fix(router-sdk): fix MixedRoute tokenpath (#287)

## PR Scope

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## Description

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## How Has This Been Tested?

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## Are there any breaking changes?

_[e.g. Type definitions, API definitions]_

If there are breaking changes, please ensure you bump the major version Bump the major version (by using the title `feat(breaking): ...`), post a notice in #eng-sdks, and explicitly notify all Uniswap Labs consumers of the SDK.

## (Optional) Feedback Focus

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## (Optional) Follow Ups

_[Things that weren't addressed in this PR, ways you plan to build on this work, or other ways this work could be extended]_

sdks/router-sdk/src/entities/mixedRoute/route.test.ts

@@ -20,6 +20,7 @@ describe('MixedRoute', () => {
   const pool_v4_0_weth = new V4Pool(token0, weth, FeeAmount.MEDIUM, 60, ADDRESS_ZERO, SQRT_RATIO_ONE, 0, 0, [])
   const pool_v4_1_eth = new V4Pool(token1, ETHER, FeeAmount.MEDIUM, 60, ADDRESS_ZERO, SQRT_RATIO_ONE, 0, 0, [])
   const pool_v4_0_1 = new V4Pool(token0, token1, FeeAmount.MEDIUM, 60, ADDRESS_ZERO, SQRT_RATIO_ONE, 0, 0, [])
+  const pool_v4_weth_eth = new V4Pool(weth, ETHER, 0, 0, ADDRESS_ZERO, SQRT_RATIO_ONE, 0, 0)
 
   const pool_v3_0_1 = new V3Pool(token0, token1, FeeAmount.MEDIUM, encodeSqrtRatioX96(1, 1), 0, 0, [])
   const pool_v3_0_weth = new V3Pool(token0, weth, FeeAmount.MEDIUM, encodeSqrtRatioX96(1, 1), 0, 0, [])
@@ -34,6 +35,11 @@ describe('MixedRoute', () => {
   const pair_2_3 = new Pair(CurrencyAmount.fromRawAmount(token2, '100'), CurrencyAmount.fromRawAmount(token3, '200'))
 
   describe('path', () => {
+    it('real v3 weth pool and fake v4 eth/weth pool', () => {
+      const route = new MixedRouteSDK([pool_v3_0_weth, pool_v4_weth_eth], token0, ETHER)
+      expect(route.path).toEqual([token0, weth, ETHER])
+    })
+
     it('wraps pure v3 route object and successfully constructs a path from the tokens', () => {
       /// @dev since the MixedRoute sdk object lives here in router-sdk we don't need to wrap it
       const routeOriginal = new MixedRouteSDK([pool_v3_0_1], token0, token1)
@@ -84,9 +90,9 @@ describe('MixedRoute', () => {
     })
 
     it('wraps mixed route object with mixed v4 route that converts WETH -> ETH ', () => {
-      const route = new MixedRouteSDK([pool_v3_0_weth, pool_v4_1_eth], token0, token1)
-      expect(route.pools).toEqual([pool_v3_0_weth, pool_v4_1_eth])
-      expect(route.path).toEqual([token0, weth, token1])
+      const route = new MixedRouteSDK([pool_v3_0_weth, pool_v4_weth_eth, pool_v4_1_eth], token0, token1)
+      expect(route.pools).toEqual([pool_v3_0_weth, pool_v4_weth_eth, pool_v4_1_eth])
+      expect(route.path).toEqual([token0, weth, ETHER, token1])
       expect(route.input).toEqual(token0)
       expect(route.output).toEqual(token1)
       expect(route.pathInput).toEqual(token0)

sdks/router-sdk/src/entities/mixedRoute/route.ts

@@ -54,9 +54,17 @@ export class MixedRouteSDK<TInput extends Currency, TOutput extends Currency> {
       const prevPool = pools[i - 1]
       const pool = pools[i]
       const inputToken = tokenPath[i]
-      const outputToken = pool.token0.wrapped.equals(inputToken.wrapped) ? pool.token1 : pool.token0
 
-      invariant(isValidTokenPath(prevPool, pool, inputToken), 'PATH')
+      const outputToken =
+        pool instanceof V4Pool
+          ? pool.token0.equals(inputToken)
+            ? pool.token1
+            : pool.token0
+          : pool.token0.wrapped.equals(inputToken.wrapped)
+          ? pool.token1
+          : pool.token0
+
+      invariant(isValidTokenPath(prevPool, pool, inputToken), `PATH`)
       tokenPath.push(outputToken)
     }