Error: Transaction reverted: function call to a non-contract account when adding liquidity through UniswapV2Router

Question

I've been testing around UniswapV2 using hardhat and I always get this error when I try to add liquidity through the addLiquidity function on UniswapV2Router.

For the test, I deployed mock tokens, deployed UniswapV2Factory, then deployed Router.

here's the deployment code

const token = await ethers.getContractFactory("Token");
weth = await token.deploy('WETH', 'WETH', 6, owner.address);
mim = await token.deploy('MIM', 'MIM', 18, owner.address);
dai = await token.deploy('DAI', 'DAI', 18, owner.address);

const Factory = await ethers.getContractFactory("UniswapV2Factory");
const factory = await Factory.deploy(owner.address);

const Router = await ethers.getContractFactory("UniswapV2Router02");
const router = await Router.deploy(factory.address, weth.address);

const mim_100000000 = ethers.BigNumber.from("100000000000000000000000000");
const dai_100000000 = ethers.BigNumber.from("100000000000000000000000000");
await dai.approve(router.address, dai_100000000);
await mim.approve(router.address, mim_100000000);

await router.addLiquidity(mim.address, dai.address, mim_100000000, dai_100000000, mim_100000000, dai_100000000, owner.address, Date.now());

and it always produces this error:

Error: Transaction reverted: function call to a non-contract account
    at UniswapV2Router02.getReserves (contracts/libraries/UniswapV2Library.sol:31)
    at UniswapV2Router02._addLiquidity (contracts/UniswapV2Router02.sol:45)
    at UniswapV2Router02.addLiquidity (contracts/UniswapV2Router02.sol:71)
    at HardhatNode._mineBlockWithPendingTxs 

Can someone help please?

Answer 1

The error is getting triggered in line 45 of the router contract https://github.com/Uniswap/v2-periphery/blob/master/contracts/UniswapV2Router02.sol#L45

It means that UniswapV2Library is not a correct contract reference inside the Smart Contract, so calling getReserves fails.

Looking at the code, you are not deploying and linking the library properly, so when it tries to call the function, it tries to call the function from address 0x0, giving that error.

You can find tons of docs on how to use libraries, I suggest this one to get started: https://www.tutorialspoint.com/solidity/solidity_libraries.htm

Although if what you want is just to make it work hardhat has tools to deploy and link libraries: https://hardhat.org/plugins/nomiclabs-hardhat-ethers#library-linking

Best of luck!

Answer 2

There is an issue with createPair UniswapV2Library function. If UniswapV2Pair is changed, it will return different pair address, different hex

First solution:

The purpose of this function is to get the contract address for 2 tokens without calling any contract (pure function). The problem is the hex value init in code hash, this hash is just a keccak256() of your compiled contract UniswapV2Pair.sol

How to solve your problem ?

Very easy. Just find your bytecode from compiled contract, hash it and replace on the contract.

For Hardhat users:

1. Find UniswapV2Pair.json in the artifacts folder
2. Find the bytecode in the file
3. Copy it on Keccak Online (Don't forget to change the input type to "hex" and delete the "0x" at the start)
4. Hash it and copy the hash in the pairFor function

Second solution:

Change this function to this and it will work:

import '@uniswap/v2-core/contracts/interfaces/IUniswapV2Factory.sol';
    function pairFor(address factory, address tokenA, address tokenB) internal view returns (address pair) {
        pair = IUniswapV2Factory(factory).getPair(tokenA,tokenB);
    }

Read more here: Testing Token with Uniswap liquidity provisioning using hardhat https://forum.openzeppelin.com/t/uniswap-fork-testing-hardhat/14472