jberc20 with a check fn

src/JBERC20.sol

@@ -43,6 +43,11 @@ contract JBERC20 is ERC20Votes, ERC20Permit, Ownable, IJBToken {
         return super.balanceOf(account);
     }
 
+    /// @notice This token can only be added to a project when its created by the `JBTokens` contract.
+    function canBeAddedTo(uint256) external pure override returns (bool) {
+        return false;
+    }
+
     /// @notice The number of decimals used for this token's fixed point accounting.
     /// @return The number of decimals.
     function decimals() public view override(ERC20, IJBToken) returns (uint8) {

src/JBTokens.sol

@@ -27,6 +27,7 @@ contract JBTokens is JBControlled, IJBTokens {
     error JBTokens_OverflowAlert(uint256 value, uint256 limit);
     error JBTokens_ProjectAlreadyHasToken(IJBToken token);
     error JBTokens_TokenAlreadyBeingUsed(uint256 projectId);
+    error JBTokens_TokenCantBeAdded(uint256 projectId);
     error JBTokens_TokenNotFound();
     error JBTokens_TokensMustHave18Decimals(uint256 decimals);
 
@@ -335,6 +336,9 @@ contract JBTokens is JBControlled, IJBTokens {
         // Can't change to a token that doesn't use 18 decimals.
         if (token.decimals() != 18) revert JBTokens_TokensMustHave18Decimals(token.decimals());
 
+        // Make sure the token can be added to the project its being added to.
+        if (!token.canBeAddedTo(projectId)) revert JBTokens_TokenCantBeAdded(projectId);
+
         // Store the new token.
         tokenOf[projectId] = token;
 

src/interfaces/IJBToken.sol

@@ -3,6 +3,7 @@ pragma solidity ^0.8.0;
 
 interface IJBToken {
     function balanceOf(address account) external view returns (uint256);
+    function canBeAddedTo(uint256 projectId) external view returns (bool);
     function decimals() external view returns (uint8);
     function totalSupply() external view returns (uint256);
 

test/TestPermissions.sol

@@ -192,6 +192,7 @@ contract TestPermissions_Local is TestBaseWorkflow, JBTest {
 
         // Will succeed when setting the correct projects token
         mockExpect(token, abi.encodeCall(MockERC20.decimals, ()), abi.encode(18));
+        mockExpect(token, abi.encodeCall(IJBToken.canBeAddedTo, (1)), abi.encode(true));
         _controller.setTokenFor(1, IJBToken(token));
     }
 

test/TestTokenFlow.sol

@@ -85,6 +85,11 @@ contract TestTokenFlow_Local is TestBaseWorkflow {
             IJBToken _newToken = IJBToken(Clones.clone(address(new JBERC20())));
             _newToken.initialize({name: "NewTestName", symbol: "NewTestSymbol", owner: address(_tokens)});
 
+            // Mock the token can be added to the project.
+            vm.mockCall(
+                address(_newToken), abi.encodeWithSelector(IJBToken.canBeAddedTo.selector, _projectId), abi.encode(true)
+            );
+
             // Set the projects token to `_newToken`.
             _controller.setTokenFor(_projectId, _newToken);
 

test/units/static/JBTokens/TestSetTokenFor.sol

@@ -73,10 +73,23 @@ contract TestSetTokenFor_Local is JBTokensSetup {
 
         //mock call to token decimals
         mockExpect(address(_token), abi.encodeCall(IJBToken.decimals, ()), abi.encode(18));
+        mockExpect(address(_token), abi.encodeCall(IJBToken.canBeAddedTo, (_projectId)), abi.encode(true));
 
         vm.expectEmit();
         emit IJBTokens.SetToken(_projectId, _token, address(this));
 
         _tokens.setTokenFor(_projectId, IJBToken(address(_token)));
     }
+
+    function test_WhenCantBeAddedTo() external whenCallerIsControllerOfProject {
+        // it will set token states and emit SetToken
+
+        //mock call to token decimals
+        mockExpect(address(_token), abi.encodeCall(IJBToken.decimals, ()), abi.encode(18));
+        mockExpect(address(_token), abi.encodeCall(IJBToken.canBeAddedTo, (_projectId)), abi.encode(false));
+
+        vm.expectRevert(abi.encodeWithSelector(JBTokens.JBTokens_TokenCantBeAdded.selector, _projectId));
+
+        _tokens.setTokenFor(_projectId, IJBToken(address(_token)));
+    }
 }