feat: Handle native ETH outputs

- In this case, the ETH needs to be manually transferred to the reactor.
- Native ETH input is not supported for UniswapX

Source:
https://support.uniswap.org/hc/en-us/articles/17544708791821-Are-there-network-costs-for-UniswapX

foundry/src/uniswap_x/UniswapXFiller.sol

@@ -20,6 +20,7 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
     // UniswapX V2DutchOrder Reactor
     IReactor public immutable reactor;
     address public immutable tychoRouter;
+    address public immutable nativeAddress;
 
     // keccak256("NAME_OF_ROLE") : save gas on deployment
     bytes32 public constant REACTOR_ROLE =
@@ -31,7 +32,11 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
         address indexed token, uint256 amount, address indexed receiver
     );
 
-    constructor(address _tychoRouter, address _reactor) {
+    constructor(
+        address _tychoRouter,
+        address _reactor,
+        address _native_address
+    ) {
         if (_tychoRouter == address(0)) revert UniswapXFiller__AddressZero();
         if (_reactor == address(0)) revert UniswapXFiller__AddressZero();
 
@@ -39,6 +44,7 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
         _grantRole(REACTOR_ROLE, address(_reactor));
         tychoRouter = _tychoRouter;
         reactor = IReactor(_reactor);
+        nativeAddress = _native_address;
     }
 
     function execute(SignedOrder calldata order, bytes calldata callbackData)
@@ -67,26 +73,14 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
 
         // The TychoRouter will take the input tokens from the filler
         if (tokenInApprovalNeeded) {
-            // TODO only if ERC20
+            // Native ETH input is not supported by UniswapX
             IERC20(order.input.token).forceApprove(
                 tychoRouter, order.input.maxAmount
             );
-        } else {
-            // The filler will manually transfer into the TychoRouter
-            // Note: We are using the balance of our filler instead of the order
-            // input amount to avoid having to do a decay calculation in the filler.
-            IERC20 inputToken = IERC20(order.input.token);
-            inputToken.transfer(
-                tychoRouter, inputToken.balanceOf(address(this))
-            );
         }
 
-        // TODO properly handle native in and out tokens
-        uint256 ethValue = 0;
-
         // slither-disable-next-line low-level-calls
-        (bool success, bytes memory result) =
+        (bool success, bytes memory result) = tychoRouter.call(tychoCalldata);
-            tychoRouter.call{value: ethValue}(tychoCalldata);
 
         if (!success) {
             revert(
@@ -101,9 +95,15 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
         if (tokenOutApprovalNeeded) {
             // Multiple outputs are possible when taking fees - but token itself should
             // not change.
-            // TODO only if ERC20
+            OutputToken memory output = order.outputs[0];
-            IERC20 token = IERC20(order.outputs[0].token);
+            if (output.token != nativeAddress) {
+                IERC20 token = IERC20(output.token);
                 token.forceApprove(address(reactor), type(uint256).max);
+            } else {
+                // With native ETH - the filler is responsible for transferring back
+                // to the reactor.
+                Address.sendValue(payable(address(reactor)), output.amount);
+            }
         }
     }
 

foundry/test/uniswap_x/UniswapXFiller.t.sol

@@ -23,7 +23,7 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
 
     function fillerSetup() public {
         vm.startPrank(ADMIN);
-        filler = new UniswapXFiller(tychoRouterAddr, REACTOR);
+        filler = new UniswapXFiller(tychoRouterAddr, REACTOR, address(0));
         fillerAddr = address(filler);
         filler.grantRole(keccak256("EXECUTOR_ROLE"), EXECUTOR);
         vm.stopPrank();
@@ -31,12 +31,12 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
 
     function testTychoAddressZeroTychoRouter() public {
         vm.expectRevert(UniswapXFiller__AddressZero.selector);
-        filler = new UniswapXFiller(address(0), REACTOR);
+        filler = new UniswapXFiller(address(0), REACTOR, address(0));
     }
 
     function testTychoAddressZeroReactor() public {
         vm.expectRevert(UniswapXFiller__AddressZero.selector);
-        filler = new UniswapXFiller(tychoRouterAddr, address(0));
+        filler = new UniswapXFiller(tychoRouterAddr, address(0), address(0));
     }
 
     function testCallback() public {