2.4.0

(cherry picked from commit bddbbe99a7)

.circleci/config.yml

@@ -0,0 +1,82 @@
+version: 2
+# 2.1 does not yet support local run
+# unless with workaround. For simplicity just use it.
+# https://github.com/CircleCI-Public/circleci-cli/issues/79
+
+aliases:
+  - &defaults
+    docker:
+      - image: circleci/node:8
+
+  - &npm_install_if_necessary
+    run:
+      name: Install npm dependencies
+      command: |
+        if [ ! -d node_modules ]; then
+          npm ci
+        fi
+
+  - &cache_key_node_modules
+    key: v1-node_modules-{{ checksum "package-lock.json" }}
+
+jobs:
+  dependencies:
+    <<: *defaults
+    steps:
+      - checkout
+      - restore_cache:
+          <<: *cache_key_node_modules
+      - *npm_install_if_necessary
+      - save_cache:
+          paths:
+            - node_modules
+          <<: *cache_key_node_modules
+
+  lint:
+    <<: *defaults
+    steps:
+      - checkout
+      - restore_cache:
+          <<: *cache_key_node_modules
+      - *npm_install_if_necessary
+      - run:
+          name: Linter
+          command: npm run lint
+  test:
+    <<: *defaults
+    steps:
+      - checkout
+      - restore_cache:
+          <<: *cache_key_node_modules
+      - *npm_install_if_necessary
+      - run:
+          name: Unit tests
+          command: npm run test
+  coverage:
+    docker:
+      - image: circleci/node:8
+    steps:
+      - checkout
+      - restore_cache:
+          <<: *cache_key_node_modules
+      - *npm_install_if_necessary
+      - run:
+          name: Unit tests with coverage report
+          command: npm run coverage
+
+      # TODO(xinbenlv, #1839): run SOLC_NIGHTLY to be run but allow it to fail.
+
+workflows:
+  version: 2
+  everything:
+    jobs:
+      - dependencies
+      - lint:
+          requires:
+            - dependencies
+      - test:
+          requires:
+            - dependencies
+      - coverage:
+          requires:
+            - dependencies

.codecov.yml

@@ -0,0 +1,3 @@
+comment: off
+coverage:
+  range: "100...100"

.editorconfig

@@ -6,7 +6,12 @@ root = true
 [*]
 charset = utf-8
 end_of_line = lf
-indent_size = 2
 indent_style = space
 insert_final_newline = true
 trim_trailing_whitespace = true
+
+[*.sol]
+indent_size = 4
+
+[*.js]
+indent_size = 2

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,16 +1,16 @@
 ---
 name: Bug report
-about: Report a bug in OpenZeppelin
+about: Report a bug in OpenZeppelin Contracts
 
 ---
 
 <!-- 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 OpenZeppelin Community Forum: https://forum.openzeppelin.com/. -->
 
 **💻 Environment**
 
-<!-- Tell us what version of OpenZeppelin you're using, and how you're using it: Truffle, Remix, etc. -->
+<!-- Tell us what version of OpenZeppelin Contracts you're using, and how you're using it: Truffle, Remix, etc. -->
 
 **📝 Details**
 

.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

@@ -13,7 +13,7 @@ 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 Solidity linter (`npm run lint:sol`) and fixed any issues,

.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

@@ -40,3 +40,11 @@ build/
 
 # IntelliJ IDE
 .idea
+
+# docs artifacts
+docs/modules/api
+openzeppelin-docs
+
+# only used to package @openzeppelin/contracts
+contracts/build/
+contracts/README.md

.solhint.json

@@ -1,11 +1,14 @@
 {
-  "extends": "default",
+  "extends": "solhint:recommended",
   "rules": {
     "indent": ["error", 4],
-
-    "bracket-align": false,
-    "compiler-fixed": false,
-    "no-simple-event-func-name": false,
-    "two-lines-top-level-separator": false
+    "func-order": "off",
+    "bracket-align": "off",
+    "compiler-fixed": "off",
+    "no-simple-event-func-name": "off",
+    "separate-by-one-line-in-contract": "off",
+    "two-lines-top-level-separator": "off",
+    "mark-callable-contracts": "off",
+    "compiler-version": ["error", "^0.5.0"]
   }
 }

.travis.yml

@@ -1,53 +0,0 @@
-dist: trusty
-sudo: false
-group: beta
-language: node_js
-node_js:
-  - "8"
-
-cache:
-  directories:
-    - node_modules
-
-jobs:
-  # XXX fast_finish doesn't work with stages yet. See
-  # https://github.com/travis-ci/travis-ci/issues/8425
-  # --elopio - 20180531
-  fast_finish: true
-  allow_failures:
-    - env: SOLC_NIGHTLY=true
-  include:
-    - stage: tests
-      name: "Linter"
-      script: npm run lint
-
-    - stage: tests
-      name: "Unit tests"
-      script: npm run test
-
-    - 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:
-      - secure: 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
-    on_success: change
-    on_failure: always
-    on_pull_requests: false

CHANGELOG.md

@@ -1,10 +1,48 @@
 # Changelog
 
+## 2.4.0 (2019-10-29)
+
+### New features:
+ * `Address.toPayable`: added a helper to convert between address types without having to resort to low-level casting. ([#1773](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1773))
+ * Facilities to make metatransaction-enabled contracts through the Gas Station Network. ([#1844](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1844))
+ * `Address.sendValue`: added a replacement to Solidity's `transfer`, removing the fixed gas stipend. ([#1962](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1962))
+ * Added replacement for functions that don't forward all gas (which have been deprecated): ([#1976](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1976))
+   * `PullPayment.withdrawPaymentsWithGas(address payable payee)`
+   * `Escrow.withdrawWithGas(address payable payee)`
+ * `SafeMath`: added support for custom error messages to `sub`, `div` and `mod` functions. ([#1828](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1828))
+
+### Improvements:
+ * `Address.isContract`: switched from `extcodesize` to `extcodehash` for less gas usage. ([#1802](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1802))
+ * `ERC20` and `ERC777` updated to throw custom errors on subtraction overflows. ([#1828](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1828))
+
+### Deprecations:
+ * Deprecated functions that don't forward all gas: ([#1976](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1976))
+   * `PullPayment.withdrawPayments(address payable payee)`
+   * `Escrow.withdraw(address payable payee)`
+
+### Breaking changes:
+ * `Address` now requires a minimum Solidity compiler version of 0.5.5. ([#1802](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1802))
+ * `SignatureBouncer` has been removed from drafts, both to avoid confusions with the GSN and `GSNRecipientSignature` (previously called `GSNBouncerSignature`) and because the API was not very clear. ([#1879](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1879))
+
+## 2.3.0 (2019-05-27)
+
+### 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/))
+ * `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.
@@ -13,14 +51,14 @@
  * 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))
- * `Counter`'s API has been improved, and is now used by `ERC721` (though it is still in `drafts`). ([#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:
- * `TokenMetadata` (in drafts) has been renamed to `ERC20Metadata`. ([#1618](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1618))
+### 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))

CODE_STYLE.md

@@ -61,3 +61,9 @@ Any exception or additions specific to our project are documented below.
 
   Some standards (e.g. ERC20) use present tense, and in those cases the
   standard specification prevails.
+  
+* Interface names should have a capital I prefix.
+
+    ```
+    interface IERC777 {
+    ```

CONTRIBUTING.md

@@ -1,30 +1,27 @@
-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
 
-Smart contracts manage value and are highly vulnerable to errors and attacks. We have very strict guidelines, please make sure to review them: ["Contribution guidelines wiki entry"](https://github.com/OpenZeppelin/openzeppelin-solidity/wiki/Contribution-guidelines).
+Smart contracts manage value and are highly vulnerable to errors and attacks. We have very strict [guidelines], please make sure to review them!
 
 ## Creating Pull Requests (PRs)
 
 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* 
-* 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
 
 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)
@@ -32,7 +29,7 @@ NOTE: The directory `openzeppelin-solidity` represents your fork's local copy.
 git checkout -b fix/some-bug-#123
 ```
 
-3) Make your changes, add your files, commit and push to your fork.
+3) Make your changes, add your files, commit, and push to your fork.
 
 ```
 git add SomeFile.js
@@ -40,19 +37,35 @@ 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) Run tests, linter, etc. This can be done by running local continuous integration and make sure it passes.
 
-*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.
+```bash
+npm test
+npm run lint
+```
 
-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.
+or you can simply run CircleCI locally
+```bash
+circleci local execute --job build
+circleci local execute --job test
+```
+*Note*: requires installing CircleCI and docker locally on your machine.
+
+5) 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
+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.
+
+6) 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.
 
 *IMPORTANT* Please pay attention to the maintainer's feedback, since its a necessary step to keep up with the standards OpenZeppelin attains to.
 
 ## 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!
+
+[guidelines]: GUIDELINES.md

DOCUMENTATION.md

@@ -0,0 +1,16 @@
+Documentation is hosted at https://docs.openzeppelin.com/contracts.
+
+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) is the
+program that extracts the API Reference from source code.
+
+The [`docs.openzeppelin.com`](https://github.com/OpenZeppelin/docs.openzeppelin.com)
+repository hosts the configuration for the entire site, which includes
+documetation for all of the OpenZeppelin projects.
+
+To run the docs locally you should run `npm run docs start` on this
+repository.

GUIDELINES.md

@@ -0,0 +1,64 @@
+Design Guidelines
+=======
+
+These are some global design goals in OpenZeppelin.
+
+#### D0 - Security in Depth
+We strive to provide secure, tested, audited code. To achieve this, we need to match intention with function. Thus, documentation, code clarity, community review and security discussions are fundamental.
+
+#### D1 - Simple and Modular
+Simpler code means easier audits, and better understanding of what each component does. We look for small files, small contracts, and small functions. If you can separate a contract into two independent functionalities you should probably do it.
+
+#### D2 - Naming Matters
+
+We take our time with picking names. Code is going to be written once, and read hundreds of times. Renaming for clarity is encouraged.
+
+#### D3 - Tests
+
+Write tests for all your code. We encourage Test Driven Development so we know when our code is right. Even though not all code in the repository is tested at the moment, we aim to test every line of code in the future.
+
+#### D4 - Check preconditions and post-conditions
+
+A very important way to prevent vulnerabilities is to catch a contract’s inconsistent state as early as possible. This is why we want functions to check pre- and post-conditions for executing its logic. When writing code, ask yourself what you are expecting to be true before and after the function runs, and express it in code.
+
+#### D5 - Code Consistency
+
+Consistency on the way classes are used is paramount to an easier understanding of the library. The codebase should be as unified as possible. Read existing code and get inspired before you write your own. Follow the style guidelines. Don’t hesitate to ask for help on how to best write a specific piece of code.
+
+#### D6 - Regular Audits
+Following good programming practices is a way to reduce the risk of vulnerabilities, but professional code audits are still needed. We will perform regular code audits on major releases, and hire security professionals to provide independent review.
+
+## Style Guidelines
+
+The design guidelines have quite a high abstraction level. These style guidelines are more concrete and easier to apply, and also more opinionated.
+
+### General
+
+#### G0 - Default to Solidity's official style guide.
+
+Follow the official Solidity style guide: https://solidity.readthedocs.io/en/latest/style-guide.html
+
+#### G1 - No Magic Constants
+
+Avoid constants in the code as much as possible. Magic strings are also magic constants.
+
+#### G2 - Code that Fails Early
+
+We ask our code to fail as soon as possible when an unexpected input was provided or unexpected state was found.
+
+#### G3 - Internal Amounts Must be Signed Integers and Represent the Smallest Units.
+
+Avoid representation errors by always dealing with weis when handling ether. GUIs can convert to more human-friendly representations. Use Signed Integers (int) to prevent underflow problems.
+
+
+### Testing
+
+#### T1 - Tests Must be Written Elegantly
+
+Style guidelines are not relaxed for tests. Tests are a good way to show how to use the library, and maintaining them is extremely necessary.
+
+Don't write long tests, write helper functions to make them be as short and concise as possible (they should take just a few lines each), and use good variable names.
+
+#### T2 - Tests Must not be Random
+
+Inputs for tests should not be generated randomly. Accounts used to create test contracts are an exception, those can be random. Also, the type and structure of outputs should be checked.

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

@@ -1,17 +1,19 @@
-# <img src="logo.png" alt="OpenZeppelin" width="400px">
+# <img src="logo.png" alt="OpenZeppelin" height="40px">
 
-[![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://circleci.com/gh/OpenZeppelin/openzeppelin-contracts.svg?style=shield)](https://circleci.com/gh/OpenZeppelin/openzeppelin-contracts)
+[![Coverage Status](https://codecov.io/gh/OpenZeppelin/openzeppelin-contracts/graph/badge.svg)](https://codecov.io/gh/OpenZeppelin/openzeppelin-contracts)
 
-**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.
+**OpenZeppelin Contracts 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 Contracts 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] document.
+
 ## Usage
 
 To write your custom contracts, import ours and extend them through inheritance.
@@ -19,8 +21,8 @@ To write your custom contracts, import ours and extend them through inheritance.
 ```solidity
 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 {
@@ -28,13 +30,13 @@ 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 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 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
 
-OpenZeppelin the project is maintained by [Zeppelin] the company, and developed following our high standards for code quality and security. OpenZeppelin is meant to provide tested and community-audited code, but please use common sense when doing anything that deals with real money! We take no responsibility for your implementation decisions and any security problems you might experience.
+This project is maintained by [OpenZeppelin], and developed following our high standards for code quality and security. OpenZeppelin is meant to provide tested and community-audited code, but please use common sense when doing anything that deals with real money! We take no responsibility for your implementation decisions and any security problems you might experience.
 
 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.
 
@@ -51,10 +53,12 @@ OpenZeppelin exists thanks to its contributors. There are many ways you can part
 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
-[Zeppelin]: https://zeppelin.solutions
+[API docs]: https://docs.openzeppelin.com/contracts/api/token/erc20
+[guides]: https://docs.openzeppelin.com/contracts
+[API Stability]: https://docs.openzeppelin.com/contracts/api-stability
+[forum]: https://forum.openzeppelin.com
+[OpenZeppelin]: https://openzeppelin.com
 [contribution guide]: CONTRIBUTING.md
 [Truffle]: https://truffleframework.com/docs/truffle/quickstart
 [Embark]: https://embark.status.im/docs/quick_start.html
+[Buidler]: https://buidler.dev/getting-started/#overview

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
 
@@ -76,7 +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/ARCHITECTURE.md

@@ -1,27 +0,0 @@
-## Architecture
-
-The following provides visibility into how OpenZeppelin's contracts are organized:
-
-- **access** - Smart contracts that enable functionality that can be used for selective restrictions and basic authorization control functions.
-- **crowdsale** - A collection of smart contracts used to manage token crowdsales that allow investors to purchase tokens with ETH. Includes a base contract which implements fundamental crowdsale functionality in its simplest form. The base contract can be extended in order to satisfy your crowdsale’s specific requirements.
-	- **distribution** - Includes extensions of the base crowdsale contract which can be used to customize the completion of a crowdsale.
-	- **emission** - Includes extensions of the base crowdsale contract which can be used to mint and manage how tokens are issued to purchasers.
-	- **price** - Includes extensions of the crowdsale contract that can be used to manage changes in token prices.
-	- **validation**  - Includes extensions of the crowdsale contract that can be used to enforce restraints and limit access to token purchases.
-- **examples** - A collection of simple smart contracts that demonstrate how to add new features to base contracts through multiple inheritance.
-- **introspection**  - An interface that can be used to make a contract comply with the ERC-165 standard as well as a contract that implements ERC-165 using a lookup table.
-- **lifecycle** - A collection of base contracts used to manage the existence and behavior of your contracts and their funds.
-- **math** - Libraries with safety checks on operations that throw on errors.
-- **mocks** - A collection of abstract contracts that are primarily used for unit testing. They also serve as good usage examples and demonstrate how to combine contracts with inheritance when developing your own custom applications.
-- **ownership** - A collection of smart contracts that can be used to manage contract and token ownership
-- **payment** - A collection of smart contracts that can be used to manage payments through escrow arrangements, withdrawals, and claims. Includes support for both single payees and multiple payees.
-- **proposals** - A collection of smart contracts that reflect community Ethereum Improvement Proposals (EIPs). These contracts are under development and standardization. They are not recommended for production, but they are useful for experimentation with pending EIP standards. Go [here](https://github.com/OpenZeppelin/openzeppelin-solidity/wiki/ERC-Process) for more information.
-
-- **token** - A collection of approved ERC standard tokens -- their interfaces and implementations.
-	- **ERC20** - A standard interface for fungible tokens:
-		- *Interfaces* - Includes the ERC-20 token standard basic interface. I.e., what the contract’s ABI can represent.
-		- *Implementations* - Includes ERC-20 token implementations that include all required and some optional ERC-20 functionality.
-	- **ERC721** - A standard interface for non-fungible tokens
-		- *Interfaces* - Includes the ERC-721 token standard basic interface. I.e., what the contract’s ABI can represent.
-		- *Implementations* - Includes ERC-721 token implementations that include all required and some optional ERC-721 functionality.
-

contracts/GSN/Context.sol

@@ -0,0 +1,27 @@
+pragma solidity ^0.5.0;
+
+/*
+ * @dev Provides information about the current execution context, including the
+ * sender of the transaction and its data. While these are generally available
+ * via msg.sender and msg.data, they should not be accessed in such a direct
+ * manner, since when dealing with GSN meta-transactions the account sending and
+ * paying for execution may not be the actual sender (as far as an application
+ * is concerned).
+ *
+ * This contract is only required for intermediate, library-like contracts.
+ */
+contract Context {
+    // Empty internal constructor, to prevent people from mistakenly deploying
+    // an instance of this contract, which should be used via inheritance.
+    constructor () internal { }
+    // solhint-disable-previous-line no-empty-blocks
+
+    function _msgSender() internal view returns (address payable) {
+        return msg.sender;
+    }
+
+    function _msgData() internal view returns (bytes memory) {
+        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
+        return msg.data;
+    }
+}

contracts/GSN/GSNRecipient.sol

@@ -0,0 +1,228 @@
+pragma solidity ^0.5.0;
+
+import "./IRelayRecipient.sol";
+import "./IRelayHub.sol";
+import "./Context.sol";
+
+/**
+ * @dev Base GSN recipient contract: includes the {IRelayRecipient} interface
+ * and enables GSN support on all contracts in the inheritance tree.
+ *
+ * TIP: This contract is abstract. The functions {acceptRelayedCall},
+ *  {_preRelayedCall}, and {_postRelayedCall} are not implemented and must be
+ * provided by derived contracts. See the
+ * xref:ROOT:gsn-strategies.adoc#gsn-strategies[GSN strategies] for more
+ * information on how to use the pre-built {GSNRecipientSignature} and
+ * {GSNRecipientERC20Fee}, or how to write your own.
+ */
+contract GSNRecipient is IRelayRecipient, Context {
+    // Default RelayHub address, deployed on mainnet and all testnets at the same address
+    address private _relayHub = 0xD216153c06E857cD7f72665E0aF1d7D82172F494;
+
+    uint256 constant private RELAYED_CALL_ACCEPTED = 0;
+    uint256 constant private RELAYED_CALL_REJECTED = 11;
+
+    // How much gas is forwarded to postRelayedCall
+    uint256 constant internal POST_RELAYED_CALL_MAX_GAS = 100000;
+
+    /**
+     * @dev Emitted when a contract changes its {IRelayHub} contract to a new one.
+     */
+    event RelayHubChanged(address indexed oldRelayHub, address indexed newRelayHub);
+
+    /**
+     * @dev Returns the address of the {IRelayHub} contract for this recipient.
+     */
+    function getHubAddr() public view returns (address) {
+        return _relayHub;
+    }
+
+    /**
+     * @dev Switches to a new {IRelayHub} instance. This method is added for future-proofing: there's no reason to not
+     * use the default instance.
+     *
+     * IMPORTANT: After upgrading, the {GSNRecipient} will no longer be able to receive relayed calls from the old
+     * {IRelayHub} instance. Additionally, all funds should be previously withdrawn via {_withdrawDeposits}.
+     */
+    function _upgradeRelayHub(address newRelayHub) internal {
+        address currentRelayHub = _relayHub;
+        require(newRelayHub != address(0), "GSNRecipient: new RelayHub is the zero address");
+        require(newRelayHub != currentRelayHub, "GSNRecipient: new RelayHub is the current one");
+
+        emit RelayHubChanged(currentRelayHub, newRelayHub);
+
+        _relayHub = newRelayHub;
+    }
+
+    /**
+     * @dev Returns the version string of the {IRelayHub} for which this recipient implementation was built. If
+     * {_upgradeRelayHub} is used, the new {IRelayHub} instance should be compatible with this version.
+     */
+    // This function is view for future-proofing, it may require reading from
+    // storage in the future.
+    function relayHubVersion() public view returns (string memory) {
+        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
+        return "1.0.0";
+    }
+
+    /**
+     * @dev Withdraws the recipient's deposits in `RelayHub`.
+     *
+     * Derived contracts should expose this in an external interface with proper access control.
+     */
+    function _withdrawDeposits(uint256 amount, address payable payee) internal {
+        IRelayHub(_relayHub).withdraw(amount, payee);
+    }
+
+    // Overrides for Context's functions: when called from RelayHub, sender and
+    // data require some pre-processing: the actual sender is stored at the end
+    // of the call data, which in turns means it needs to be removed from it
+    // when handling said data.
+
+    /**
+     * @dev Replacement for msg.sender. Returns the actual sender of a transaction: msg.sender for regular transactions,
+     * and the end-user for GSN relayed calls (where msg.sender is actually `RelayHub`).
+     *
+     * IMPORTANT: Contracts derived from {GSNRecipient} should never use `msg.sender`, and use {_msgSender} instead.
+     */
+    function _msgSender() internal view returns (address payable) {
+        if (msg.sender != _relayHub) {
+            return msg.sender;
+        } else {
+            return _getRelayedCallSender();
+        }
+    }
+
+    /**
+     * @dev Replacement for msg.data. Returns the actual calldata of a transaction: msg.data for regular transactions,
+     * and a reduced version for GSN relayed calls (where msg.data contains additional information).
+     *
+     * IMPORTANT: Contracts derived from {GSNRecipient} should never use `msg.data`, and use {_msgData} instead.
+     */
+    function _msgData() internal view returns (bytes memory) {
+        if (msg.sender != _relayHub) {
+            return msg.data;
+        } else {
+            return _getRelayedCallData();
+        }
+    }
+
+    // Base implementations for pre and post relayedCall: only RelayHub can invoke them, and data is forwarded to the
+    // internal hook.
+
+    /**
+     * @dev See `IRelayRecipient.preRelayedCall`.
+     *
+     * This function should not be overriden directly, use `_preRelayedCall` instead.
+     *
+     * * Requirements:
+     *
+     * - the caller must be the `RelayHub` contract.
+     */
+    function preRelayedCall(bytes calldata context) external returns (bytes32) {
+        require(msg.sender == getHubAddr(), "GSNRecipient: caller is not RelayHub");
+        return _preRelayedCall(context);
+    }
+
+    /**
+     * @dev See `IRelayRecipient.preRelayedCall`.
+     *
+     * Called by `GSNRecipient.preRelayedCall`, which asserts the caller is the `RelayHub` contract. Derived contracts
+     * must implement this function with any relayed-call preprocessing they may wish to do.
+     *
+     */
+    function _preRelayedCall(bytes memory context) internal returns (bytes32);
+
+    /**
+     * @dev See `IRelayRecipient.postRelayedCall`.
+     *
+     * This function should not be overriden directly, use `_postRelayedCall` instead.
+     *
+     * * Requirements:
+     *
+     * - the caller must be the `RelayHub` contract.
+     */
+    function postRelayedCall(bytes calldata context, bool success, uint256 actualCharge, bytes32 preRetVal) external {
+        require(msg.sender == getHubAddr(), "GSNRecipient: caller is not RelayHub");
+        _postRelayedCall(context, success, actualCharge, preRetVal);
+    }
+
+    /**
+     * @dev See `IRelayRecipient.postRelayedCall`.
+     *
+     * Called by `GSNRecipient.postRelayedCall`, which asserts the caller is the `RelayHub` contract. Derived contracts
+     * must implement this function with any relayed-call postprocessing they may wish to do.
+     *
+     */
+    function _postRelayedCall(bytes memory context, bool success, uint256 actualCharge, bytes32 preRetVal) internal;
+
+    /**
+     * @dev Return this in acceptRelayedCall to proceed with the execution of a relayed call. Note that this contract
+     * will be charged a fee by RelayHub
+     */
+    function _approveRelayedCall() internal pure returns (uint256, bytes memory) {
+        return _approveRelayedCall("");
+    }
+
+    /**
+     * @dev See `GSNRecipient._approveRelayedCall`.
+     *
+     * This overload forwards `context` to _preRelayedCall and _postRelayedCall.
+     */
+    function _approveRelayedCall(bytes memory context) internal pure returns (uint256, bytes memory) {
+        return (RELAYED_CALL_ACCEPTED, context);
+    }
+
+    /**
+     * @dev Return this in acceptRelayedCall to impede execution of a relayed call. No fees will be charged.
+     */
+    function _rejectRelayedCall(uint256 errorCode) internal pure returns (uint256, bytes memory) {
+        return (RELAYED_CALL_REJECTED + errorCode, "");
+    }
+
+    /*
+     * @dev Calculates how much RelayHub will charge a recipient for using `gas` at a `gasPrice`, given a relayer's
+     * `serviceFee`.
+     */
+    function _computeCharge(uint256 gas, uint256 gasPrice, uint256 serviceFee) internal pure returns (uint256) {
+        // The fee is expressed as a percentage. E.g. a value of 40 stands for a 40% fee, so the recipient will be
+        // charged for 1.4 times the spent amount.
+        return (gas * gasPrice * (100 + serviceFee)) / 100;
+    }
+
+    function _getRelayedCallSender() private pure returns (address payable result) {
+        // We need to read 20 bytes (an address) located at array index msg.data.length - 20. In memory, the array
+        // is prefixed with a 32-byte length value, so we first add 32 to get the memory read index. However, doing
+        // so would leave the address in the upper 20 bytes of the 32-byte word, which is inconvenient and would
+        // require bit shifting. We therefore subtract 12 from the read index so the address lands on the lower 20
+        // bytes. This can always be done due to the 32-byte prefix.
+
+        // The final memory read index is msg.data.length - 20 + 32 - 12 = msg.data.length. Using inline assembly is the
+        // easiest/most-efficient way to perform this operation.
+
+        // These fields are not accessible from assembly
+        bytes memory array = msg.data;
+        uint256 index = msg.data.length;
+
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            // Load the 32 bytes word from memory with the address on the lower 20 bytes, and mask those.
+            result := and(mload(add(array, index)), 0xffffffffffffffffffffffffffffffffffffffff)
+        }
+        return result;
+    }
+
+    function _getRelayedCallData() private pure returns (bytes memory) {
+        // RelayHub appends the sender address at the end of the calldata, so in order to retrieve the actual msg.data,
+        // we must strip the last 20 bytes (length of an address type) from it.
+
+        uint256 actualDataLength = msg.data.length - 20;
+        bytes memory actualData = new bytes(actualDataLength);
+
+        for (uint256 i = 0; i < actualDataLength; ++i) {
+            actualData[i] = msg.data[i];
+        }
+
+        return actualData;
+    }
+}

contracts/GSN/GSNRecipientERC20Fee.sol

@@ -0,0 +1,151 @@
+pragma solidity ^0.5.0;
+
+import "./GSNRecipient.sol";
+import "../math/SafeMath.sol";
+import "../ownership/Secondary.sol";
+import "../token/ERC20/SafeERC20.sol";
+import "../token/ERC20/ERC20.sol";
+import "../token/ERC20/ERC20Detailed.sol";
+
+/**
+ * @dev A xref:ROOT:gsn-strategies.adoc#gsn-strategies[GSN strategy] that charges transaction fees in a special purpose ERC20
+ * token, which we refer to as the gas payment token. The amount charged is exactly the amount of Ether charged to the
+ * recipient. This means that the token is essentially pegged to the value of Ether.
+ *
+ * The distribution strategy of the gas payment token to users is not defined by this contract. It's a mintable token
+ * whose only minter is the recipient, so the strategy must be implemented in a derived contract, making use of the
+ * internal {_mint} function.
+ */
+contract GSNRecipientERC20Fee is GSNRecipient {
+    using SafeERC20 for __unstable__ERC20PrimaryAdmin;
+    using SafeMath for uint256;
+
+    enum GSNRecipientERC20FeeErrorCodes {
+        INSUFFICIENT_BALANCE
+    }
+
+    __unstable__ERC20PrimaryAdmin private _token;
+
+    /**
+     * @dev The arguments to the constructor are the details that the gas payment token will have: `name` and `symbol`. `decimals` is hard-coded to 18.
+     */
+    constructor(string memory name, string memory symbol) public {
+        _token = new __unstable__ERC20PrimaryAdmin(name, symbol, 18);
+    }
+
+    /**
+     * @dev Returns the gas payment token.
+     */
+    function token() public view returns (IERC20) {
+        return IERC20(_token);
+    }
+
+    /**
+     * @dev Internal function that mints the gas payment token. Derived contracts should expose this function in their public API, with proper access control mechanisms.
+     */
+    function _mint(address account, uint256 amount) internal {
+        _token.mint(account, amount);
+    }
+
+    /**
+     * @dev Ensures that only users with enough gas payment token balance can have transactions relayed through the GSN.
+     */
+    function acceptRelayedCall(
+        address,
+        address from,
+        bytes calldata,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256,
+        uint256,
+        bytes calldata,
+        uint256 maxPossibleCharge
+    )
+        external
+        view
+        returns (uint256, bytes memory)
+    {
+        if (_token.balanceOf(from) < maxPossibleCharge) {
+            return _rejectRelayedCall(uint256(GSNRecipientERC20FeeErrorCodes.INSUFFICIENT_BALANCE));
+        }
+
+        return _approveRelayedCall(abi.encode(from, maxPossibleCharge, transactionFee, gasPrice));
+    }
+
+    /**
+     * @dev Implements the precharge to the user. The maximum possible charge (depending on gas limit, gas price, and
+     * fee) will be deducted from the user balance of gas payment token. Note that this is an overestimation of the
+     * actual charge, necessary because we cannot predict how much gas the execution will actually need. The remainder
+     * is returned to the user in {_postRelayedCall}.
+     */
+    function _preRelayedCall(bytes memory context) internal returns (bytes32) {
+        (address from, uint256 maxPossibleCharge) = abi.decode(context, (address, uint256));
+
+        // The maximum token charge is pre-charged from the user
+        _token.safeTransferFrom(from, address(this), maxPossibleCharge);
+    }
+
+    /**
+     * @dev Returns to the user the extra amount that was previously charged, once the actual execution cost is known.
+     */
+    function _postRelayedCall(bytes memory context, bool, uint256 actualCharge, bytes32) internal {
+        (address from, uint256 maxPossibleCharge, uint256 transactionFee, uint256 gasPrice) =
+            abi.decode(context, (address, uint256, uint256, uint256));
+
+        // actualCharge is an _estimated_ charge, which assumes postRelayedCall will use all available gas.
+        // This implementation's gas cost can be roughly estimated as 10k gas, for the two SSTORE operations in an
+        // ERC20 transfer.
+        uint256 overestimation = _computeCharge(POST_RELAYED_CALL_MAX_GAS.sub(10000), gasPrice, transactionFee);
+        actualCharge = actualCharge.sub(overestimation);
+
+        // After the relayed call has been executed and the actual charge estimated, the excess pre-charge is returned
+        _token.safeTransfer(from, maxPossibleCharge.sub(actualCharge));
+    }
+}
+
+/**
+ * @title __unstable__ERC20PrimaryAdmin
+ * @dev An ERC20 token owned by another contract, which has minting permissions and can use transferFrom to receive
+ * anyone's tokens. This contract is an internal helper for GSNRecipientERC20Fee, and should not be used
+ * outside of this context.
+ */
+// solhint-disable-next-line contract-name-camelcase
+contract __unstable__ERC20PrimaryAdmin is ERC20, ERC20Detailed, Secondary {
+    uint256 private constant UINT256_MAX = 2**256 - 1;
+
+    constructor(string memory name, string memory symbol, uint8 decimals) public ERC20Detailed(name, symbol, decimals) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
+
+    // The primary account (GSNRecipientERC20Fee) can mint tokens
+    function mint(address account, uint256 amount) public onlyPrimary {
+        _mint(account, amount);
+    }
+
+    // The primary account has 'infinite' allowance for all token holders
+    function allowance(address owner, address spender) public view returns (uint256) {
+        if (spender == primary()) {
+            return UINT256_MAX;
+        } else {
+            return super.allowance(owner, spender);
+        }
+    }
+
+    // Allowance for the primary account cannot be changed (it is always 'infinite')
+    function _approve(address owner, address spender, uint256 value) internal {
+        if (spender == primary()) {
+            return;
+        } else {
+            super._approve(owner, spender, value);
+        }
+    }
+
+    function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
+        if (recipient == primary()) {
+            _transfer(sender, recipient, amount);
+            return true;
+        } else {
+            return super.transferFrom(sender, recipient, amount);
+        }
+    }
+}

contracts/GSN/GSNRecipientSignature.sol

@@ -0,0 +1,72 @@
+pragma solidity ^0.5.0;
+
+import "./GSNRecipient.sol";
+import "../cryptography/ECDSA.sol";
+
+/**
+ * @dev A xref:ROOT:gsn-strategies.adoc#gsn-strategies[GSN strategy] that allows relayed transactions through when they are
+ * accompanied by the signature of a trusted signer. The intent is for this signature to be generated by a server that
+ * performs validations off-chain. Note that nothing is charged to the user in this scheme. Thus, the server should make
+ * sure to account for this in their economic and threat model.
+ */
+contract GSNRecipientSignature is GSNRecipient {
+    using ECDSA for bytes32;
+
+    address private _trustedSigner;
+
+    enum GSNRecipientSignatureErrorCodes {
+        INVALID_SIGNER
+    }
+
+    /**
+     * @dev Sets the trusted signer that is going to be producing signatures to approve relayed calls.
+     */
+    constructor(address trustedSigner) public {
+        require(trustedSigner != address(0), "GSNRecipientSignature: trusted signer is the zero address");
+        _trustedSigner = trustedSigner;
+    }
+
+    /**
+     * @dev Ensures that only transactions with a trusted signature can be relayed through the GSN.
+     */
+    function acceptRelayedCall(
+        address relay,
+        address from,
+        bytes calldata encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes calldata approvalData,
+        uint256
+    )
+        external
+        view
+        returns (uint256, bytes memory)
+    {
+        bytes memory blob = abi.encodePacked(
+            relay,
+            from,
+            encodedFunction,
+            transactionFee,
+            gasPrice,
+            gasLimit,
+            nonce, // Prevents replays on RelayHub
+            getHubAddr(), // Prevents replays in multiple RelayHubs
+            address(this) // Prevents replays in multiple recipients
+        );
+        if (keccak256(blob).toEthSignedMessageHash().recover(approvalData) == _trustedSigner) {
+            return _approveRelayedCall();
+        } else {
+            return _rejectRelayedCall(uint256(GSNRecipientSignatureErrorCodes.INVALID_SIGNER));
+        }
+    }
+
+    function _preRelayedCall(bytes memory) internal returns (bytes32) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
+
+    function _postRelayedCall(bytes memory, bool, uint256, bytes32) internal {
+        // solhint-disable-previous-line no-empty-blocks
+    }
+}

contracts/GSN/IRelayHub.sol

@@ -0,0 +1,267 @@
+pragma solidity ^0.5.0;
+
+/**
+ * @dev Interface for `RelayHub`, the core contract of the GSN. Users should not need to interact with this contract
+ * directly.
+ *
+ * See the https://github.com/OpenZeppelin/openzeppelin-gsn-helpers[OpenZeppelin GSN helpers] for more information on
+ * how to deploy an instance of `RelayHub` on your local test network.
+ */
+contract IRelayHub {
+    // Relay management
+
+    /**
+     * @dev Adds stake to a relay and sets its `unstakeDelay`. If the relay does not exist, it is created, and the caller
+     * of this function becomes its owner. If the relay already exists, only the owner can call this function. A relay
+     * cannot be its own owner.
+     *
+     * All Ether in this function call will be added to the relay's stake.
+     * Its unstake delay will be assigned to `unstakeDelay`, but the new value must be greater or equal to the current one.
+     *
+     * Emits a {Staked} event.
+     */
+    function stake(address relayaddr, uint256 unstakeDelay) external payable;
+
+    /**
+     * @dev Emitted when a relay's stake or unstakeDelay are increased
+     */
+    event Staked(address indexed relay, uint256 stake, uint256 unstakeDelay);
+
+    /**
+     * @dev Registers the caller as a relay.
+     * The relay must be staked for, and not be a contract (i.e. this function must be called directly from an EOA).
+     *
+     * This function can be called multiple times, emitting new {RelayAdded} events. Note that the received
+     * `transactionFee` is not enforced by {relayCall}.
+     *
+     * Emits a {RelayAdded} event.
+     */
+    function registerRelay(uint256 transactionFee, string memory url) public;
+
+    /**
+     * @dev Emitted when a relay is registered or re-registerd. Looking at these events (and filtering out
+     * {RelayRemoved} events) lets a client discover the list of available relays.
+     */
+    event RelayAdded(address indexed relay, address indexed owner, uint256 transactionFee, uint256 stake, uint256 unstakeDelay, string url);
+
+    /**
+     * @dev Removes (deregisters) a relay. Unregistered (but staked for) relays can also be removed.
+     *
+     * Can only be called by the owner of the relay. After the relay's `unstakeDelay` has elapsed, {unstake} will be
+     * callable.
+     *
+     * Emits a {RelayRemoved} event.
+     */
+    function removeRelayByOwner(address relay) public;
+
+    /**
+     * @dev Emitted when a relay is removed (deregistered). `unstakeTime` is the time when unstake will be callable.
+     */
+    event RelayRemoved(address indexed relay, uint256 unstakeTime);
+
+    /** Deletes the relay from the system, and gives back its stake to the owner.
+     *
+     * Can only be called by the relay owner, after `unstakeDelay` has elapsed since {removeRelayByOwner} was called.
+     *
+     * Emits an {Unstaked} event.
+     */
+    function unstake(address relay) public;
+
+    /**
+     * @dev Emitted when a relay is unstaked for, including the returned stake.
+     */
+    event Unstaked(address indexed relay, uint256 stake);
+
+    // States a relay can be in
+    enum RelayState {
+        Unknown, // The relay is unknown to the system: it has never been staked for
+        Staked, // The relay has been staked for, but it is not yet active
+        Registered, // The relay has registered itself, and is active (can relay calls)
+        Removed    // The relay has been removed by its owner and can no longer relay calls. It must wait for its unstakeDelay to elapse before it can unstake
+    }
+
+    /**
+     * @dev Returns a relay's status. Note that relays can be deleted when unstaked or penalized, causing this function
+     * to return an empty entry.
+     */
+    function getRelay(address relay) external view returns (uint256 totalStake, uint256 unstakeDelay, uint256 unstakeTime, address payable owner, RelayState state);
+
+    // Balance management
+
+    /**
+     * @dev Deposits Ether for a contract, so that it can receive (and pay for) relayed transactions.
+     *
+     * Unused balance can only be withdrawn by the contract itself, by calling {withdraw}.
+     *
+     * Emits a {Deposited} event.
+     */
+    function depositFor(address target) public payable;
+
+    /**
+     * @dev Emitted when {depositFor} is called, including the amount and account that was funded.
+     */
+    event Deposited(address indexed recipient, address indexed from, uint256 amount);
+
+    /**
+     * @dev Returns an account's deposits. These can be either a contracts's funds, or a relay owner's revenue.
+     */
+    function balanceOf(address target) external view returns (uint256);
+
+    /**
+     * Withdraws from an account's balance, sending it back to it. Relay owners call this to retrieve their revenue, and
+     * contracts can use it to reduce their funding.
+     *
+     * Emits a {Withdrawn} event.
+     */
+    function withdraw(uint256 amount, address payable dest) public;
+
+    /**
+     * @dev Emitted when an account withdraws funds from `RelayHub`.
+     */
+    event Withdrawn(address indexed account, address indexed dest, uint256 amount);
+
+    // Relaying
+
+    /**
+     * @dev Checks if the `RelayHub` will accept a relayed operation.
+     * Multiple things must be true for this to happen:
+     *  - all arguments must be signed for by the sender (`from`)
+     *  - the sender's nonce must be the current one
+     *  - the recipient must accept this transaction (via {acceptRelayedCall})
+     *
+     * Returns a `PreconditionCheck` value (`OK` when the transaction can be relayed), or a recipient-specific error
+     * code if it returns one in {acceptRelayedCall}.
+     */
+    function canRelay(
+        address relay,
+        address from,
+        address to,
+        bytes memory encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes memory signature,
+        bytes memory approvalData
+    ) public view returns (uint256 status, bytes memory recipientContext);
+
+    // Preconditions for relaying, checked by canRelay and returned as the corresponding numeric values.
+    enum PreconditionCheck {
+        OK,                         // All checks passed, the call can be relayed
+        WrongSignature,             // The transaction to relay is not signed by requested sender
+        WrongNonce,                 // The provided nonce has already been used by the sender
+        AcceptRelayedCallReverted,  // The recipient rejected this call via acceptRelayedCall
+        InvalidRecipientStatusCode  // The recipient returned an invalid (reserved) status code
+    }
+
+    /**
+     * @dev Relays a transaction.
+     *
+     * For this to succeed, multiple conditions must be met:
+     *  - {canRelay} must `return PreconditionCheck.OK`
+     *  - the sender must be a registered relay
+     *  - the transaction's gas price must be larger or equal to the one that was requested by the sender
+     *  - the transaction must have enough gas to not run out of gas if all internal transactions (calls to the
+     * recipient) use all gas available to them
+     *  - the recipient must have enough balance to pay the relay for the worst-case scenario (i.e. when all gas is
+     * spent)
+     *
+     * If all conditions are met, the call will be relayed and the recipient charged. {preRelayedCall}, the encoded
+     * function and {postRelayedCall} will be called in that order.
+     *
+     * Parameters:
+     *  - `from`: the client originating the request
+     *  - `to`: the target {IRelayRecipient} contract
+     *  - `encodedFunction`: the function call to relay, including data
+     *  - `transactionFee`: fee (%) the relay takes over actual gas cost
+     *  - `gasPrice`: gas price the client is willing to pay
+     *  - `gasLimit`: gas to forward when calling the encoded function
+     *  - `nonce`: client's nonce
+     *  - `signature`: client's signature over all previous params, plus the relay and RelayHub addresses
+     *  - `approvalData`: dapp-specific data forwared to {acceptRelayedCall}. This value is *not* verified by the
+     * `RelayHub`, but it still can be used for e.g. a signature.
+     *
+     * Emits a {TransactionRelayed} event.
+     */
+    function relayCall(
+        address from,
+        address to,
+        bytes memory encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes memory signature,
+        bytes memory approvalData
+    ) public;
+
+    /**
+     * @dev Emitted when an attempt to relay a call failed.
+     *
+     * This can happen due to incorrect {relayCall} arguments, or the recipient not accepting the relayed call. The
+     * actual relayed call was not executed, and the recipient not charged.
+     *
+     * The `reason` parameter contains an error code: values 1-10 correspond to `PreconditionCheck` entries, and values
+     * over 10 are custom recipient error codes returned from {acceptRelayedCall}.
+     */
+    event CanRelayFailed(address indexed relay, address indexed from, address indexed to, bytes4 selector, uint256 reason);
+
+    /**
+     * @dev Emitted when a transaction is relayed. 
+     * Useful when monitoring a relay's operation and relayed calls to a contract
+     *
+     * Note that the actual encoded function might be reverted: this is indicated in the `status` parameter.
+     *
+     * `charge` is the Ether value deducted from the recipient's balance, paid to the relay's owner.
+     */
+    event TransactionRelayed(address indexed relay, address indexed from, address indexed to, bytes4 selector, RelayCallStatus status, uint256 charge);
+
+    // Reason error codes for the TransactionRelayed event
+    enum RelayCallStatus {
+        OK,                      // The transaction was successfully relayed and execution successful - never included in the event
+        RelayedCallFailed,       // The transaction was relayed, but the relayed call failed
+        PreRelayedFailed,        // The transaction was not relayed due to preRelatedCall reverting
+        PostRelayedFailed,       // The transaction was relayed and reverted due to postRelatedCall reverting
+        RecipientBalanceChanged  // The transaction was relayed and reverted due to the recipient's balance changing
+    }
+
+    /**
+     * @dev Returns how much gas should be forwarded to a call to {relayCall}, in order to relay a transaction that will
+     * spend up to `relayedCallStipend` gas.
+     */
+    function requiredGas(uint256 relayedCallStipend) public view returns (uint256);
+
+    /**
+     * @dev Returns the maximum recipient charge, given the amount of gas forwarded, gas price and relay fee.
+     */
+    function maxPossibleCharge(uint256 relayedCallStipend, uint256 gasPrice, uint256 transactionFee) public view returns (uint256);
+
+     // Relay penalization. 
+     // Any account can penalize relays, removing them from the system immediately, and rewarding the
+    // reporter with half of the relay's stake. The other half is burned so that, even if the relay penalizes itself, it
+    // still loses half of its stake.
+
+    /**
+     * @dev Penalize a relay that signed two transactions using the same nonce (making only the first one valid) and
+     * different data (gas price, gas limit, etc. may be different).
+     *
+     * The (unsigned) transaction data and signature for both transactions must be provided.
+     */
+    function penalizeRepeatedNonce(bytes memory unsignedTx1, bytes memory signature1, bytes memory unsignedTx2, bytes memory signature2) public;
+
+    /**
+     * @dev Penalize a relay that sent a transaction that didn't target `RelayHub`'s {registerRelay} or {relayCall}.
+     */
+    function penalizeIllegalTransaction(bytes memory unsignedTx, bytes memory signature) public;
+
+    /**
+     * @dev Emitted when a relay is penalized.
+     */
+    event Penalized(address indexed relay, address sender, uint256 amount);
+
+    /**
+     * @dev Returns an account's nonce in `RelayHub`.
+     */
+    function getNonce(address from) external view returns (uint256);
+}
+

contracts/GSN/IRelayRecipient.sol

@@ -0,0 +1,74 @@
+pragma solidity ^0.5.0;
+
+/**
+ * @dev Base interface for a contract that will be called via the GSN from {IRelayHub}.
+ *
+ * TIP: You don't need to write an implementation yourself! Inherit from {GSNRecipient} instead.
+ */
+contract IRelayRecipient {
+    /**
+     * @dev Returns the address of the {IRelayHub} instance this recipient interacts with.
+     */
+    function getHubAddr() public view returns (address);
+
+    /**
+     * @dev Called by {IRelayHub} to validate if this recipient accepts being charged for a relayed call. Note that the
+     * recipient will be charged regardless of the execution result of the relayed call (i.e. if it reverts or not).
+     *
+     * The relay request was originated by `from` and will be served by `relay`. `encodedFunction` is the relayed call
+     * calldata, so its first four bytes are the function selector. The relayed call will be forwarded `gasLimit` gas,
+     * and the transaction executed with a gas price of at least `gasPrice`. `relay`'s fee is `transactionFee`, and the
+     * recipient will be charged at most `maxPossibleCharge` (in wei). `nonce` is the sender's (`from`) nonce for
+     * replay attack protection in {IRelayHub}, and `approvalData` is a optional parameter that can be used to hold a signature
+     * over all or some of the previous values.
+     *
+     * Returns a tuple, where the first value is used to indicate approval (0) or rejection (custom non-zero error code,
+     * values 1 to 10 are reserved) and the second one is data to be passed to the other {IRelayRecipient} functions.
+     *
+     * {acceptRelayedCall} is called with 50k gas: if it runs out during execution, the request will be considered
+     * rejected. A regular revert will also trigger a rejection.
+     */
+    function acceptRelayedCall(
+        address relay,
+        address from,
+        bytes calldata encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes calldata approvalData,
+        uint256 maxPossibleCharge
+    )
+        external
+        view
+        returns (uint256, bytes memory);
+
+    /**
+     * @dev Called by {IRelayHub} on approved relay call requests, before the relayed call is executed. This allows to e.g.
+     * pre-charge the sender of the transaction.
+     *
+     * `context` is the second value returned in the tuple by {acceptRelayedCall}.
+     *
+     * Returns a value to be passed to {postRelayedCall}.
+     *
+     * {preRelayedCall} is called with 100k gas: if it runs out during exection or otherwise reverts, the relayed call
+     * will not be executed, but the recipient will still be charged for the transaction's cost.
+     */
+    function preRelayedCall(bytes calldata context) external returns (bytes32);
+
+    /**
+     * @dev Called by {IRelayHub} on approved relay call requests, after the relayed call is executed. This allows to e.g.
+     * charge the user for the relayed call costs, return any overcharges from {preRelayedCall}, or perform
+     * contract-specific bookkeeping.
+     *
+     * `context` is the second value returned in the tuple by {acceptRelayedCall}. `success` is the execution status of
+     * the relayed call. `actualCharge` is an estimate of how much the recipient will be charged for the transaction,
+     * not including any gas used by {postRelayedCall} itself. `preRetVal` is {preRelayedCall}'s return value.
+     *
+     *
+     * {postRelayedCall} is called with 100k gas: if it runs out during execution or otherwise reverts, the relayed call
+     * and the call to {preRelayedCall} will be reverted retroactively, but the recipient will still be charged for the
+     * transaction's cost.
+     */
+    function postRelayedCall(bytes calldata context, bool success, uint256 actualCharge, bytes32 preRetVal) external;
+}

contracts/GSN/README.adoc

@@ -0,0 +1,30 @@
+= Gas Station Network (GSN)
+
+_Available since v2.4.0._
+
+This set of contracts provide all the tools required to make a contract callable via the https://gsn.openzeppelin.com[Gas Station Network].
+
+TIP: If you're new to the GSN, head over to our xref:openzeppelin::gsn/what-is-the-gsn.adoc[overview of the system] and basic guide to xref:ROOT:gsn.adoc[creating a GSN-capable contract].
+
+The core contract a recipient must inherit from is {GSNRecipient}: it includes all necessary interfaces, as well as some helper methods to make interacting with the GSN easier.
+
+Utilities to make writing xref:ROOT:gsn-strategies.adoc[GSN strategies] easy are available in {GSNRecipient}, or you can simply use one of our pre-made strategies:
+
+* {GSNRecipientERC20Fee} charges the end user for gas costs in an application-specific xref:ROOT:tokens.adoc#ERC20[ERC20 token]
+* {GSNRecipientSignature} accepts all relayed calls that have been signed by a trusted third party (e.g. a private key in a backend)
+
+You can also take a look at the two contract interfaces that make up the GSN protocol: {IRelayRecipient} and {IRelayHub}, but you won't need to use those directly.
+
+== Recipient
+
+{{GSNRecipient}}
+
+== Strategies
+
+{{GSNRecipientSignature}}
+{{GSNRecipientERC20Fee}}
+
+== Protocol
+
+{{IRelayRecipient}}
+{{IRelayHub}}

contracts/access/README.adoc

@@ -0,0 +1,21 @@
+= Access
+
+NOTE: This page is incomplete. We're working to improve it for the next release. Stay tuned!
+
+== Library
+
+{{Roles}}
+
+== Roles
+
+{{CapperRole}}
+
+{{MinterRole}}
+
+{{PauserRole}}
+
+{{SignerRole}}
+
+{{WhitelistAdminRole}}
+
+{{WhitelistedRole}}

contracts/access/Roles.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 /**
  * @title Roles
@@ -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

@@ -1,8 +1,9 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../Roles.sol";
 
-contract CapperRole {
+contract CapperRole is Context {
     using Roles for Roles.Role;
 
     event CapperAdded(address indexed account);
@@ -11,11 +12,11 @@ contract CapperRole {
     Roles.Role private _cappers;
 
     constructor () internal {
-        _addCapper(msg.sender);
+        _addCapper(_msgSender());
     }
 
     modifier onlyCapper() {
-        require(isCapper(msg.sender));
+        require(isCapper(_msgSender()), "CapperRole: caller does not have the Capper role");
         _;
     }
 
@@ -28,7 +29,7 @@ contract CapperRole {
     }
 
     function renounceCapper() public {
-        _removeCapper(msg.sender);
+        _removeCapper(_msgSender());
     }
 
     function _addCapper(address account) internal {

contracts/access/roles/MinterRole.sol

@@ -1,8 +1,9 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../Roles.sol";
 
-contract MinterRole {
+contract MinterRole is Context {
     using Roles for Roles.Role;
 
     event MinterAdded(address indexed account);
@@ -11,11 +12,11 @@ contract MinterRole {
     Roles.Role private _minters;
 
     constructor () internal {
-        _addMinter(msg.sender);
+        _addMinter(_msgSender());
     }
 
     modifier onlyMinter() {
-        require(isMinter(msg.sender));
+        require(isMinter(_msgSender()), "MinterRole: caller does not have the Minter role");
         _;
     }
 
@@ -28,7 +29,7 @@ contract MinterRole {
     }
 
     function renounceMinter() public {
-        _removeMinter(msg.sender);
+        _removeMinter(_msgSender());
     }
 
     function _addMinter(address account) internal {

contracts/access/roles/PauserRole.sol

@@ -1,8 +1,9 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../Roles.sol";
 
-contract PauserRole {
+contract PauserRole is Context {
     using Roles for Roles.Role;
 
     event PauserAdded(address indexed account);
@@ -11,11 +12,11 @@ contract PauserRole {
     Roles.Role private _pausers;
 
     constructor () internal {
-        _addPauser(msg.sender);
+        _addPauser(_msgSender());
     }
 
     modifier onlyPauser() {
-        require(isPauser(msg.sender));
+        require(isPauser(_msgSender()), "PauserRole: caller does not have the Pauser role");
         _;
     }
 
@@ -28,7 +29,7 @@ contract PauserRole {
     }
 
     function renouncePauser() public {
-        _removePauser(msg.sender);
+        _removePauser(_msgSender());
     }
 
     function _addPauser(address account) internal {

contracts/access/roles/SignerRole.sol

@@ -1,8 +1,9 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../Roles.sol";
 
-contract SignerRole {
+contract SignerRole is Context {
     using Roles for Roles.Role;
 
     event SignerAdded(address indexed account);
@@ -11,11 +12,11 @@ contract SignerRole {
     Roles.Role private _signers;
 
     constructor () internal {
-        _addSigner(msg.sender);
+        _addSigner(_msgSender());
     }
 
     modifier onlySigner() {
-        require(isSigner(msg.sender));
+        require(isSigner(_msgSender()), "SignerRole: caller does not have the Signer role");
         _;
     }
 
@@ -28,7 +29,7 @@ contract SignerRole {
     }
 
     function renounceSigner() public {
-        _removeSigner(msg.sender);
+        _removeSigner(_msgSender());
     }
 
     function _addSigner(address account) internal {

contracts/access/roles/WhitelistAdminRole.sol

@@ -1,12 +1,13 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../Roles.sol";
 
 /**
  * @title WhitelistAdminRole
  * @dev WhitelistAdmins are responsible for assigning and removing Whitelisted accounts.
  */
-contract WhitelistAdminRole {
+contract WhitelistAdminRole is Context {
     using Roles for Roles.Role;
 
     event WhitelistAdminAdded(address indexed account);
@@ -15,11 +16,11 @@ contract WhitelistAdminRole {
     Roles.Role private _whitelistAdmins;
 
     constructor () internal {
-        _addWhitelistAdmin(msg.sender);
+        _addWhitelistAdmin(_msgSender());
     }
 
     modifier onlyWhitelistAdmin() {
-        require(isWhitelistAdmin(msg.sender));
+        require(isWhitelistAdmin(_msgSender()), "WhitelistAdminRole: caller does not have the WhitelistAdmin role");
         _;
     }
 
@@ -32,7 +33,7 @@ contract WhitelistAdminRole {
     }
 
     function renounceWhitelistAdmin() public {
-        _removeWhitelistAdmin(msg.sender);
+        _removeWhitelistAdmin(_msgSender());
     }
 
     function _addWhitelistAdmin(address account) internal {

contracts/access/roles/WhitelistedRole.sol

@@ -1,5 +1,6 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../Roles.sol";
 import "./WhitelistAdminRole.sol";
 
@@ -9,7 +10,7 @@ import "./WhitelistAdminRole.sol";
  * crowdsale). This role is special in that the only accounts that can add it are WhitelistAdmins (who can also remove
  * it), and not Whitelisteds themselves.
  */
-contract WhitelistedRole is WhitelistAdminRole {
+contract WhitelistedRole is Context, WhitelistAdminRole {
     using Roles for Roles.Role;
 
     event WhitelistedAdded(address indexed account);
@@ -18,7 +19,7 @@ contract WhitelistedRole is WhitelistAdminRole {
     Roles.Role private _whitelisteds;
 
     modifier onlyWhitelisted() {
-        require(isWhitelisted(msg.sender));
+        require(isWhitelisted(_msgSender()), "WhitelistedRole: caller does not have the Whitelisted role");
         _;
     }
 
@@ -35,7 +36,7 @@ contract WhitelistedRole is WhitelistAdminRole {
     }
 
     function renounceWhitelisted() public {
-        _removeWhitelisted(msg.sender);
+        _removeWhitelisted(_msgSender());
     }
 
     function _addWhitelisted(address account) internal {

contracts/crowdsale/Crowdsale.sol

@@ -1,5 +1,6 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../GSN/Context.sol";
 import "../token/ERC20/IERC20.sol";
 import "../math/SafeMath.sol";
 import "../token/ERC20/SafeERC20.sol";
@@ -17,7 +18,7 @@ import "../utils/ReentrancyGuard.sol";
  * the methods to add functionality. Consider using 'super' where appropriate to concatenate
  * behavior.
  */
-contract Crowdsale is ReentrancyGuard {
+contract Crowdsale is Context, ReentrancyGuard {
     using SafeMath for uint256;
     using SafeERC20 for IERC20;
 
@@ -54,9 +55,9 @@ 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;
@@ -70,7 +71,7 @@ contract Crowdsale is ReentrancyGuard {
      * buyTokens directly when purchasing tokens from a contract.
      */
     function () external payable {
-        buyTokens(msg.sender);
+        buyTokens(_msgSender());
     }
 
     /**
@@ -118,7 +119,7 @@ contract Crowdsale is ReentrancyGuard {
         _weiRaised = _weiRaised.add(weiAmount);
 
         _processPurchase(beneficiary, tokens);
-        emit TokensPurchased(msg.sender, beneficiary, weiAmount, tokens);
+        emit TokensPurchased(_msgSender(), beneficiary, weiAmount, tokens);
 
         _updatePurchasingState(beneficiary, weiAmount);
 
@@ -136,8 +137,9 @@ 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");
+        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
     }
 
     /**

contracts/crowdsale/README.adoc

@@ -0,0 +1,35 @@
+= Crowdsales
+
+NOTE: This page is incomplete. We're working to improve it for the next release. Stay tuned!
+
+== Core
+
+{{Crowdsale}}
+
+== Emission
+
+{{AllowanceCrowdsale}}
+
+{{MintedCrowdsale}}
+
+== Validation
+
+{{CappedCrowdsale}}
+
+{{IndividuallyCappedCrowdsale}}
+
+{{PausableCrowdsale}}
+
+{{TimedCrowdsale}}
+
+{{WhitelistCrowdsale}}
+
+== Distribution
+
+{{FinalizableCrowdsale}}
+
+{{PostDeliveryCrowdsale}}
+
+{{RefundableCrowdsale}}
+
+{{RefundablePostDeliveryCrowdsale}}

contracts/crowdsale/distribution/FinalizableCrowdsale.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../../math/SafeMath.sol";
 import "../validation/TimedCrowdsale.sol";
@@ -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

@@ -1,7 +1,9 @@
-pragma solidity ^0.5.2;
+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

@@ -1,20 +1,21 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../../GSN/Context.sol";
 import "../../math/SafeMath.sol";
 import "./FinalizableCrowdsale.sol";
 import "../../payment/escrow/RefundEscrow.sol";
 
 /**
  * @title RefundableCrowdsale
- * @dev Extension of FinalizableCrowdsale contract that adds a funding goal, and the possibility of users
+ * @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.
  */
-contract RefundableCrowdsale is FinalizableCrowdsale {
+contract RefundableCrowdsale is Context, FinalizableCrowdsale {
     using SafeMath for uint256;
 
     // minimum amount of funds to be raised in weis
@@ -28,7 +29,7 @@ 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 +42,12 @@ 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 +61,7 @@ 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()) {
@@ -77,6 +78,6 @@ contract RefundableCrowdsale is FinalizableCrowdsale {
      * @dev Overrides Crowdsale fund forwarding, sending funds to escrow.
      */
     function _forwardFunds() internal {
-        _escrow.deposit.value(msg.value)(msg.sender);
+        _escrow.deposit.value(msg.value)(_msgSender());
     }
 }

contracts/crowdsale/distribution/RefundablePostDeliveryCrowdsale.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "./RefundableCrowdsale.sol";
 import "./PostDeliveryCrowdsale.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

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../Crowdsale.sol";
 import "../../token/ERC20/IERC20.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

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../Crowdsale.sol";
 import "../../token/ERC20/ERC20Mintable.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

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../validation/TimedCrowdsale.sol";
 import "../../math/SafeMath.sol";
@@ -21,8 +21,9 @@ 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;
     }
@@ -32,7 +33,7 @@ contract IncreasingPriceCrowdsale is TimedCrowdsale {
      * all calls to it are a mistake.
      */
     function rate() public view returns (uint256) {
-        revert();
+        revert("IncreasingPriceCrowdsale: rate() called");
     }
 
     /**

contracts/crowdsale/validation/CappedCrowdsale.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../../math/SafeMath.sol";
 import "../Crowdsale.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

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../../math/SafeMath.sol";
 import "../Crowdsale.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/PausableCrowdsale.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../Crowdsale.sol";
 import "../../lifecycle/Pausable.sol";

contracts/crowdsale/validation/TimedCrowdsale.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../../math/SafeMath.sol";
 import "../Crowdsale.sol";
@@ -24,7 +24,7 @@ contract TimedCrowdsale is Crowdsale {
      * @dev Reverts if not in crowdsale time range.
      */
     modifier onlyWhileOpen {
-        require(isOpen());
+        require(isOpen(), "TimedCrowdsale: not open");
         _;
     }
 
@@ -35,8 +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;
@@ -74,7 +75,7 @@ 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
      */
@@ -83,12 +84,13 @@ contract TimedCrowdsale is Crowdsale {
     }
 
     /**
-     * @dev Extend crowdsale
+     * @dev Extend crowdsale.
      * @param newClosingTime Crowdsale closing time
      */
     function _extendTime(uint256 newClosingTime) internal {
-        require(!hasClosed());
-        require(newClosingTime > _closingTime);
+        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

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 import "../Crowdsale.sol";
 import "../../access/roles/WhitelistedRole.sol";
 
@@ -15,7 +15,7 @@ contract WhitelistCrowdsale is WhitelistedRole, Crowdsale {
      * @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

@@ -1,17 +1,29 @@
-pragma solidity ^0.5.2;
+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.
+     *
+     * IMPORTANT: `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) {
         // Check the signature length
@@ -55,9 +67,12 @@ library ECDSA {
     }
 
     /**
-     * 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
+     * https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign[`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 ^0.5.2;
+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.adoc

@@ -0,0 +1,9 @@
+= Cryptography
+
+This collection of libraries provides simple and safe ways to use different cryptographic primitives.
+
+== Libraries
+
+{{ECDSA}}
+
+{{MerkleProof}}

contracts/drafts/Counters.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../math/SafeMath.sol";
 
@@ -6,10 +6,10 @@ 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
+ * 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
+ * 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.
  */

contracts/drafts/ERC1046/ERC20Metadata.sol

@@ -1,11 +1,11 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../../token/ERC20/IERC20.sol";
 
 /**
  * @title ERC-1047 Token Metadata
  * @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
+ * @dev {tokenURI} must respond with a URI that implements https://eips.ethereum.org/EIPS/eip-1047
  */
 contract ERC20Metadata {
     string private _tokenURI;

contracts/drafts/ERC20Migrator.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/IERC20.sol";
 import "../token/ERC20/ERC20Mintable.sol";
@@ -14,8 +14,8 @@ import "../math/Math.sol";
  * migration to the new token contract. In this way, token holders "turn in"
  * their old balance and will be minted an equal amount in the new token.
  * The new token contract must be mintable. For the precise interface refer to
- * OpenZeppelin's ERC20Mintable, but the only functions that are needed are
- * `isMinter(address)` and `mint(address, amount)`. The migrator will check
+ * OpenZeppelin's {ERC20Mintable}, but the only functions that are needed are
+ * {MinterRole-isMinter} and {ERC20Mintable-mint}. The migrator will check
  * that it is a minter for the token.
  * The balance from the legacy token will be transferred to the migrator, as it
  * is migrated, and remain there forever.
@@ -24,6 +24,7 @@ import "../math/Math.sol";
  * version of it using ZeppelinOS. To read more about how this can be done
  * using this implementation, please follow the official documentation site of
  * ZeppelinOS: https://docs.zeppelinos.org/docs/erc20_onboarding.html
+ *
  * Example of usage:
  * ```
  * const migrator = await ERC20Migrator.new(legacyToken.address);
@@ -44,7 +45,7 @@ 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;
     }
 
@@ -68,9 +69,10 @@ contract ERC20Migrator {
      * @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)));
+        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_;
     }
@@ -82,7 +84,7 @@ contract ERC20Migrator {
      * @param amount amount of tokens to be migrated
      */
     function migrate(address account, uint256 amount) public {
-        require(address(_newToken) != address(0));
+        require(address(_newToken) != address(0), "ERC20Migrator: migration not started");
         _legacyToken.safeTransferFrom(account, address(this), amount);
         _newToken.mint(account, amount);
     }

contracts/drafts/ERC20Snapshot.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../math/SafeMath.sol";
 import "../utils/Arrays.sol";
@@ -7,14 +7,16 @@ 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
+ * @dev Inspired by Jordi Baylina's
+ * https://github.com/Giveth/minimd/blob/ea04d950eea153a04c51fa510b068b9dded390cb/contracts/MiniMeToken.sol[MiniMeToken]
+ * to record historical balances.
+ *
+ * When a snapshot is made, the balances and total supply 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
+ * 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>
  */
@@ -31,7 +33,7 @@ contract ERC20Snapshot is ERC20 {
     }
 
     mapping (address => Snapshots) private _accountBalanceSnapshots;
-    Snapshots private _totalSupplySnaphots;
+    Snapshots private _totalSupplySnapshots;
 
     // Snapshot ids increase monotonically, with the first value being 1. An id of 0 is invalid.
     Counters.Counter private _currentSnapshotId;
@@ -56,7 +58,7 @@ contract ERC20Snapshot is ERC20 {
     }
 
     function totalSupplyAt(uint256 snapshotId) public view returns(uint256) {
-        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _totalSupplySnaphots);
+        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _totalSupplySnapshots);
 
         return snapshotted ? value : totalSupply();
     }
@@ -101,8 +103,9 @@ contract ERC20Snapshot is ERC20 {
     function _valueAt(uint256 snapshotId, Snapshots storage snapshots)
         private view returns (bool, uint256)
     {
-        require(snapshotId > 0);
-        require(snapshotId <= _currentSnapshotId.current());
+        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);
 
@@ -118,7 +121,7 @@ contract ERC20Snapshot is ERC20 {
     }
 
     function _updateTotalSupplySnapshot() private {
-        _updateSnapshot(_totalSupplySnaphots, totalSupply());
+        _updateSnapshot(_totalSupplySnapshots, totalSupply());
     }
 
     function _updateSnapshot(Snapshots storage snapshots, uint256 currentValue) private {

contracts/drafts/README.adoc

@@ -0,0 +1,21 @@
+= Drafts
+
+NOTE: This page is incomplete. We're working to improve it for the next release. Stay tuned!
+
+== ERC 20
+
+{{ERC20Migrator}}
+
+{{ERC20Snapshot}}
+
+{{TokenVesting}}
+
+== Miscellaneous
+
+{{Counters}}
+
+{{SignedSafeMath}}
+
+== ERC 1046
+
+{{ERC1046}}

contracts/drafts/SignatureBouncer.sol

@@ -1,120 +0,0 @@
-pragma solidity ^0.5.2;
-
-import "../access/roles/SignerRole.sol";
-import "../cryptography/ECDSA.sol";
-
-/**
- * @title SignatureBouncer
- * @author PhABC, Shrugs and aflesher
- * @dev SignatureBouncer allows users to submit a signature as a permission to
- * do an action.
- * If the signature is from one of the authorized signer addresses, the
- * signature is valid.
- * Note that SignatureBouncer offers no protection against replay attacks, users
- * must add this themselves!
- *
- * Signer addresses can be individual servers signing grants or different
- * users within a decentralized club that have permission to invite other
- * members. This technique is useful for whitelists and airdrops; instead of
- * putting all valid addresses on-chain, simply sign a grant of the form
- * keccak256(abi.encodePacked(`:contractAddress` + `:granteeAddress`)) using a
- * valid signer address.
- * Then restrict access to your crowdsale/whitelist/airdrop using the
- * `onlyValidSignature` modifier (or implement your own using _isValidSignature).
- * In addition to `onlyValidSignature`, `onlyValidSignatureAndMethod` and
- * `onlyValidSignatureAndData` can be used to restrict access to only a given
- * method or a given method with given parameters respectively.
- * See the tests in SignatureBouncer.test.js for specific usage examples.
- *
- * @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 _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.
- */
-contract SignatureBouncer is SignerRole {
-    using ECDSA for bytes32;
-
-    // Function selectors are 4 bytes long, as documented in
-    // https://solidity.readthedocs.io/en/v0.4.24/abi-spec.html#function-selector
-    uint256 private constant _METHOD_ID_SIZE = 4;
-    // Signature size is 65 bytes (tightly packed v + r + s), but gets padded to 96 bytes
-    uint256 private constant _SIGNATURE_SIZE = 96;
-
-    constructor () internal {
-        // solhint-disable-previous-line no-empty-blocks
-    }
-
-    /**
-     * @dev requires that a valid signature of a signer was provided
-     */
-    modifier onlyValidSignature(bytes memory signature) {
-        require(_isValidSignature(msg.sender, signature));
-        _;
-    }
-
-    /**
-     * @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));
-        _;
-    }
-
-    /**
-     * @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));
-        _;
-    }
-
-    /**
-     * @dev is the signature of `this + account` from a signer?
-     * @return bool
-     */
-    function _isValidSignature(address account, bytes memory signature) internal view returns (bool) {
-        return _isValidDataHash(keccak256(abi.encodePacked(address(this), account)), signature);
-    }
-
-    /**
-     * @dev is the signature of `this + account + methodId` from a signer?
-     * @return bool
-     */
-    function _isValidSignatureAndMethod(address account, bytes memory signature) internal view returns (bool) {
-        bytes memory data = new bytes(_METHOD_ID_SIZE);
-        for (uint i = 0; i < data.length; i++) {
-            data[i] = msg.data[i];
-        }
-        return _isValidDataHash(keccak256(abi.encodePacked(address(this), account, data)), signature);
-    }
-
-    /**
-     * @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);
-
-        bytes memory data = new bytes(msg.data.length - _SIGNATURE_SIZE);
-        for (uint i = 0; i < data.length; i++) {
-            data[i] = msg.data[i];
-        }
-
-        return _isValidDataHash(keccak256(abi.encodePacked(address(this), account, data)), signature);
-    }
-
-    /**
-     * @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) {
-        address signer = hash.toEthSignedMessageHash().recover(signature);
-
-        return signer != address(0) && isSigner(signer);
-    }
-}

contracts/drafts/SignedSafeMath.sol

@@ -1,8 +1,8 @@
-pragma solidity ^0.5.2;
+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;
@@ -13,15 +13,15 @@ library SignedSafeMath {
     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;
     }
@@ -30,8 +30,8 @@ library SignedSafeMath {
      * @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;
 
@@ -43,7 +43,7 @@ library SignedSafeMath {
      */
     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;
     }
@@ -53,7 +53,7 @@ library SignedSafeMath {
      */
     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/Strings.sol

@@ -0,0 +1,32 @@
+pragma solidity ^0.5.0;
+
+/**
+ * @title Strings
+ * @dev String operations.
+ */
+library Strings {
+    /**
+     * @dev Converts a `uint256` to a `string`.
+     * via OraclizeAPI - MIT licence
+     * https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol
+     */
+    function fromUint256(uint256 value) internal pure returns (string memory) {
+        if (value == 0) {
+            return "0";
+        }
+        uint256 temp = value;
+        uint256 digits;
+        while (temp != 0) {
+            digits++;
+            temp /= 10;
+        }
+        bytes memory buffer = new bytes(digits);
+        uint256 index = digits - 1;
+        temp = value;
+        while (temp != 0) {
+            buffer[index--] = byte(uint8(48 + temp % 10));
+            temp /= 10;
+        }
+        return string(buffer);
+    }
+}

contracts/drafts/TokenVesting.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/SafeERC20.sol";
 import "../ownership/Ownable.sol";
@@ -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

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../crowdsale/validation/CappedCrowdsale.sol";
 import "../crowdsale/distribution/RefundableCrowdsale.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/examples/SimpleToken.sol

@@ -1,5 +1,6 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../GSN/Context.sol";
 import "../token/ERC20/ERC20.sol";
 import "../token/ERC20/ERC20Detailed.sol";
 
@@ -9,14 +10,12 @@ import "../token/ERC20/ERC20Detailed.sol";
  * Note they can later distribute these tokens as they wish using `transfer` and other
  * `ERC20` functions.
  */
-contract SimpleToken is ERC20, ERC20Detailed {
-    uint8 public constant DECIMALS = 18;
-    uint256 public constant INITIAL_SUPPLY = 10000 * (10 ** uint256(DECIMALS));
+contract SimpleToken is Context, ERC20, ERC20Detailed {
 
     /**
-     * @dev Constructor that gives msg.sender all of existing tokens.
+     * @dev Constructor that gives _msgSender() all of existing tokens.
      */
-    constructor () public ERC20Detailed("SimpleToken", "SIM", DECIMALS) {
-        _mint(msg.sender, INITIAL_SUPPLY);
+    constructor () public ERC20Detailed("SimpleToken", "SIM", 18) {
+        _mint(_msgSender(), 10000 * (10 ** uint256(decimals())));
     }
 }

contracts/introspection/ERC165.sol

@@ -1,44 +1,52 @@
-pragma solidity ^0.5.2;
+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 ^0.5.2;
+pragma solidity ^0.5.0;
 
 /**
- * @title ERC165Checker
- * @dev Use `using ERC165Checker for address`; to include this library
- * https://eips.ethereum.org/EIPS/eip-165
+ * @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

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 ^0.5.2;
+pragma solidity ^0.5.0;
 
 /**
- * @title IERC165
- * @dev https://eips.ethereum.org/EIPS/eip-165
+ * @dev Interface of the ERC165 standard, as defined in the
+ * https://eips.ethereum.org/EIPS/eip-165[EIP].
+ *
+ * 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
+     * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
+     * 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
+ * https://eips.ethereum.org/EIPS/eip-1820#interface-implementation-erc1820implementerinterface[EIP].
+ * 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
+ * https://eips.ethereum.org/EIPS/eip-1820[EIP]. 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
+     * https://eips.ethereum.org/EIPS/eip-1820#interface-name[section of the EIP].
+     */
+    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` 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` 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.adoc

@@ -0,0 +1,28 @@
+= Introspection
+
+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.
+
+== Local
+
+{{IERC165}}
+
+{{ERC165}}
+
+{{ERC165Checker}}
+
+== Global
+
+{{IERC1820Registry}}
+
+{{IERC1820Implementer}}
+
+{{ERC1820Implementer}}

contracts/lifecycle/Pausable.sol

@@ -1,23 +1,40 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
+import "../GSN/Context.sol";
 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 {
+contract Pausable is Context, 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 +44,7 @@ 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");
         _;
     }
 
@@ -35,23 +52,23 @@ 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;
-        emit Paused(msg.sender);
+        emit Paused(_msgSender());
     }
 
     /**
-     * @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;
-        emit Unpaused(msg.sender);
+        emit Unpaused(_msgSender());
     }
 }

contracts/math/Math.sol

@@ -1,8 +1,7 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 /**
- * @title Math
- * @dev Assorted math operations
+ * @dev Standard math utilities missing in the Solidity language.
  */
 library Math {
     /**
@@ -20,9 +19,8 @@ library Math {
     }
 
     /**
-     * @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

contracts/math/README.adoc

@@ -0,0 +1,9 @@
+= Math
+
+These are math-related utilities.
+
+== Libraries
+
+{{SafeMath}}
+
+{{Math}}

contracts/math/SafeMath.sol

@@ -1,33 +1,120 @@
-pragma solidity ^0.5.2;
+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.
+     * @dev Returns the addition of two unsigned integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     * - Addition 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-solidity/pull/522
-        if (a == 0) {
-            return 0;
-        }
-
-        uint256 c = a * b;
-        require(c / a == b);
+    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) {
+        return sub(a, b, "SafeMath: subtraction overflow");
+    }
+
+    /**
+     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
+     * overflow (when the result is negative).
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     * - Subtraction cannot overflow.
+     *
+     * _Available since v2.4.0._
+     */
+    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b <= a, errorMessage);
+        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) {
+        return div(a, b, "SafeMath: division by zero");
+    }
+
+    /**
+     * @dev Returns the integer division of two unsigned integers. Reverts with custom message 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.
+     *
+     * _Available since v2.4.0._
+     */
+    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         // Solidity only automatically asserts when dividing by 0
-        require(b > 0);
+        require(b > 0, errorMessage);
         uint256 c = a / b;
         // assert(a == b * c + a % b); // There is no case in which this doesn't hold
 
@@ -35,31 +122,35 @@ 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);
+        return mod(a, b, "SafeMath: modulo by zero");
+    }
+
+    /**
+     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
+     * Reverts with custom message 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.
+     *
+     * _Available since v2.4.0._
+     */
+    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b != 0, errorMessage);
         return a % b;
     }
 }

contracts/mocks/Acknowledger.sol

@@ -1,19 +0,0 @@
-pragma solidity ^0.5.2;
-
-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/AddressImpl.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../utils/Address.sol";
 
@@ -6,4 +6,14 @@ contract AddressImpl {
     function isContract(address account) external view returns (bool) {
         return Address.isContract(account);
     }
+
+    function toPayable(address account) external pure returns (address payable) {
+        return Address.toPayable(account);
+    }
+
+    function sendValue(address payable receiver, uint256 amount) external {
+        Address.sendValue(receiver, amount);
+    }
+
+    function () external payable { } // sendValue's tests require the contract to hold Ether
 }

contracts/mocks/AllowanceCrowdsaleImpl.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/IERC20.sol";
 import "../crowdsale/emission/AllowanceCrowdsale.sol";

contracts/mocks/ArraysImpl.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../utils/Arrays.sol";
 

contracts/mocks/CappedCrowdsaleImpl.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/IERC20.sol";
 import "../crowdsale/validation/CappedCrowdsale.sol";

contracts/mocks/CapperRoleMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../access/roles/CapperRole.sol";
 

contracts/mocks/ConditionalEscrowMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../payment/escrow/ConditionalEscrow.sol";
 

contracts/mocks/ContextMock.sol

@@ -0,0 +1,27 @@
+pragma solidity ^0.5.0;
+
+import "../GSN/Context.sol";
+
+contract ContextMock is Context {
+    event Sender(address sender);
+
+    function msgSender() public {
+        emit Sender(_msgSender());
+    }
+
+    event Data(bytes data, uint256 integerValue, string stringValue);
+
+    function msgData(uint256 integerValue, string memory stringValue) public {
+        emit Data(_msgData(), integerValue, stringValue);
+    }
+}
+
+contract ContextMockCaller {
+    function callSender(ContextMock context) public {
+        context.msgSender();
+    }
+
+    function callData(ContextMock context, uint256 integerValue, string memory stringValue) public {
+        context.msgData(integerValue, stringValue);
+    }
+}

contracts/mocks/CountersImpl.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../drafts/Counters.sol";
 

contracts/mocks/CrowdsaleMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../crowdsale/Crowdsale.sol";
 

contracts/mocks/ECDSAMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../cryptography/ECDSA.sol";
 

contracts/mocks/ERC165/ERC165InterfacesSupported.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../../introspection/IERC165.sol";
 
@@ -13,37 +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/ERC165/ERC165NotSupported.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 contract ERC165NotSupported {
     // solhint-disable-previous-line no-empty-blocks

contracts/mocks/ERC165CheckerMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../introspection/ERC165Checker.sol";
 

contracts/mocks/ERC165Mock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../introspection/ERC165.sol";
 

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/ERC20BurnableMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/ERC20Burnable.sol";
 

contracts/mocks/ERC20DetailedMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/ERC20.sol";
 import "../token/ERC20/ERC20Detailed.sol";

contracts/mocks/ERC20MetadataMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/ERC20.sol";
 import "../drafts/ERC1046/ERC20Metadata.sol";

contracts/mocks/ERC20MintableMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/ERC20Mintable.sol";
 import "./MinterRoleMock.sol";

contracts/mocks/ERC20Mock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/ERC20.sol";
 
@@ -20,6 +20,10 @@ contract ERC20Mock is ERC20 {
         _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/ERC20PausableMock.sol

@@ -1,11 +1,11 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC20/ERC20Pausable.sol";
 import "./PauserRoleMock.sol";
 
 // mock class using ERC20Pausable
 contract ERC20PausableMock is ERC20Pausable, PauserRoleMock {
-    constructor (address initialAccount, uint initialBalance) public {
+    constructor (address initialAccount, uint256 initialBalance) public {
         _mint(initialAccount, initialBalance);
     }
 }

contracts/mocks/ERC20SnapshotMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../drafts/ERC20Snapshot.sol";
 

contracts/mocks/ERC721FullMock.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC721/ERC721Full.sol";
 import "../token/ERC721/ERC721Mintable.sol";

contracts/mocks/ERC721GSNRecipientMock.sol

@@ -0,0 +1,18 @@
+pragma solidity ^0.5.0;
+
+import "../token/ERC721/ERC721.sol";
+import "../GSN/GSNRecipient.sol";
+import "../GSN/GSNRecipientSignature.sol";
+
+/**
+ * @title ERC721GSNRecipientMock
+ * A simple ERC721 mock that has GSN support enabled
+ */
+contract ERC721GSNRecipientMock is ERC721, GSNRecipient, GSNRecipientSignature {
+    constructor(address trustedSigner) public GSNRecipientSignature(trustedSigner) { }
+    // solhint-disable-previous-line no-empty-blocks
+
+    function mint(uint256 tokenId) public {
+        _mint(_msgSender(), tokenId);
+    }
+}

contracts/mocks/ERC721MintableBurnableImpl.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.5.2;
+pragma solidity ^0.5.0;
 
 import "../token/ERC721/ERC721Full.sol";
 import "../token/ERC721/ERC721Mintable.sol";