Implement SBT signaling standard (#6)

* Implement EIP-5192

* Add tests

* Update docs

contracts/BasicGuildRewardNFT.sol

@@ -67,6 +67,8 @@ contract BasicGuildRewardNFT is
 
         _safeMint(receiver, tokenId);
 
+        emit Locked(tokenId);
+
         emit Claimed(receiver, tokenId);
     }
 

contracts/interfaces/IBasicGuildRewardNFT.sol

@@ -76,8 +76,4 @@ interface IBasicGuildRewardNFT {
 
     /// @notice Error thrown when the supplied signature is invalid.
     error IncorrectSignature();
-
-    /// @notice Error thrown when trying to query info about a token that's not (yet) minted.
-    /// @param tokenId The queried id.
-    error NonExistentToken(uint256 tokenId);
 }

contracts/interfaces/IERC5192.sol

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: CC0-1.0
+pragma solidity ^0.8.0;
+
+interface IERC5192 {
+    /// @notice Emitted when the locking status is changed to locked.
+    /// @dev If a token is minted and the status is locked, this event should be emitted.
+    /// @param tokenId The identifier for a token.
+    event Locked(uint256 tokenId);
+
+    /// @notice Emitted when the locking status is changed to unlocked.
+    /// @dev If a token is minted and the status is unlocked, this event should be emitted.
+    /// @param tokenId The identifier for a token.
+    event Unlocked(uint256 tokenId);
+
+    /// @notice Returns the locking status of an Soulbound Token
+    /// @dev SBTs assigned to zero address are considered invalid, and queries
+    /// about them do throw.
+    /// @param tokenId The identifier for an SBT.
+    function locked(uint256 tokenId) external view returns (bool);
+}

contracts/token/SoulboundERC721.sol

@@ -3,6 +3,7 @@ pragma solidity 0.8.19;
 
 /* solhint-disable max-line-length */
 
+import { IERC5192 } from "../interfaces/IERC5192.sol";
 import { ERC721Upgradeable } from "@openzeppelin/contracts-upgradeable/token/ERC721/ERC721Upgradeable.sol";
 import { IERC721Upgradeable } from "@openzeppelin/contracts-upgradeable/token/ERC721/IERC721Upgradeable.sol";
 import { ERC721EnumerableUpgradeable } from "@openzeppelin/contracts-upgradeable/token/ERC721/extensions/ERC721EnumerableUpgradeable.sol";
@@ -14,7 +15,11 @@ import { IERC721EnumerableUpgradeable } from "@openzeppelin/contracts-upgradeabl
 /// @notice Allowance and transfer-related functions are disabled.
 /// @dev Inheriting from upgradeable contracts here - even though we're using it in a non-upgradeable way,
 /// we still want it to be initializable
-contract SoulboundERC721 is ERC721Upgradeable, ERC721EnumerableUpgradeable {
+contract SoulboundERC721 is ERC721Upgradeable, ERC721EnumerableUpgradeable, IERC5192 {
+    /// @notice Error thrown when trying to query info about a token that's not (yet) minted.
+    /// @param tokenId The queried id.
+    error NonExistentToken(uint256 tokenId);
+
     /// @notice Error thrown when a function's execution is not possible, because this is a soulbound NFT.
     error Soulbound();
 
@@ -28,7 +33,15 @@ contract SoulboundERC721 is ERC721Upgradeable, ERC721EnumerableUpgradeable {
     function supportsInterface(
         bytes4 interfaceId
     ) public view virtual override(ERC721EnumerableUpgradeable, ERC721Upgradeable) returns (bool) {
-        return interfaceId == type(IERC721EnumerableUpgradeable).interfaceId || super.supportsInterface(interfaceId);
+        return
+            interfaceId == type(IERC5192).interfaceId ||
+            interfaceId == type(IERC721EnumerableUpgradeable).interfaceId ||
+            super.supportsInterface(interfaceId);
+    }
+
+    function locked(uint256 tokenId) external view returns (bool) {
+        if (!_exists(tokenId)) revert NonExistentToken(tokenId);
+        return true;
     }
 
     function approve(

docs/contracts/interfaces/IBasicGuildRewardNFT.md

@@ -220,17 +220,3 @@ error IncorrectSignature()
 
 Error thrown when the supplied signature is invalid.
 
-### NonExistentToken
-
-```solidity
-error NonExistentToken(uint256 tokenId)
-```
-
-Error thrown when trying to query info about a token that's not (yet) minted.
-
-#### Parameters
-
-| Name | Type | Description |
-| ---- | ---- | ----------- |
-| tokenId | uint256 | The queried id. |
-

docs/contracts/interfaces/IERC5192.md

@@ -0,0 +1,60 @@
+# IERC5192
+
+## Functions
+
+### locked
+
+```solidity
+function locked(
+    uint256 tokenId
+) external returns (bool)
+```
+
+Returns the locking status of an Soulbound Token
+
+SBTs assigned to zero address are considered invalid, and queries
+about them do throw.
+
+#### Parameters
+
+| Name | Type | Description |
+| :--- | :--- | :---------- |
+| `tokenId` | uint256 | The identifier for an SBT. |
+
+## Events
+
+### Locked
+
+```solidity
+event Locked(
+    uint256 tokenId
+)
+```
+
+Emitted when the locking status is changed to locked.
+
+If a token is minted and the status is locked, this event should be emitted.
+
+#### Parameters
+
+| Name | Type | Description |
+| :--- | :--- | :---------- |
+| `tokenId` | uint256 | The identifier for a token. |
+### Unlocked
+
+```solidity
+event Unlocked(
+    uint256 tokenId
+)
+```
+
+Emitted when the locking status is changed to unlocked.
+
+If a token is minted and the status is unlocked, this event should be emitted.
+
+#### Parameters
+
+| Name | Type | Description |
+| :--- | :--- | :---------- |
+| `tokenId` | uint256 | The identifier for a token. |
+

docs/contracts/token/SoulboundERC721.md

@@ -41,6 +41,25 @@ See {IERC165-supportsInterface}.
 | :--- | :--- | :---------- |
 | `interfaceId` | bytes4 |  |
 
+### locked
+
+```solidity
+function locked(
+    uint256 tokenId
+) external returns (bool)
+```
+
+Returns the locking status of an Soulbound Token
+
+SBTs assigned to zero address are considered invalid, and queries
+about them do throw.
+
+#### Parameters
+
+| Name | Type | Description |
+| :--- | :--- | :---------- |
+| `tokenId` | uint256 | The identifier for an SBT. |
+
 ### approve
 
 ```solidity
@@ -169,6 +188,20 @@ Still used for minting/burning.
 
 ## Custom errors
 
+### NonExistentToken
+
+```solidity
+error NonExistentToken(uint256 tokenId)
+```
+
+Error thrown when trying to query info about a token that's not (yet) minted.
+
+#### Parameters
+
+| Name | Type | Description |
+| ---- | ---- | ----------- |
+| tokenId | uint256 | The queried id. |
+
 ### Soulbound
 
 ```solidity

test/BasicGuildRewardNFT.spec.ts

@@ -201,6 +201,13 @@ describe("BasicGuildRewardNFT", () => {
         expect(await nft.ownerOf(tokenId)).to.eq(wallet0.address);
       });
 
+      it("should emit Locked event when minting", async () => {
+        const tokenId = await nft.totalSupply();
+        await expect(nft.claim(wallet0.address, sampleUserId, sampleSignature, { value: fee + price }))
+          .to.emit(nft, "Locked")
+          .withArgs(tokenId);
+      });
+
       it("should emit Claimed event", async () => {
         await expect(
           nft.claim(wallet0.address, sampleUserId, sampleSignature, {

test/SoulboundERC721.spec.ts

@@ -53,4 +53,11 @@ describe("SoulboundERC721", () => {
       nft["safeTransferFrom(address,address,uint256,bytes)"](wallet0.address, randomWallet.address, 0, ethers.ZeroHash)
     ).to.be.revertedWithCustomError(BasicGuildRewardNFT, "Soulbound");
   });
+
+  it("should have a locked function that throws for not minted tokens", async () => {
+    const tokenId = 1;
+    await expect(nft.locked(tokenId))
+      .to.be.revertedWithCustomError(BasicGuildRewardNFT, "NonExistentToken")
+      .withArgs(tokenId);
+  });
 });