     *
+     * - the caller must be the current manager for `account`.
+     */
+    function setManager(address account, address newManager) external;
+
+    /**
+     * @dev Returns the manager for `account`.
+     *
+     * See `setManager`.
+     */
+    function getManager(address account) external view returns (address);
+
+    /**
+     * @dev Sets the `implementer` contract as `account`'s implementer for
+     * `interfaceHash`.
+     *
+     * `account` being the zero address is an alias for the caller's address.
+     * The zero address can also be used in `implementer` to remove an old one.
+     *
+     * See `interfaceHash` to learn how these are created.
+     *
+     * Emits an `InterfaceImplementerSet` event.
+     *
+     * Requirements:
+     *
+     * - the caller must be the current manager for `account`.
+     * - `interfaceHash` must not be an `IERC165` interface id (i.e. it must not
+     * end in 28 zeroes).
+     * - `implementer` must implement `IERC1820Implementer` and return true when
+     * queried for support, unless `implementer` is the caller. See
+     * `IERC1820Implementer.canImplementInterfaceForAddress`.
+     */
+    function setInterfaceImplementer(address account, bytes32 interfaceHash, address implementer) external;
+
+    /**
+     * @dev Returns the implementer of `interfaceHash` for `account`. If no such
+     * implementer is registered, returns the zero address.
+     *
+     * If `interfaceHash` is an `IERC165` interface id (i.e. it ends with 28
+     * zeroes), `account` will be queried for support of it.
+     *
+     * `account` being the zero address is an alias for the caller's address.
+     */
+    function getInterfaceImplementer(address account, bytes32 interfaceHash) external view returns (address);
+
+    /**
+     * @dev Returns the interface hash for an `interfaceName`, as defined in the
+     * corresponding
+     * [section of the EIP](https://eips.ethereum.org/EIPS/eip-1820#interface-name).
+     */
+    function interfaceHash(string calldata interfaceName) external pure returns (bytes32);
+
+    /**
+     *  @notice Updates the cache with whether the contract implements an ERC165 interface or not.
+     *  @param account Address of the contract for which to update the cache.
+     *  @param interfaceId ERC165 interface for which to update the cache.
+     */
+    function updateERC165Cache(address account, bytes4 interfaceId) external;
+
+    /**
+     *  @notice Checks whether a contract implements an ERC165 interface or not.
+     *  If the result is not cached a direct lookup on the contract address is performed.
+     *  If the result is not cached or the cached value is out-of-date, the cache MUST be updated manually by calling
+     *  'updateERC165Cache' with the contract address.
+     *  @param account Address of the contract to check.
+     *  @param interfaceId ERC165 interface to check.
+     *  @return True if `account.address()` implements `interfaceId`, false otherwise.
+     */
+    function implementsERC165Interface(address account, bytes4 interfaceId) external view returns (bool);
+
+    /**
+     *  @notice Checks whether a contract implements an ERC165 interface or not without using nor updating the cache.
+     *  @param account Address of the contract to check.
+     *  @param interfaceId ERC165 interface to check.
+     *  @return True if `account.address()` implements `interfaceId`, false otherwise.
+     */
+    function implementsERC165InterfaceNoCache(address account, bytes4 interfaceId) external view returns (bool);
+
+    event InterfaceImplementerSet(address indexed account, bytes32 indexed interfaceHash, address indexed implementer);
+
+    event ManagerChanged(address indexed account, address indexed newManager);
+}

contracts/introspection/README.md

@@ -0,0 +1,23 @@
+---
+sections:
+  - title: Local
+    contracts:
+      - IERC165
+      - ERC165
+      - ERC165Checker
+  - title: Global
+    contracts:
+      - IERC1820Registry
+      - IERC1820Implementer
+      - ERC1820Implementer
+---
+
+This set of interfaces and contracts deal with [type introspection](https://en.wikipedia.org/wiki/Type_introspection) of contracts, that is, examining which functions can be called on them. This is usually referred to as a contract's _interface_.
+
+Ethereum contracts have no native concept of an interface, so applications must usually simply trust they are not making an incorrect call. For trusted setups this is a non-issue, but often unknown and untrusted third-party addresses need to be interacted with. There may even not be any direct calls to them! (e.g. `ERC20` tokens may be sent to a contract that lacks a way to transfer them out of it, locking them forever). In these cases, a contract _declaring_ its interface can be very helpful in preventing errors.
+
+There are two main ways to approach this.
+ - Locally, where a contract implements `IERC165` and declares an interface, and a second one queries it directly via `ERC165Checker`.
+ - Globally, where a global and unique registry (`IERC1820Registry`) is used to register implementers of a certain interface (`IERC1820Implementer`). It is then the registry that is queried, which allows for more complex setups, like contracts implementing interfaces for externally-owned accounts.
+
+Note that, in all cases, accounts simply _declare_ their interfaces, but they are not required to actually implement them. This mechanism can therefore be used to both prevent errors and allow for complex interactions (see `ERC777`), but it must not be relied on for security.

contracts/lifecycle/Pausable.sol

@@ -3,21 +3,37 @@ pragma solidity ^0.5.0;
 import "../access/roles/PauserRole.sol";
 
 /**
- * @title Pausable
- * @dev Base contract which allows children to implement an emergency stop mechanism.
+ * @dev Contract module which allows children to implement an emergency stop
+ * mechanism that can be triggered by an authorized account.
+ *
+ * This module is used through inheritance. It will make available the
+ * modifiers `whenNotPaused` and `whenPaused`, which can be applied to
+ * the functions of your contract. Note that they will not be pausable by
+ * simply including this module, only once the modifiers are put in place.
  */
 contract Pausable is PauserRole {
+    /**
+     * @dev Emitted when the pause is triggered by a pauser (`account`).
+     */
     event Paused(address account);
+
+    /**
+     * @dev Emitted when the pause is lifted by a pauser (`account`).
+     */
     event Unpaused(address account);
 
     bool private _paused;
 
+    /**
+     * @dev Initializes the contract in unpaused state. Assigns the Pauser role
+     * to the deployer.
+     */
     constructor () internal {
         _paused = false;
     }
 
     /**
-     * @return true if the contract is paused, false otherwise.
+     * @dev Returns true if the contract is paused, and false otherwise.
      */
     function paused() public view returns (bool) {
         return _paused;
@@ -27,7 +43,7 @@ contract Pausable is PauserRole {
      * @dev Modifier to make a function callable only when the contract is not paused.
      */
     modifier whenNotPaused() {
-        require(!_paused);
+        require(!_paused, "Pausable: paused");
         _;
     }
 
@@ -35,12 +51,12 @@ contract Pausable is PauserRole {
      * @dev Modifier to make a function callable only when the contract is paused.
      */
     modifier whenPaused() {
-        require(_paused);
+        require(_paused, "Pausable: not paused");
         _;
     }
 
     /**
-     * @dev called by the owner to pause, triggers stopped state
+     * @dev Called by a pauser to pause, triggers stopped state.
      */
     function pause() public onlyPauser whenNotPaused {
         _paused = true;
@@ -48,7 +64,7 @@ contract Pausable is PauserRole {
     }
 
     /**
-     * @dev called by the owner to unpause, returns to normal state
+     * @dev Called by a pauser to unpause, returns to normal state.
      */
     function unpause() public onlyPauser whenPaused {
         _paused = false;

contracts/math/Math.sol

@@ -1,29 +1,27 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title Math
- * @dev Assorted math operations
+ * @dev Standard math utilities missing in the Solidity language.
  */
 library Math {
     /**
-    * @dev Returns the largest of two numbers.
-    */
+     * @dev Returns the largest of two numbers.
+     */
     function max(uint256 a, uint256 b) internal pure returns (uint256) {
         return a >= b ? a : b;
     }
 
     /**
-    * @dev Returns the smallest of two numbers.
-    */
+     * @dev Returns the smallest of two numbers.
+     */
     function min(uint256 a, uint256 b) internal pure returns (uint256) {
         return a < b ? a : b;
     }
 
     /**
-    * @dev Calculates the average of two numbers. Since these are integers,
-    * averages of an even and odd number cannot be represented, and will be
-    * rounded down.
-    */
+     * @dev Returns the average of two numbers. The result is rounded towards
+     * zero.
+     */
     function average(uint256 a, uint256 b) internal pure returns (uint256) {
         // (a + b) / 2 can overflow, so we distribute
         return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);

contracts/math/README.md

@@ -0,0 +1,10 @@
+---
+title: Math
+sections:
+  - title: Libraries
+    contracts:
+      - SafeMath
+      - Math
+---
+
+These are math-related utilities.

contracts/math/SafeMath.sol

@@ -1,33 +1,88 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title SafeMath
- * @dev Unsigned math operations with safety checks that revert on error
+ * @dev Wrappers over Solidity's arithmetic operations with added overflow
+ * checks.
+ *
+ * Arithmetic operations in Solidity wrap on overflow. This can easily result
+ * in bugs, because programmers usually assume that an overflow raises an
+ * error, which is the standard behavior in high level programming languages.
+ * `SafeMath` restores this intuition by reverting the transaction when an
+ * operation overflows.
+ *
+ * Using this library instead of the unchecked operations eliminates an entire
+ * class of bugs, so it's recommended to use it always.
  */
 library SafeMath {
     /**
-    * @dev Multiplies two unsigned integers, reverts on overflow.
-    */
-    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
-        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
-        // benefit is lost if 'b' is also tested.
-        // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
-        if (a == 0) {
-            return 0;
-        }
-
-        uint256 c = a * b;
-        require(c / a == b);
+     * @dev Returns the addition of two unsigned integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     * - Addition cannot overflow.
+     */
+    function add(uint256 a, uint256 b) internal pure returns (uint256) {
+        uint256 c = a + b;
+        require(c >= a, "SafeMath: addition overflow");
 
         return c;
     }
 
     /**
-    * @dev Integer division of two unsigned integers truncating the quotient, reverts on division by zero.
-    */
+     * @dev Returns the subtraction of two unsigned integers, reverting on
+     * overflow (when the result is negative).
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     * - Subtraction cannot overflow.
+     */
+    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
+        require(b <= a, "SafeMath: subtraction overflow");
+        uint256 c = a - b;
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the multiplication of two unsigned integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `*` operator.
+     *
+     * Requirements:
+     * - Multiplication cannot overflow.
+     */
+    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
+        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
+        // benefit is lost if 'b' is also tested.
+        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
+        if (a == 0) {
+            return 0;
+        }
+
+        uint256 c = a * b;
+        require(c / a == b, "SafeMath: multiplication overflow");
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the integer division of two unsigned integers. Reverts on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator. Note: this function uses a
+     * `revert` opcode (which leaves remaining gas untouched) while Solidity
+     * uses an invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     * - The divisor cannot be zero.
+     */
     function div(uint256 a, uint256 b) internal pure returns (uint256) {
         // Solidity only automatically asserts when dividing by 0
-        require(b > 0);
+        require(b > 0, "SafeMath: division by zero");
         uint256 c = a / b;
         // assert(a == b * c + a % b); // There is no case in which this doesn't hold
 
@@ -35,31 +90,18 @@ library SafeMath {
     }
 
     /**
-    * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
-    */
-    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
-        require(b <= a);
-        uint256 c = a - b;
-
-        return c;
-    }
-
-    /**
-    * @dev Adds two unsigned integers, reverts on overflow.
-    */
-    function add(uint256 a, uint256 b) internal pure returns (uint256) {
-        uint256 c = a + b;
-        require(c >= a);
-
-        return c;
-    }
-
-    /**
-    * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
-    * reverts when dividing by zero.
-    */
+     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
+     * Reverts when dividing by zero.
+     *
+     * Counterpart to Solidity's `%` operator. This function uses a `revert`
+     * opcode (which leaves remaining gas untouched) while Solidity uses an
+     * invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     * - The divisor cannot be zero.
+     */
     function mod(uint256 a, uint256 b) internal pure returns (uint256) {
-        require(b != 0);
+        require(b != 0, "SafeMath: modulo by zero");
         return a % b;
     }
 }

contracts/mocks/Acknowledger.sol

@@ -1,19 +0,0 @@
-pragma solidity ^0.5.0;
-
-contract Acknowledger {
-    event AcknowledgeFoo(uint256 a);
-    event AcknowledgeBarSingle(uint256 a);
-    event AcknowledgeBarDouble(uint256 a, uint256 b);
-
-    function foo(uint256 a) public {
-        emit AcknowledgeFoo(a);
-    }
-
-    function bar(uint256 a) public {
-        emit AcknowledgeBarSingle(a);
-    }
-
-    function bar(uint256 a, uint256 b) public {
-        emit AcknowledgeBarDouble(a, b);
-    }
-}

contracts/mocks/CounterImpl.sol

@@ -1,17 +0,0 @@
-pragma solidity ^0.5.0;
-
-import "../drafts/Counter.sol";
-
-contract CounterImpl {
-    using Counter for Counter.Counter;
-
-    uint256 public theId;
-
-    // use whatever key you want to track your counters
-    mapping(string => Counter.Counter) private _counters;
-
-    function doThing(string memory key) public returns (uint256) {
-        theId = _counters[key].next();
-        return theId;
-    }
-}

contracts/mocks/CountersImpl.sol

@@ -0,0 +1,21 @@
+pragma solidity ^0.5.0;
+
+import "../drafts/Counters.sol";
+
+contract CountersImpl {
+    using Counters for Counters.Counter;
+
+    Counters.Counter private _counter;
+
+    function current() public view returns (uint256) {
+        return _counter.current();
+    }
+
+    function increment() public {
+        _counter.increment();
+    }
+
+    function decrement() public {
+        _counter.decrement();
+    }
+}

contracts/mocks/ERC165/ERC165InterfacesSupported.sol

@@ -3,7 +3,8 @@ pragma solidity ^0.5.0;
 import "../../introspection/IERC165.sol";
 
 /**
- * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-214.md#specification
+ * https://eips.ethereum.org/EIPS/eip-214#specification
+ * From the specification:
  * > Any attempts to make state-changing operations inside an execution instance with STATIC set to true will instead
  * throw an exception.
  * > These operations include [...], LOG0, LOG1, LOG2, [...]
@@ -12,37 +13,36 @@ import "../../introspection/IERC165.sol";
  * solidity-coverage ignores the /mocks folder, so we duplicate its implementation here to avoid instrumenting it
  */
 contract SupportsInterfaceWithLookupMock is IERC165 {
-    bytes4 public constant INTERFACE_ID_ERC165 = 0x01ffc9a7;
-    /**
-     * 0x01ffc9a7 ===
-     *     bytes4(keccak256('supportsInterface(bytes4)'))
+    /*
+     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
      */
+    bytes4 public constant INTERFACE_ID_ERC165 = 0x01ffc9a7;
 
     /**
-     * @dev a mapping of interface id to whether or not it's supported
+     * @dev A mapping of interface id to whether or not it's supported.
      */
     mapping(bytes4 => bool) private _supportedInterfaces;
 
     /**
      * @dev A contract implementing SupportsInterfaceWithLookup
-     * implement ERC165 itself
+     * implement ERC165 itself.
      */
     constructor () public {
         _registerInterface(INTERFACE_ID_ERC165);
     }
 
     /**
-     * @dev implement supportsInterface(bytes4) using a lookup table
+     * @dev Implement supportsInterface(bytes4) using a lookup table.
      */
     function supportsInterface(bytes4 interfaceId) external view returns (bool) {
         return _supportedInterfaces[interfaceId];
     }
 
     /**
-     * @dev private method for registering an interface
+     * @dev Private method for registering an interface.
      */
     function _registerInterface(bytes4 interfaceId) internal {
-        require(interfaceId != 0xffffffff);
+        require(interfaceId != 0xffffffff, "ERC165InterfacesSupported: invalid interface id");
         _supportedInterfaces[interfaceId] = true;
     }
 }

contracts/mocks/ERC1820ImplementerMock.sol

@@ -0,0 +1,9 @@
+pragma solidity ^0.5.0;
+
+import "../introspection/ERC1820Implementer.sol";
+
+contract ERC1820ImplementerMock is ERC1820Implementer {
+    function registerInterfaceForAddress(bytes32 interfaceHash, address account) public {
+        _registerInterfaceForAddress(interfaceHash, account);
+    }
+}

contracts/mocks/ERC20MetadataMock.sol

@@ -0,0 +1,14 @@
+pragma solidity ^0.5.0;
+
+import "../token/ERC20/ERC20.sol";
+import "../drafts/ERC1046/ERC20Metadata.sol";
+
+contract ERC20MetadataMock is ERC20, ERC20Metadata {
+    constructor (string memory tokenURI) public ERC20Metadata(tokenURI) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
+
+    function setTokenURI(string memory tokenURI) public {
+        _setTokenURI(tokenURI);
+    }
+}

contracts/mocks/ERC20Mock.sol

@@ -19,4 +19,12 @@ contract ERC20Mock is ERC20 {
     function burnFrom(address account, uint256 amount) public {
         _burnFrom(account, amount);
     }
+
+    function transferInternal(address from, address to, uint256 value) public {
+        _transfer(from, to, value);
+    }
+
+    function approveInternal(address owner, address spender, uint256 value) public {
+        _approve(owner, spender, value);
+    }
 }

contracts/mocks/ERC20SnapshotMock.sol

@@ -0,0 +1,18 @@
+pragma solidity ^0.5.0;
+
+import "../drafts/ERC20Snapshot.sol";
+
+
+contract ERC20SnapshotMock is ERC20Snapshot {
+    constructor(address initialAccount, uint256 initialBalance) public {
+        _mint(initialAccount, initialBalance);
+    }
+
+    function mint(address account, uint256 amount) public {
+        _mint(account, amount);
+    }
+
+    function burn(address account, uint256 amount) public {
+        _burn(account, amount);
+    }
+}

contracts/mocks/ERC20WithMetadataMock.sol

@@ -1,10 +0,0 @@
-pragma solidity ^0.5.0;
-
-import "../token/ERC20/ERC20.sol";
-import "../drafts/ERC1046/TokenMetadata.sol";
-
-contract ERC20WithMetadataMock is ERC20, ERC20WithMetadata {
-    constructor (string memory tokenURI) public ERC20WithMetadata(tokenURI) {
-        // solhint-disable-previous-line no-empty-blocks
-    }
-}

contracts/mocks/ERC721ReceiverMock.sol

@@ -16,7 +16,7 @@ contract ERC721ReceiverMock is IERC721Receiver {
     function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data)
         public returns (bytes4)
     {
-        require(!_reverts);
+        require(!_reverts, "ERC721ReceiverMock: reverting");
         emit Received(operator, from, tokenId, data, gasleft());
         return _retval;
     }

contracts/mocks/ERC777Mock.sol

@@ -0,0 +1,25 @@
+pragma solidity ^0.5.0;
+
+import "../token/ERC777/ERC777.sol";
+
+contract ERC777Mock is ERC777 {
+    constructor(
+        address initialHolder,
+        uint256 initialBalance,
+        string memory name,
+        string memory symbol,
+        address[] memory defaultOperators
+    ) public ERC777(name, symbol, defaultOperators) {
+        _mint(msg.sender, initialHolder, initialBalance, "", "");
+    }
+
+    function mintInternal (
+        address operator,
+        address to,
+        uint256 amount,
+        bytes memory userData,
+        bytes memory operatorData
+    ) public {
+        _mint(operator, to, amount, userData, operatorData);
+    }
+}

contracts/mocks/ERC777SenderRecipientMock.sol

@@ -0,0 +1,147 @@
+pragma solidity ^0.5.0;
+
+import "../token/ERC777/IERC777.sol";
+import "../token/ERC777/IERC777Sender.sol";
+import "../token/ERC777/IERC777Recipient.sol";
+import "../introspection/IERC1820Registry.sol";
+import "../introspection/ERC1820Implementer.sol";
+
+contract ERC777SenderRecipientMock is IERC777Sender, IERC777Recipient, ERC1820Implementer {
+    event TokensToSendCalled(
+        address operator,
+        address from,
+        address to,
+        uint256 amount,
+        bytes data,
+        bytes operatorData,
+        address token,
+        uint256 fromBalance,
+        uint256 toBalance
+    );
+
+    event TokensReceivedCalled(
+        address operator,
+        address from,
+        address to,
+        uint256 amount,
+        bytes data,
+        bytes operatorData,
+        address token,
+        uint256 fromBalance,
+        uint256 toBalance
+    );
+
+    bool private _shouldRevertSend;
+    bool private _shouldRevertReceive;
+
+    IERC1820Registry private _erc1820 = IERC1820Registry(0x1820a4B7618BdE71Dce8cdc73aAB6C95905faD24);
+
+    bytes32 constant private TOKENS_SENDER_INTERFACE_HASH = keccak256("ERC777TokensSender");
+    bytes32 constant private TOKENS_RECIPIENT_INTERFACE_HASH = keccak256("ERC777TokensRecipient");
+
+    function tokensToSend(
+        address operator,
+        address from,
+        address to,
+        uint amount,
+        bytes calldata userData,
+        bytes calldata operatorData
+    ) external {
+        if (_shouldRevertSend) {
+            revert();
+        }
+
+        IERC777 token = IERC777(msg.sender);
+
+        uint256 fromBalance = token.balanceOf(from);
+        // when called due to burn, to will be the zero address, which will have a balance of 0
+        uint256 toBalance = token.balanceOf(to);
+
+        emit TokensToSendCalled(
+            operator,
+            from,
+            to,
+            amount,
+            userData,
+            operatorData,
+            address(token),
+            fromBalance,
+            toBalance
+        );
+    }
+
+    function tokensReceived(
+        address operator,
+        address from,
+        address to,
+        uint amount,
+        bytes calldata userData,
+        bytes calldata operatorData
+    ) external{
+        if (_shouldRevertReceive) {
+            revert();
+        }
+
+        IERC777 token = IERC777(msg.sender);
+
+        uint256 fromBalance = token.balanceOf(from);
+        // when called due to burn, to will be the zero address, which will have a balance of 0
+        uint256 toBalance = token.balanceOf(to);
+
+        emit TokensReceivedCalled(
+            operator,
+            from,
+            to,
+            amount,
+            userData,
+            operatorData,
+            address(token),
+            fromBalance,
+            toBalance
+        );
+    }
+
+    function senderFor(address account) public {
+        _registerInterfaceForAddress(TOKENS_SENDER_INTERFACE_HASH, account);
+
+        address self = address(this);
+        if (account == self) {
+            registerSender(self);
+        }
+    }
+
+    function registerSender(address sender) public {
+        _erc1820.setInterfaceImplementer(address(this), TOKENS_SENDER_INTERFACE_HASH, sender);
+    }
+
+    function recipientFor(address account) public {
+        _registerInterfaceForAddress(TOKENS_RECIPIENT_INTERFACE_HASH, account);
+
+        address self = address(this);
+        if (account == self) {
+            registerRecipient(self);
+        }
+    }
+
+    function registerRecipient(address recipient) public {
+        _erc1820.setInterfaceImplementer(address(this), TOKENS_RECIPIENT_INTERFACE_HASH, recipient);
+    }
+
+    function setShouldRevertSend(bool shouldRevert) public {
+        _shouldRevertSend = shouldRevert;
+    }
+
+    function setShouldRevertReceive(bool shouldRevert) public {
+        _shouldRevertReceive = shouldRevert;
+    }
+
+    function send(IERC777 token, address to, uint256 amount, bytes memory data) public {
+        // This is 777's send function, not the Solidity send function
+        token.send(to, amount, data); // solhint-disable-line check-send-result
+    }
+
+    function burn(IERC777 token, uint256 amount, bytes memory data) public {
+        token.burn(amount, data);
+    }
+}
+

contracts/mocks/EventEmitter.sol

@@ -1,73 +0,0 @@
-pragma solidity ^0.5.0;
-
-contract EventEmitter {
-    event Argumentless();
-    event ShortUint(uint8 value);
-    event ShortInt(int8 value);
-    event LongUint(uint256 value);
-    event LongInt(int256 value);
-    event Address(address value);
-    event Boolean(bool value);
-    event String(string value);
-    event LongUintBooleanString(uint256 uintValue, bool booleanValue, string stringValue);
-
-    constructor (uint8 uintValue, bool booleanValue, string memory stringValue) public {
-        emit ShortUint(uintValue);
-        emit Boolean(booleanValue);
-        emit String(stringValue);
-    }
-
-    function emitArgumentless() public {
-        emit Argumentless();
-    }
-
-    function emitShortUint(uint8 value) public {
-        emit ShortUint(value);
-    }
-
-    function emitShortInt(int8 value) public {
-        emit ShortInt(value);
-    }
-
-    function emitLongUint(uint256 value) public {
-        emit LongUint(value);
-    }
-
-    function emitLongInt(int256 value) public {
-        emit LongInt(value);
-    }
-
-    function emitAddress(address value) public {
-        emit Address(value);
-    }
-
-    function emitBoolean(bool value) public {
-        emit Boolean(value);
-    }
-
-    function emitString(string memory value) public {
-        emit String(value);
-    }
-
-    function emitLongUintBooleanString(uint256 uintValue, bool booleanValue, string memory stringValue) public {
-        emit LongUintBooleanString(uintValue, booleanValue, stringValue);
-    }
-
-    function emitLongUintAndBoolean(uint256 uintValue, bool boolValue) public {
-        emit LongUint(uintValue);
-        emit Boolean(boolValue);
-    }
-
-    function emitStringAndEmitIndirectly(string memory value, IndirectEventEmitter emitter) public {
-        emit String(value);
-        emitter.emitStringIndirectly(value);
-    }
-}
-
-contract IndirectEventEmitter {
-    event IndirectString(string value);
-
-    function emitStringIndirectly(string memory value) public {
-        emit IndirectString(value);
-    }
-}

contracts/mocks/Failer.sol

@@ -1,23 +0,0 @@
-pragma solidity ^0.5.0;
-
-contract Failer {
-    uint256[] private array;
-
-    function dontFail() public pure {
-        // solhint-disable-previous-line no-empty-blocks
-    }
-
-    function failWithRevert() public pure {
-        revert();
-    }
-
-    function failWithThrow() public pure {
-        assert(false);
-    }
-
-    function failWithOutOfGas() public {
-        for (uint256 i = 0; i < 2**200; ++i) {
-            array.push(i);
-        }
-    }
-}

contracts/mocks/OwnableInterfaceId.sol

@@ -5,7 +5,7 @@ import "../ownership/Ownable.sol";
 /**
  * @title Ownable interface id calculator.
  * @dev See the EIP165 specification for more information:
- * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md#specification
+ * https://eips.ethereum.org/EIPS/eip-165#specification
  */
 contract OwnableInterfaceId {
     function getInterfaceId() public pure returns (bytes4) {

contracts/mocks/ReentrancyAttack.sol

@@ -4,6 +4,6 @@ contract ReentrancyAttack {
     function callSender(bytes4 data) public {
         // solhint-disable-next-line avoid-low-level-calls
         (bool success,) = msg.sender.call(abi.encodeWithSelector(data));
-        require(success);
+        require(success, "ReentrancyAttack: failed call");
     }
 }

contracts/mocks/ReentrancyMock.sol

@@ -26,7 +26,7 @@ contract ReentrancyMock is ReentrancyGuard {
             count();
             // solhint-disable-next-line avoid-low-level-calls
             (bool success,) = address(this).call(abi.encodeWithSignature("countThisRecursive(uint256)", n - 1));
-            require(success);
+            require(success, "ReentrancyMock: failed call");
         }
     }
 

contracts/mocks/SafeERC20Helper.sol

@@ -3,106 +3,127 @@ pragma solidity ^0.5.0;
 import "../token/ERC20/IERC20.sol";
 import "../token/ERC20/SafeERC20.sol";
 
-contract ERC20FailingMock {
+contract ERC20ReturnFalseMock {
     uint256 private _allowance;
 
+    // IERC20's functions are not pure, but these mock implementations are: to prevent Solidity from issuing warnings,
+    // we write to a dummy state variable.
+    uint256 private _dummy;
+
     function transfer(address, uint256) public returns (bool) {
+        _dummy = 0;
         return false;
     }
 
     function transferFrom(address, address, uint256) public returns (bool) {
+        _dummy = 0;
         return false;
     }
 
     function approve(address, uint256) public returns (bool) {
+        _dummy = 0;
         return false;
     }
 
     function allowance(address, address) public view returns (uint256) {
+        require(_dummy == 0); // Duummy read from a state variable so that the function is view
         return 0;
     }
 }
 
-contract ERC20SucceedingMock {
-    uint256 private _allowance;
+contract ERC20ReturnTrueMock {
+    mapping (address => uint256) private _allowances;
+
+    // IERC20's functions are not pure, but these mock implementations are: to prevent Solidity from issuing warnings,
+    // we write to a dummy state variable.
+    uint256 private _dummy;
 
     function transfer(address, uint256) public returns (bool) {
+        _dummy = 0;
         return true;
     }
 
     function transferFrom(address, address, uint256) public returns (bool) {
+        _dummy = 0;
         return true;
     }
 
     function approve(address, uint256) public returns (bool) {
+        _dummy = 0;
         return true;
     }
 
     function setAllowance(uint256 allowance_) public {
-        _allowance = allowance_;
+        _allowances[msg.sender] = allowance_;
     }
 
-    function allowance(address, address) public view returns (uint256) {
-        return _allowance;
+    function allowance(address owner, address) public view returns (uint256) {
+        return _allowances[owner];
     }
 }
 
-contract SafeERC20Helper {
-    using SafeERC20 for IERC20;
+contract ERC20NoReturnMock {
+    mapping (address => uint256) private _allowances;
 
-    IERC20 private _failing;
-    IERC20 private _succeeding;
+    // IERC20's functions are not pure, but these mock implementations are: to prevent Solidity from issuing warnings,
+    // we write to a dummy state variable.
+    uint256 private _dummy;
 
-    constructor () public {
-        _failing = IERC20(address(new ERC20FailingMock()));
-        _succeeding = IERC20(address(new ERC20SucceedingMock()));
+    function transfer(address, uint256) public {
+        _dummy = 0;
     }
 
-    function doFailingTransfer() public {
-        _failing.safeTransfer(address(0), 0);
+    function transferFrom(address, address, uint256) public {
+        _dummy = 0;
     }
 
-    function doFailingTransferFrom() public {
-        _failing.safeTransferFrom(address(0), address(0), 0);
-    }
-
-    function doFailingApprove() public {
-        _failing.safeApprove(address(0), 0);
-    }
-
-    function doFailingIncreaseAllowance() public {
-        _failing.safeIncreaseAllowance(address(0), 0);
-    }
-
-    function doFailingDecreaseAllowance() public {
-        _failing.safeDecreaseAllowance(address(0), 0);
-    }
-
-    function doSucceedingTransfer() public {
-        _succeeding.safeTransfer(address(0), 0);
-    }
-
-    function doSucceedingTransferFrom() public {
-        _succeeding.safeTransferFrom(address(0), address(0), 0);
-    }
-
-    function doSucceedingApprove(uint256 amount) public {
-        _succeeding.safeApprove(address(0), amount);
-    }
-
-    function doSucceedingIncreaseAllowance(uint256 amount) public {
-        _succeeding.safeIncreaseAllowance(address(0), amount);
-    }
-
-    function doSucceedingDecreaseAllowance(uint256 amount) public {
-        _succeeding.safeDecreaseAllowance(address(0), amount);
+    function approve(address, uint256) public {
+        _dummy = 0;
     }
 
     function setAllowance(uint256 allowance_) public {
-        ERC20SucceedingMock(address(_succeeding)).setAllowance(allowance_);
+        _allowances[msg.sender] = allowance_;
+    }
+
+    function allowance(address owner, address) public view returns (uint256) {
+        return _allowances[owner];
+    }
+}
+
+contract SafeERC20Wrapper {
+    using SafeERC20 for IERC20;
+
+    IERC20 private _token;
+
+    constructor (IERC20 token) public {
+        _token = token;
+    }
+
+    function transfer() public {
+        _token.safeTransfer(address(0), 0);
+    }
+
+    function transferFrom() public {
+        _token.safeTransferFrom(address(0), address(0), 0);
+    }
+
+    function approve(uint256 amount) public {
+        _token.safeApprove(address(0), amount);
+    }
+
+    function increaseAllowance(uint256 amount) public {
+        _token.safeIncreaseAllowance(address(0), amount);
+    }
+
+    function decreaseAllowance(uint256 amount) public {
+        _token.safeDecreaseAllowance(address(0), amount);
+    }
+
+    function setAllowance(uint256 allowance_) public {
+        ERC20ReturnTrueMock(address(_token)).setAllowance(allowance_);
     }
 
     function allowance() public view returns (uint256) {
-        return _succeeding.allowance(address(0), address(0));
+        return _token.allowance(address(0), address(0));
     }
 }

contracts/mocks/TimedCrowdsaleImpl.sol

@@ -11,4 +11,8 @@ contract TimedCrowdsaleImpl is TimedCrowdsale {
     {
         // solhint-disable-previous-line no-empty-blocks
     }
+
+    function extendTime(uint256 closingTime) public {
+        _extendTime(closingTime);
+    }
 }

contracts/ownership/Ownable.sol

@@ -1,9 +1,13 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title Ownable
- * @dev The Ownable contract has an owner address, and provides basic authorization control
- * functions, this simplifies the implementation of "user permissions".
+ * @dev Contract module which provides a basic access control mechanism, where
+ * there is an account (an owner) that can be granted exclusive access to
+ * specific functions.
+ *
+ * This module is used through inheritance. It will make available the modifier
+ * `onlyOwner`, which can be aplied to your functions to restrict their use to
+ * the owner.
  */
 contract Ownable {
     address private _owner;
@@ -11,8 +15,7 @@ contract Ownable {
     event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
 
     /**
-     * @dev The Ownable constructor sets the original `owner` of the contract to the sender
-     * account.
+     * @dev Initializes the contract setting the deployer as the initial owner.
      */
     constructor () internal {
         _owner = msg.sender;
@@ -20,7 +23,7 @@ contract Ownable {
     }
 
     /**
-     * @return the address of the owner.
+     * @dev Returns the address of the current owner.
      */
     function owner() public view returns (address) {
         return _owner;
@@ -30,22 +33,23 @@ contract Ownable {
      * @dev Throws if called by any account other than the owner.
      */
     modifier onlyOwner() {
-        require(isOwner());
+        require(isOwner(), "Ownable: caller is not the owner");
         _;
     }
 
     /**
-     * @return true if `msg.sender` is the owner of the contract.
+     * @dev Returns true if the caller is the current owner.
      */
     function isOwner() public view returns (bool) {
         return msg.sender == _owner;
     }
 
     /**
-     * @dev Allows the current owner to relinquish control of the contract.
-     * @notice Renouncing to ownership will leave the contract without an owner.
-     * It will not be possible to call the functions with the `onlyOwner`
-     * modifier anymore.
+     * @dev Leaves the contract without owner. It will not be possible to call
+     * `onlyOwner` functions anymore. Can only be called by the current owner.
+     *
+     * > Note: Renouncing ownership will leave the contract without an owner,
+     * thereby removing any functionality that is only available to the owner.
      */
     function renounceOwnership() public onlyOwner {
         emit OwnershipTransferred(_owner, address(0));
@@ -53,19 +57,18 @@ contract Ownable {
     }
 
     /**
-     * @dev Allows the current owner to transfer control of the contract to a newOwner.
-     * @param newOwner The address to transfer ownership to.
+     * @dev Transfers ownership of the contract to a new account (`newOwner`).
+     * Can only be called by the current owner.
      */
     function transferOwnership(address newOwner) public onlyOwner {
         _transferOwnership(newOwner);
     }
 
     /**
-     * @dev Transfers control of the contract to a newOwner.
-     * @param newOwner The address to transfer ownership to.
+     * @dev Transfers ownership of the contract to a new account (`newOwner`).
      */
     function _transferOwnership(address newOwner) internal {
-        require(newOwner != address(0));
+        require(newOwner != address(0), "Ownable: new owner is the zero address");
         emit OwnershipTransferred(_owner, newOwner);
         _owner = newOwner;
     }

contracts/ownership/README.md

@@ -0,0 +1,3 @@
+Contract modules for simple authorization and access control mechanisms.
+
+For more complex needs see [Access](access).

contracts/ownership/Secondary.sol

@@ -1,12 +1,14 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title Secondary
- * @dev A Secondary contract can only be used by its primary account (the one that created it)
+ * @dev A Secondary contract can only be used by its primary account (the one that created it).
  */
 contract Secondary {
     address private _primary;
 
+    /**
+     * @dev Emitted when the primary contract changes.
+     */
     event PrimaryTransferred(
         address recipient
     );
@@ -23,7 +25,7 @@ contract Secondary {
      * @dev Reverts if called from any account other than the primary.
      */
     modifier onlyPrimary() {
-        require(msg.sender == _primary);
+        require(msg.sender == _primary, "Secondary: caller is not the primary account");
         _;
     }
 
@@ -39,7 +41,7 @@ contract Secondary {
      * @param recipient The address of new primary.
      */
     function transferPrimary(address recipient) public onlyPrimary {
-        require(recipient != address(0));
+        require(recipient != address(0), "Secondary: new primary is the zero address");
         _primary = recipient;
         emit PrimaryTransferred(_primary);
     }

contracts/package.json

@@ -0,0 +1,32 @@
+{
+  "name": "@openzeppelin/contracts",
+  "version": "2.3.0",
+  "description": "Secure Smart Contract library for Solidity",
+  "files": [
+    "**/*.sol",
+    "/build/*.json",
+    "!/mocks",
+    "!/examples"
+  ],
+  "scripts": {
+    "prepare": "bash ../scripts/prepare-contracts-package.sh"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/OpenZeppelin/openzeppelin-contracts.git"
+  },
+  "keywords": [
+    "solidity",
+    "ethereum",
+    "smart",
+    "contracts",
+    "security",
+    "zeppelin"
+  ],
+  "author": "OpenZeppelin Community <maintainers@openzeppelin.org>",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/OpenZeppelin/openzeppelin-contracts/issues"
+  },
+  "homepage": "https://github.com/OpenZeppelin/openzeppelin-contracts"
+}

contracts/payment/PaymentSplitter.sol

@@ -4,8 +4,16 @@ import "../math/SafeMath.sol";
 
 /**
  * @title PaymentSplitter
- * @dev This contract can be used when payments need to be received by a group
- * of people and split proportionately to some number of shares they own.
+ * @dev This contract allows to split Ether payments among a group of accounts. The sender does not need to be aware
+ * that the Ether will be split in this way, since it is handled transparently by the contract.
+ *
+ * The split can be in equal parts or in any other arbitrary proportion. The way this is specified is by assigning each
+ * account to a number of shares. Of all the Ether that this contract receives, each account will then be able to claim
+ * an amount proportional to the percentage of total shares they were assigned.
+ *
+ * `PaymentSplitter` follows a _pull payment_ model. This means that payments are not automatically forwarded to the
+ * accounts but kept in this contract, and the actual transfer is triggered as a separate step by calling the `release`
+ * function.
  */
 contract PaymentSplitter {
     using SafeMath for uint256;
@@ -22,11 +30,16 @@ contract PaymentSplitter {
     address[] private _payees;
 
     /**
-     * @dev Constructor
+     * @dev Creates an instance of `PaymentSplitter` where each account in `payees` is assigned the number of shares at
+     * the matching position in the `shares` array.
+     *
+     * All addresses in `payees` must be non-zero. Both arrays must have the same non-zero length, and there must be no
+     * duplicates in `payees`.
      */
     constructor (address[] memory payees, uint256[] memory shares) public payable {
-        require(payees.length == shares.length);
-        require(payees.length > 0);
+        // solhint-disable-next-line max-line-length
+        require(payees.length == shares.length, "PaymentSplitter: payees and shares length mismatch");
+        require(payees.length > 0, "PaymentSplitter: no payees");
 
         for (uint256 i = 0; i < payees.length; i++) {
             _addPayee(payees[i], shares[i]);
@@ -34,58 +47,64 @@ contract PaymentSplitter {
     }
 
     /**
-     * @dev payable fallback
+     * @dev The Ether received will be logged with `PaymentReceived` events. Note that these events are not fully
+     * reliable: it's possible for a contract to receive Ether without triggering this function. This only affects the
+     * reliability of the events, and not the actual splitting of Ether.
+     *
+     * To learn more about this see the Solidity documentation for [fallback functions].
+     *
+     * [fallback functions]: https://solidity.readthedocs.io/en/latest/contracts.html#fallback-function
      */
     function () external payable {
         emit PaymentReceived(msg.sender, msg.value);
     }
 
     /**
-     * @return the total shares of the contract.
+     * @dev Getter for the total shares held by payees.
      */
     function totalShares() public view returns (uint256) {
         return _totalShares;
     }
 
     /**
-     * @return the total amount already released.
+     * @dev Getter for the total amount of Ether already released.
      */
     function totalReleased() public view returns (uint256) {
         return _totalReleased;
     }
 
     /**
-     * @return the shares of an account.
+     * @dev Getter for the amount of shares held by an account.
      */
     function shares(address account) public view returns (uint256) {
         return _shares[account];
     }
 
     /**
-     * @return the amount already released to an account.
+     * @dev Getter for the amount of Ether already released to a payee.
      */
     function released(address account) public view returns (uint256) {
         return _released[account];
     }
 
     /**
-     * @return the address of a payee.
+     * @dev Getter for the address of the payee number `index`.
      */
     function payee(uint256 index) public view returns (address) {
         return _payees[index];
     }
 
     /**
-     * @dev Release one of the payee's proportional payment.
-     * @param account Whose payments will be released.
+     * @dev Triggers a transfer to `account` of the amount of Ether they are owed, according to their percentage of the
+     * total shares and their previous withdrawals.
      */
     function release(address payable account) public {
-        require(_shares[account] > 0);
+        require(_shares[account] > 0, "PaymentSplitter: account has no shares");
 
         uint256 totalReceived = address(this).balance.add(_totalReleased);
         uint256 payment = totalReceived.mul(_shares[account]).div(_totalShares).sub(_released[account]);
 
-        require(payment != 0);
+        require(payment != 0, "PaymentSplitter: account is not due payment");
 
         _released[account] = _released[account].add(payment);
         _totalReleased = _totalReleased.add(payment);
@@ -100,9 +119,9 @@ contract PaymentSplitter {
      * @param shares_ The number of shares owned by the payee.
      */
     function _addPayee(address account, uint256 shares_) private {
-        require(account != address(0));
-        require(shares_ > 0);
-        require(_shares[account] == 0);
+        require(account != address(0), "PaymentSplitter: account is the zero address");
+        require(shares_ > 0, "PaymentSplitter: shares are 0");
+        require(_shares[account] == 0, "PaymentSplitter: account already has shares");
 
         _payees.push(account);
         _shares[account] = shares_;

contracts/payment/PullPayment.sol

@@ -15,26 +15,26 @@ contract PullPayment {
     }
 
     /**
-    * @dev Withdraw accumulated balance.
-    * @param payee Whose balance will be withdrawn.
-    */
+     * @dev Withdraw accumulated balance.
+     * @param payee Whose balance will be withdrawn.
+     */
     function withdrawPayments(address payable payee) public {
         _escrow.withdraw(payee);
     }
 
     /**
-    * @dev Returns the credit owed to an address.
-    * @param dest The creditor's address.
-    */
+     * @dev Returns the credit owed to an address.
+     * @param dest The creditor's address.
+     */
     function payments(address dest) public view returns (uint256) {
         return _escrow.depositsOf(dest);
     }
 
     /**
-    * @dev Called by the payer to store the sent amount as credit to be pulled.
-    * @param dest The destination address of the funds.
-    * @param amount The amount to transfer.
-    */
+     * @dev Called by the payer to store the sent amount as credit to be pulled.
+     * @param dest The destination address of the funds.
+     * @param amount The amount to transfer.
+     */
     function _asyncTransfer(address dest, uint256 amount) internal {
         _escrow.deposit.value(amount)(dest);
     }

contracts/payment/README.md

@@ -0,0 +1,10 @@
+---
+sections:
+  - title: Payment Utilities
+    contracts:
+      - PaymentSplitter
+      - PullPayment
+  - subdirectory: escrow
+---
+
+> This page is incomplete. We're working to improve it for the next release. Stay tuned!

contracts/payment/escrow/ConditionalEscrow.sol

@@ -9,14 +9,14 @@ import "./Escrow.sol";
  */
 contract ConditionalEscrow is Escrow {
     /**
-    * @dev Returns whether an address is allowed to withdraw their funds. To be
-    * implemented by derived contracts.
-    * @param payee The destination address of the funds.
-    */
+     * @dev Returns whether an address is allowed to withdraw their funds. To be
+     * implemented by derived contracts.
+     * @param payee The destination address of the funds.
+     */
     function withdrawalAllowed(address payee) public view returns (bool);
 
     function withdraw(address payable payee) public {
-        require(withdrawalAllowed(payee));
+        require(withdrawalAllowed(payee), "ConditionalEscrow: payee is not allowed to withdraw");
         super.withdraw(payee);
     }
 }

contracts/payment/escrow/Escrow.sol

@@ -4,17 +4,17 @@ import "../../math/SafeMath.sol";
 import "../../ownership/Secondary.sol";
 
  /**
- * @title Escrow
- * @dev Base escrow contract, holds funds designated for a payee until they
- * withdraw them.
- * @dev Intended usage: This contract (and derived escrow contracts) should be a
- * standalone contract, that only interacts with the contract that instantiated
- * it. That way, it is guaranteed that all Ether will be handled according to
- * the Escrow rules, and there is no need to check for payable functions or
- * transfers in the inheritance tree. The contract that uses the escrow as its
- * payment method should be its primary, and provide public methods redirecting
- * to the escrow's deposit and withdraw.
- */
+  * @title Escrow
+  * @dev Base escrow contract, holds funds designated for a payee until they
+  * withdraw them.
+  * @dev Intended usage: This contract (and derived escrow contracts) should be a
+  * standalone contract, that only interacts with the contract that instantiated
+  * it. That way, it is guaranteed that all Ether will be handled according to
+  * the Escrow rules, and there is no need to check for payable functions or
+  * transfers in the inheritance tree. The contract that uses the escrow as its
+  * payment method should be its primary, and provide public methods redirecting
+  * to the escrow's deposit and withdraw.
+  */
 contract Escrow is Secondary {
     using SafeMath for uint256;
 
@@ -28,9 +28,9 @@ contract Escrow is Secondary {
     }
 
     /**
-    * @dev Stores the sent amount as credit to be withdrawn.
-    * @param payee The destination address of the funds.
-    */
+     * @dev Stores the sent amount as credit to be withdrawn.
+     * @param payee The destination address of the funds.
+     */
     function deposit(address payee) public onlyPrimary payable {
         uint256 amount = msg.value;
         _deposits[payee] = _deposits[payee].add(amount);
@@ -39,9 +39,9 @@ contract Escrow is Secondary {
     }
 
     /**
-    * @dev Withdraw accumulated balance for a payee.
-    * @param payee The address whose funds will be withdrawn and transferred to.
-    */
+     * @dev Withdraw accumulated balance for a payee.
+     * @param payee The address whose funds will be withdrawn and transferred to.
+     */
     function withdraw(address payable payee) public onlyPrimary {
         uint256 payment = _deposits[payee];
 

contracts/payment/escrow/README.md

@@ -0,0 +1,8 @@
+---
+title: Escrows
+sections:
+  - contracts:
+    - Escrow
+    - ConditionalEscrow
+    - RefundEscrow
+---

contracts/payment/escrow/RefundEscrow.sol

@@ -27,20 +27,20 @@ contract RefundEscrow is ConditionalEscrow {
      * @param beneficiary The beneficiary of the deposits.
      */
     constructor (address payable beneficiary) public {
-        require(beneficiary != address(0));
+        require(beneficiary != address(0), "RefundEscrow: beneficiary is the zero address");
         _beneficiary = beneficiary;
         _state = State.Active;
     }
 
     /**
-     * @return the current state of the escrow.
+     * @return The current state of the escrow.
      */
     function state() public view returns (State) {
         return _state;
     }
 
     /**
-     * @return the beneficiary of the escrow.
+     * @return The beneficiary of the escrow.
      */
     function beneficiary() public view returns (address) {
         return _beneficiary;
@@ -51,7 +51,7 @@ contract RefundEscrow is ConditionalEscrow {
      * @param refundee The address funds will be sent to if a refund occurs.
      */
     function deposit(address refundee) public payable {
-        require(_state == State.Active);
+        require(_state == State.Active, "RefundEscrow: can only deposit while active");
         super.deposit(refundee);
     }
 
@@ -60,7 +60,7 @@ contract RefundEscrow is ConditionalEscrow {
      * further deposits.
      */
     function close() public onlyPrimary {
-        require(_state == State.Active);
+        require(_state == State.Active, "RefundEscrow: can only close while active");
         _state = State.Closed;
         emit RefundsClosed();
     }
@@ -69,7 +69,7 @@ contract RefundEscrow is ConditionalEscrow {
      * @dev Allows for refunds to take place, rejecting further deposits.
      */
     function enableRefunds() public onlyPrimary {
-        require(_state == State.Active);
+        require(_state == State.Active, "RefundEscrow: can only enable refunds while active");
         _state = State.Refunding;
         emit RefundsEnabled();
     }
@@ -78,12 +78,12 @@ contract RefundEscrow is ConditionalEscrow {
      * @dev Withdraws the beneficiary's funds.
      */
     function beneficiaryWithdraw() public {
-        require(_state == State.Closed);
+        require(_state == State.Closed, "RefundEscrow: beneficiary can only withdraw while closed");
         _beneficiary.transfer(address(this).balance);
     }
 
     /**
-     * @dev Returns whether refundees can withdraw their deposits (be refunded). The overriden function receives a
+     * @dev Returns whether refundees can withdraw their deposits (be refunded). The overridden function receives a
      * 'payee' argument, but we ignore it here since the condition is global, not per-payee.
      */
     function withdrawalAllowed(address) public view returns (bool) {

contracts/token/ERC20/ERC20.sol

@@ -4,167 +4,190 @@ import "./IERC20.sol";
 import "../../math/SafeMath.sol";
 
 /**
- * @title Standard ERC20 token
+ * @dev Implementation of the `IERC20` interface.
  *
- * @dev Implementation of the basic standard token.
- * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
- * Originally based on code by FirstBlood:
- * https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
+ * This implementation is agnostic to the way tokens are created. This means
+ * that a supply mechanism has to be added in a derived contract using `_mint`.
+ * For a generic mechanism see `ERC20Mintable`.
  *
- * This implementation emits additional Approval events, allowing applications to reconstruct the allowance status for
- * all accounts just by listening to said events. Note that this isn't required by the specification, and other
- * compliant implementations may not do it.
+ * *For a detailed writeup see our guide [How to implement supply
+ * mechanisms](https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226).*
+ *
+ * We have followed general OpenZeppelin guidelines: functions revert instead
+ * of returning `false` on failure. This behavior is nonetheless conventional
+ * and does not conflict with the expectations of ERC20 applications.
+ *
+ * Additionally, an `Approval` event is emitted on calls to `transferFrom`.
+ * This allows applications to reconstruct the allowance for all accounts just
+ * by listening to said events. Other implementations of the EIP may not emit
+ * these events, as it isn't required by the specification.
+ *
+ * Finally, the non-standard `decreaseAllowance` and `increaseAllowance`
+ * functions have been added to mitigate the well-known issues around setting
+ * allowances. See `IERC20.approve`.
  */
 contract ERC20 is IERC20 {
     using SafeMath for uint256;
 
     mapping (address => uint256) private _balances;
 
-    mapping (address => mapping (address => uint256)) private _allowed;
+    mapping (address => mapping (address => uint256)) private _allowances;
 
     uint256 private _totalSupply;
 
     /**
-    * @dev Total number of tokens in existence
-    */
+     * @dev See `IERC20.totalSupply`.
+     */
     function totalSupply() public view returns (uint256) {
         return _totalSupply;
     }
 
     /**
-    * @dev Gets the balance of the specified address.
-    * @param owner The address to query the balance of.
-    * @return An uint256 representing the amount owned by the passed address.
-    */
-    function balanceOf(address owner) public view returns (uint256) {
-        return _balances[owner];
+     * @dev See `IERC20.balanceOf`.
+     */
+    function balanceOf(address account) public view returns (uint256) {
+        return _balances[account];
     }
 
     /**
-     * @dev Function to check the amount of tokens that an owner allowed to a spender.
-     * @param owner address The address which owns the funds.
-     * @param spender address The address which will spend the funds.
-     * @return A uint256 specifying the amount of tokens still available for the spender.
+     * @dev See `IERC20.transfer`.
+     *
+     * Requirements:
+     *
+     * - `recipient` cannot be the zero address.
+     * - the caller must have a balance of at least `amount`.
+     */
+    function transfer(address recipient, uint256 amount) public returns (bool) {
+        _transfer(msg.sender, recipient, amount);
+        return true;
+    }
+
+    /**
+     * @dev See `IERC20.allowance`.
      */
     function allowance(address owner, address spender) public view returns (uint256) {
-        return _allowed[owner][spender];
+        return _allowances[owner][spender];
     }
 
     /**
-    * @dev Transfer token for a specified address
-    * @param to The address to transfer to.
-    * @param value The amount to be transferred.
-    */
-    function transfer(address to, uint256 value) public returns (bool) {
-        _transfer(msg.sender, to, value);
-        return true;
-    }
-
-    /**
-     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
-     * Beware that changing an allowance with this method brings the risk that someone may use both the old
-     * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
-     * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
-     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
-     * @param spender The address which will spend the funds.
-     * @param value The amount of tokens to be spent.
+     * @dev See `IERC20.approve`.
+     *
+     * Requirements:
+     *
+     * - `spender` cannot be the zero address.
      */
     function approve(address spender, uint256 value) public returns (bool) {
-        require(spender != address(0));
-
-        _allowed[msg.sender][spender] = value;
-        emit Approval(msg.sender, spender, value);
+        _approve(msg.sender, spender, value);
         return true;
     }
 
     /**
-     * @dev Transfer tokens from one address to another.
-     * Note that while this function emits an Approval event, this is not required as per the specification,
-     * and other compliant implementations may not emit the event.
-     * @param from address The address which you want to send tokens from
-     * @param to address The address which you want to transfer to
-     * @param value uint256 the amount of tokens to be transferred
+     * @dev See `IERC20.transferFrom`.
+     *
+     * Emits an `Approval` event indicating the updated allowance. This is not
+     * required by the EIP. See the note at the beginning of `ERC20`;
+     *
+     * Requirements:
+     * - `sender` and `recipient` cannot be the zero address.
+     * - `sender` must have a balance of at least `value`.
+     * - the caller must have allowance for `sender`'s tokens of at least
+     * `amount`.
      */
-    function transferFrom(address from, address to, uint256 value) public returns (bool) {
-        _allowed[from][msg.sender] = _allowed[from][msg.sender].sub(value);
-        _transfer(from, to, value);
-        emit Approval(from, msg.sender, _allowed[from][msg.sender]);
+    function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
+        _transfer(sender, recipient, amount);
+        _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount));
         return true;
     }
 
     /**
-     * @dev Increase the amount of tokens that an owner allowed to a spender.
-     * approve should be called when allowed_[_spender] == 0. To increment
-     * allowed value is better to use this function to avoid 2 calls (and wait until
-     * the first transaction is mined)
-     * From MonolithDAO Token.sol
-     * Emits an Approval event.
-     * @param spender The address which will spend the funds.
-     * @param addedValue The amount of tokens to increase the allowance by.
+     * @dev Atomically increases the allowance granted to `spender` by the caller.
+     *
+     * This is an alternative to `approve` that can be used as a mitigation for
+     * problems described in `IERC20.approve`.
+     *
+     * Emits an `Approval` event indicating the updated allowance.
+     *
+     * Requirements:
+     *
+     * - `spender` cannot be the zero address.
      */
     function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
-        require(spender != address(0));
-
-        _allowed[msg.sender][spender] = _allowed[msg.sender][spender].add(addedValue);
-        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
+        _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
         return true;
     }
 
     /**
-     * @dev Decrease the amount of tokens that an owner allowed to a spender.
-     * approve should be called when allowed_[_spender] == 0. To decrement
-     * allowed value is better to use this function to avoid 2 calls (and wait until
-     * the first transaction is mined)
-     * From MonolithDAO Token.sol
-     * Emits an Approval event.
-     * @param spender The address which will spend the funds.
-     * @param subtractedValue The amount of tokens to decrease the allowance by.
+     * @dev Atomically decreases the allowance granted to `spender` by the caller.
+     *
+     * This is an alternative to `approve` that can be used as a mitigation for
+     * problems described in `IERC20.approve`.
+     *
+     * Emits an `Approval` event indicating the updated allowance.
+     *
+     * Requirements:
+     *
+     * - `spender` cannot be the zero address.
+     * - `spender` must have allowance for the caller of at least
+     * `subtractedValue`.
      */
     function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
-        require(spender != address(0));
-
-        _allowed[msg.sender][spender] = _allowed[msg.sender][spender].sub(subtractedValue);
-        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
+        _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue));
         return true;
     }
 
     /**
-    * @dev Transfer token for a specified addresses
-    * @param from The address to transfer from.
-    * @param to The address to transfer to.
-    * @param value The amount to be transferred.
-    */
-    function _transfer(address from, address to, uint256 value) internal {
-        require(to != address(0));
-
-        _balances[from] = _balances[from].sub(value);
-        _balances[to] = _balances[to].add(value);
-        emit Transfer(from, to, value);
-    }
-
-    /**
-     * @dev Internal function that mints an amount of the token and assigns it to
-     * an account. This encapsulates the modification of balances such that the
-     * proper events are emitted.
-     * @param account The account that will receive the created tokens.
-     * @param value The amount that will be created.
+     * @dev Moves tokens `amount` from `sender` to `recipient`.
+     *
+     * This is internal function is equivalent to `transfer`, and can be used to
+     * e.g. implement automatic token fees, slashing mechanisms, etc.
+     *
+     * Emits a `Transfer` event.
+     *
+     * Requirements:
+     *
+     * - `sender` cannot be the zero address.
+     * - `recipient` cannot be the zero address.
+     * - `sender` must have a balance of at least `amount`.
      */
-    function _mint(address account, uint256 value) internal {
-        require(account != address(0));
+    function _transfer(address sender, address recipient, uint256 amount) internal {
+        require(sender != address(0), "ERC20: transfer from the zero address");
+        require(recipient != address(0), "ERC20: transfer to the zero address");
 
-        _totalSupply = _totalSupply.add(value);
-        _balances[account] = _balances[account].add(value);
-        emit Transfer(address(0), account, value);
+        _balances[sender] = _balances[sender].sub(amount);
+        _balances[recipient] = _balances[recipient].add(amount);
+        emit Transfer(sender, recipient, amount);
     }
 
-    /**
-     * @dev Internal function that burns an amount of the token of a given
-     * account.
-     * @param account The account whose tokens will be burnt.
-     * @param value The amount that will be burnt.
+    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
+     * the total supply.
+     *
+     * Emits a `Transfer` event with `from` set to the zero address.
+     *
+     * Requirements
+     *
+     * - `to` cannot be the zero address.
+     */
+    function _mint(address account, uint256 amount) internal {
+        require(account != address(0), "ERC20: mint to the zero address");
+
+        _totalSupply = _totalSupply.add(amount);
+        _balances[account] = _balances[account].add(amount);
+        emit Transfer(address(0), account, amount);
+    }
+
+     /**
+     * @dev Destoys `amount` tokens from `account`, reducing the
+     * total supply.
+     *
+     * Emits a `Transfer` event with `to` set to the zero address.
+     *
+     * Requirements
+     *
+     * - `account` cannot be the zero address.
+     * - `account` must have at least `amount` tokens.
      */
     function _burn(address account, uint256 value) internal {
-        require(account != address(0));
+        require(account != address(0), "ERC20: burn from the zero address");
 
         _totalSupply = _totalSupply.sub(value);
         _balances[account] = _balances[account].sub(value);
@@ -172,16 +195,34 @@ contract ERC20 is IERC20 {
     }
 
     /**
-     * @dev Internal function that burns an amount of the token of a given
-     * account, deducting from the sender's allowance for said account. Uses the
-     * internal burn function.
-     * Emits an Approval event (reflecting the reduced allowance).
-     * @param account The account whose tokens will be burnt.
-     * @param value The amount that will be burnt.
+     * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
+     *
+     * This is internal function is equivalent to `approve`, and can be used to
+     * e.g. set automatic allowances for certain subsystems, etc.
+     *
+     * Emits an `Approval` event.
+     *
+     * Requirements:
+     *
+     * - `owner` cannot be the zero address.
+     * - `spender` cannot be the zero address.
      */
-    function _burnFrom(address account, uint256 value) internal {
-        _allowed[account][msg.sender] = _allowed[account][msg.sender].sub(value);
-        _burn(account, value);
-        emit Approval(account, msg.sender, _allowed[account][msg.sender]);
+    function _approve(address owner, address spender, uint256 value) internal {
+        require(owner != address(0), "ERC20: approve from the zero address");
+        require(spender != address(0), "ERC20: approve to the zero address");
+
+        _allowances[owner][spender] = value;
+        emit Approval(owner, spender, value);
+    }
+
+    /**
+     * @dev Destoys `amount` tokens from `account`.`amount` is then deducted
+     * from the caller's allowance.
+     *
+     * See `_burn` and `_approve`.
+     */
+    function _burnFrom(address account, uint256 amount) internal {
+        _burn(account, amount);
+        _approve(account, msg.sender, _allowances[account][msg.sender].sub(amount));
     }
 }

contracts/token/ERC20/ERC20Burnable.sol

@@ -3,24 +3,24 @@ pragma solidity ^0.5.0;
 import "./ERC20.sol";
 
 /**
- * @title Burnable Token
- * @dev Token that can be irreversibly burned (destroyed).
+ * @dev Extension of `ERC20` that allows token holders to destroy both their own
+ * tokens and those that they have an allowance for, in a way that can be
+ * recognized off-chain (via event analysis).
  */
 contract ERC20Burnable is ERC20 {
     /**
-     * @dev Burns a specific amount of tokens.
-     * @param value The amount of token to be burned.
+     * @dev Destoys `amount` tokens from the caller.
+     *
+     * See `ERC20._burn`.
      */
-    function burn(uint256 value) public {
-        _burn(msg.sender, value);
+    function burn(uint256 amount) public {
+        _burn(msg.sender, amount);
     }
 
     /**
-     * @dev Burns a specific amount of tokens from the target address and decrements allowance
-     * @param from address The address which you want to send tokens from
-     * @param value uint256 The amount of token to be burned
+     * @dev See `ERC20._burnFrom`.
      */
-    function burnFrom(address from, uint256 value) public {
-        _burnFrom(from, value);
+    function burnFrom(address account, uint256 amount) public {
+        _burnFrom(account, amount);
     }
 }

contracts/token/ERC20/ERC20Capped.sol

@@ -3,26 +3,36 @@ pragma solidity ^0.5.0;
 import "./ERC20Mintable.sol";
 
 /**
- * @title Capped token
- * @dev Mintable token with a token cap.
+ * @dev Extension of `ERC20Mintable` that adds a cap to the supply of tokens.
  */
 contract ERC20Capped is ERC20Mintable {
     uint256 private _cap;
 
+    /**
+     * @dev Sets the value of the `cap`. This value is immutable, it can only be
+     * set once during construction.
+     */
     constructor (uint256 cap) public {
-        require(cap > 0);
+        require(cap > 0, "ERC20Capped: cap is 0");
         _cap = cap;
     }
 
     /**
-     * @return the cap for the token minting.
+     * @dev Returns the cap on the token's total supply.
      */
     function cap() public view returns (uint256) {
         return _cap;
     }
 
+    /**
+     * @dev See `ERC20Mintable.mint`.
+     *
+     * Requirements:
+     *
+     * - `value` must not cause the total supply to go over the cap.
+     */
     function _mint(address account, uint256 value) internal {
-        require(totalSupply().add(value) <= _cap);
+        require(totalSupply().add(value) <= _cap, "ERC20Capped: cap exceeded");
         super._mint(account, value);
     }
 }

contracts/token/ERC20/ERC20Detailed.sol

@@ -3,16 +3,18 @@ pragma solidity ^0.5.0;
 import "./IERC20.sol";
 
 /**
- * @title ERC20Detailed token
- * @dev The decimals are only for visualization purposes.
- * All the operations are done using the smallest and indivisible token unit,
- * just as on Ethereum all the operations are done in wei.
+ * @dev Optional functions from the ERC20 standard.
  */
 contract ERC20Detailed is IERC20 {
     string private _name;
     string private _symbol;
     uint8 private _decimals;
 
+    /**
+     * @dev Sets the values for `name`, `symbol`, and `decimals`. All three of
+     * these values are immutable: they can only be set once during
+     * construction.
+     */
     constructor (string memory name, string memory symbol, uint8 decimals) public {
         _name = name;
         _symbol = symbol;
@@ -20,21 +22,31 @@ contract ERC20Detailed is IERC20 {
     }
 
     /**
-     * @return the name of the token.
+     * @dev Returns the name of the token.
      */
     function name() public view returns (string memory) {
         return _name;
     }
 
     /**
-     * @return the symbol of the token.
+     * @dev Returns the symbol of the token, usually a shorter version of the
+     * name.
      */
     function symbol() public view returns (string memory) {
         return _symbol;
     }
 
     /**
-     * @return the number of decimals of the token.
+     * @dev Returns the number of decimals used to get its user representation.
+     * For example, if `decimals` equals `2`, a balance of `505` tokens should
+     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
+     *
+     * Tokens usually opt for a value of 18, imitating the relationship between
+     * Ether and Wei.
+     *
+     * > Note that this information is only used for _display_ purposes: it in
+     * no way affects any of the arithmetic of the contract, including
+     * `IERC20.balanceOf` and `IERC20.transfer`.
      */
     function decimals() public view returns (uint8) {
         return _decimals;

contracts/token/ERC20/ERC20Mintable.sol

@@ -4,18 +4,21 @@ import "./ERC20.sol";
 import "../../access/roles/MinterRole.sol";
 
 /**
- * @title ERC20Mintable
- * @dev ERC20 minting logic
+ * @dev Extension of `ERC20` that adds a set of accounts with the `MinterRole`,
+ * which have permission to mint (create) new tokens as they see fit.
+ *
+ * At construction, the deployer of the contract is the only minter.
  */
 contract ERC20Mintable is ERC20, MinterRole {
     /**
-     * @dev Function to mint tokens
-     * @param to The address that will receive the minted tokens.
-     * @param value The amount of tokens to mint.
-     * @return A boolean that indicates if the operation was successful.
+     * @dev See `ERC20._mint`.
+     *
+     * Requirements:
+     *
+     * - the caller must have the `MinterRole`.
      */
-    function mint(address to, uint256 value) public onlyMinter returns (bool) {
-        _mint(to, value);
+    function mint(address account, uint256 amount) public onlyMinter returns (bool) {
+        _mint(account, amount);
         return true;
     }
 }

contracts/token/ERC20/ERC20Pausable.sol

@@ -6,7 +6,7 @@ import "../../lifecycle/Pausable.sol";
 /**
  * @title Pausable token
  * @dev ERC20 modified with pausable transfers.
- **/
+ */
 contract ERC20Pausable is ERC20, Pausable {
     function transfer(address to, uint256 value) public whenNotPaused returns (bool) {
         return super.transfer(to, value);
@@ -20,11 +20,11 @@ contract ERC20Pausable is ERC20, Pausable {
         return super.approve(spender, value);
     }
 
-    function increaseAllowance(address spender, uint addedValue) public whenNotPaused returns (bool success) {
+    function increaseAllowance(address spender, uint addedValue) public whenNotPaused returns (bool) {
         return super.increaseAllowance(spender, addedValue);
     }
 
-    function decreaseAllowance(address spender, uint subtractedValue) public whenNotPaused returns (bool success) {
+    function decreaseAllowance(address spender, uint subtractedValue) public whenNotPaused returns (bool) {
         return super.decreaseAllowance(spender, subtractedValue);
     }
 }

contracts/token/ERC20/IERC20.sol

@@ -1,23 +1,76 @@
 pragma solidity ^0.5.0;
 
 /**
- * @title ERC20 interface
- * @dev see https://github.com/ethereum/EIPs/issues/20
+ * @dev Interface of the ERC20 standard as defined in the EIP. Does not include
+ * the optional functions; to access them see `ERC20Detailed`.
  */
 interface IERC20 {
-    function transfer(address to, uint256 value) external returns (bool);
-
-    function approve(address spender, uint256 value) external returns (bool);
-
-    function transferFrom(address from, address to, uint256 value) external returns (bool);
-
+    /**
+     * @dev Returns the amount of tokens in existence.
+     */
     function totalSupply() external view returns (uint256);
 
-    function balanceOf(address who) external view returns (uint256);
+    /**
+     * @dev Returns the amount of tokens owned by `account`.
+     */
+    function balanceOf(address account) external view returns (uint256);
 
+    /**
+     * @dev Moves `amount` tokens from the caller's account to `recipient`.
+     *
+     * Returns a boolean value indicating whether the operation succeeded.
+     *
+     * Emits a `Transfer` event.
+     */
+    function transfer(address recipient, uint256 amount) external returns (bool);
+
+    /**
+     * @dev Returns the remaining number of tokens that `spender` will be
+     * allowed to spend on behalf of `owner` through `transferFrom`. This is
+     * zero by default.
+     *
+     * This value changes when `approve` or `transferFrom` are called.
+     */
     function allowance(address owner, address spender) external view returns (uint256);
 
+    /**
+     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
+     *
+     * Returns a boolean value indicating whether the operation succeeded.
+     *
+     * > Beware that changing an allowance with this method brings the risk
+     * that someone may use both the old and the new allowance by unfortunate
+     * transaction ordering. One possible solution to mitigate this race
+     * condition is to first reduce the spender's allowance to 0 and set the
+     * desired value afterwards:
+     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
+     *
+     * Emits an `Approval` event.
+     */
+    function approve(address spender, uint256 amount) external returns (bool);
+
+    /**
+     * @dev Moves `amount` tokens from `sender` to `recipient` using the
+     * allowance mechanism. `amount` is then deducted from the caller's
+     * allowance.
+     *
+     * Returns a boolean value indicating whether the operation succeeded.
+     *
+     * Emits a `Transfer` event.
+     */
+    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
+
+    /**
+     * @dev Emitted when `value` tokens are moved from one account (`from`) to
+     * another (`to`).
+     *
+     * Note that `value` may be zero.
+     */
     event Transfer(address indexed from, address indexed to, uint256 value);
 
+    /**
+     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
+     * a call to `approve`. `value` is the new allowance.
+     */
     event Approval(address indexed owner, address indexed spender, uint256 value);
 }