# Windows can't compile when using GSNReceipient

**URL:** <https://forum.openzeppelin.com/t/windows-cant-compile-when-using-gsnreceipient/2886>\
**Category:** Contracts\
**Tags:** erc20\
**Created:** [May 18, 2020, 9:28pm UTC](https://forum.openzeppelin.com/t/windows-cant-compile-when-using-gsnreceipient/2886 "2020-05-18T21:28:17Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![abcoathup](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/abcoathup/32/415_2.png) [@abcoathup](https://forum.openzeppelin.com/u/abcoathup)\
**Post date:** [May 19, 2020, 1:20am UTC](https://forum.openzeppelin.com/t/windows-cant-compile-when-using-gsnreceipient/2886/2 "2020-05-19T01:20:03Z")

</div>

Hi @medma05,

Welcome to the community 👋

I would recommend using WSL, I use WSL2.

I got the following errors when compiling (ignoring the [SPDX warnings](https://forum.openzeppelin.com/t/solidity-0-6-8-introduces-spdx-license-identifiers-warning-spdx-license-identifier-not-provided-in-source-file/2859))

```auto
$ npx oz compile
✖ Compiling contracts with solc 0.6.8 (commit.0bbfe453)
Compilation warnings:
@openzeppelin/contracts/GSN/Context.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/GSN/GSNRecipient.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/GSN/IRelayHub.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/GSN/IRelayRecipient.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/math/SafeMath.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/token/ERC20/ERC20.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/token/ERC20/IERC20.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

@openzeppelin/contracts/utils/Address.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

contracts/MyToken.sol: Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.

Compilation errors:
contracts/MyToken.sol:7:1: TypeError: Derived contract must override function "_msgData". Two or more base classes define function with same name and parameter types.
contract MyToken is ERC20, GSNRecipient {
^ (Relevant source part starts here and spans across multiple lines).
@openzeppelin/contracts/GSN/Context.sol:22:5: Definition in "Context":
    function _msgData() internal view virtual returns (bytes memory) {
    ^ (Relevant source part starts here and spans across multiple lines).
@openzeppelin/contracts/GSN/GSNRecipient.sol:102:5: Definition in "GSNRecipient":
    function _msgData() internal view virtual override returns (bytes memory) {
    ^ (Relevant source part starts here and spans across multiple lines).

contracts/MyToken.sol:7:1: TypeError: Derived contract must override function "_msgSender". Two or more base classes define function with same name and parameter types.
contract MyToken is ERC20, GSNRecipient {
^ (Relevant source part starts here and spans across multiple lines).
@openzeppelin/contracts/GSN/Context.sol:18:5: Definition in "Context":
    function _msgSender() internal view virtual returns (address payable) {
    ^ (Relevant source part starts here and spans across multiple lines).
@openzeppelin/contracts/GSN/GSNRecipient.sol:88:5: Definition in "GSNRecipient":
    function _msgSender() internal view virtual override returns (address payable) {
    ^ (Relevant source part starts here and spans across multiple lines).

```

The OpenZeppelin Contracts v3.0 [release notes](https://github.com/OpenZeppelin/openzeppelin-contracts/releases/tag/v3.0.0) has the following note:

> The other exception to this are contracts that use the [**Gas Station Network (GSN)**](https://docs.openzeppelin.com/contracts/3.x/gsn): if you're inheriting from `GSNRecipient` or one of the other GSN contracts, you'll need to add the following snippet to your contracts:
> 
> ```auto
> function _msgSender() internal view override(Context, GSNRecipient) returns (address payable) {
> return GSNRecipient._msgSender();
> }
> 
> function _msgData() internal view override(Context, GSNRecipient) returns (bytes memory) {
> return GSNRecipient._msgData();
> } 
> 
> ```

Updating the contract to include these overrides and add the override keyword where necessary and I was able to compile:

```auto
pragma solidity ^0.6.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/GSN/GSNRecipient.sol";

contract MyToken is ERC20, GSNRecipient {
    constructor() public ERC20("My Token", "TKN") {
        uint256 initialSupply = 1000 * (10**18);
        _mint(msg.sender, initialSupply);
    }

    function mint(address account, uint256 amount) public {
        _mint(account, amount);
    }

    function acceptRelayedCall(
        address relay,
        address from,
        bytes calldata encodedFunction,
        uint256 transactionFee,
        uint256 gasPrice,
        uint256 gasLimit,
        uint256 nonce,
        bytes calldata approvalData,
        uint256 maxPossibleCharge
    ) external view override returns (uint256, bytes memory) {
        return _approveRelayedCall();
    }

    // We won't do any pre or post processing, so leave _preRelayedCall and _postRelayedCall empty
    function _preRelayedCall(bytes memory context) internal override returns (bytes32) {}

    function _postRelayedCall(
        bytes memory context,
        bool,
        uint256 actualCharge,
        bytes32
    ) internal override {}

    function _msgSender() internal view override(Context, GSNRecipient) returns (address payable) {
        return GSNRecipient._msgSender();
    }

    function _msgData() internal view override(Context, GSNRecipient) returns (bytes memory) {
        return GSNRecipient._msgData();
    }
}

```

* * *

One other point, depending on your development roadmap, GSNv2 is under development at [https://www.opengsn.org/](https://www.opengsn.org/). You may want to look at the upcoming improvements. You can still use GSNv1, though you may want to run your own relayer. See [Doubling down on security](https://forum.openzeppelin.com/t/doubling-down-on-security/2712) for more details.

---

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