unexpected revert behaviour using custom errors in pure function

Question

in the code below, if a revert using the custom error inside the pure function it reverts with a different error (below). If I do the same things without pure, it reverts with the custom error as expected

After @hroussille comment I tried it in remix but couldn't recreate the issue, so it seems I'm having an issue with hardhat.

---

test contract

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.10;

contract RevertProblem  {
    error aCustomError(string errorText);

    function test1() public pure {
        revert ("BadHappened");
    }

    function test2() public {
        revert aCustomError("BadHappened");
    }

    function test3() public pure {
        revert aCustomError("BadHappened");
    }
}

---

test script

const { expect } = require("chai");
const { ethers } = require("hardhat");

describe("Should revert with expected message", async function() {
 

    it("not so pure", async function () {
        const rpContractFactory = await ethers.getContractFactory('RevertProblem')
        const rpContract = await rpContractFactory.deploy()
        await rpContract.deployed()

        await expect(rpContract.test1()).to.be.revertedWith("BadHappened")
        await expect(rpContract.test2()).to.be.revertedWith('aCustomError("BadHappened")')
        await expect(rpContract.test3()).to.be.revertedWith('aCustomError("BadHappened")')
    })
});

the first two functions are fine. the third one reverts with

 AssertionError: Expected transaction to be reverted with aCustomError("BadHappened"), but other exception was thrown: Error: call revert exception [ See: https://links.ethers.org/v5-errors-CALL_EXCEPTION ] (method="test3()", data="0x062089cf0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000b42616448617070656e6564000000000000000000000000000000000000000000", errorArgs=["BadHappened"], errorName="aCustomError", errorSignature="aCustomError(string)", reason=null, code=CALL_EXCEPTION, version=abi/5.6.3)

It should revert with something like

Error: VM Exception while processing transaction: reverted with custom error 'aCustomError("BadHappened")'

If I take the pure keyword out of my non-test code the function reverts correctly, but even though it compiles OK (just get a warning about should use pure), the non-test function which calculates and returns a number went crazy haywire.

├── @nomiclabs/[email protected]
├── @nomiclabs/[email protected]
├── @openzeppelin/[email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
└── [email protected]

Answer 1

That is due to how Ethers dynamically handles transactions.

If you need to call a method that does not change the state of a smart contract (i.e. test1 and test3), Ethers uses a call. Calls are used to read the smart contract's state, and they follow a specific execution path that leads to particular results or errors (https://docs.ethers.io/v5/api/utils/logger/#errors--call-exception).

In particular, the tx is not sent to the blockchain - it's a read, it's not needed - so the execution result is returned directly to the client. In your case, test1 and test3 both return: Error: call revert exception; ...

When calling a method that can potentially change the blockchain's state, a tx is actually sent to the blockchain, it is mined, and then the result is read from the tx's receipt. There you can find the custom error.

And this is why only test2 reverts with Error: VM Exception while processing transaction: reverted with custom error 'aCustomError("BadHappened")'

If you want the 3rd test to pass, you can use:

await expect(rpContract.test3()).to.be.revertedWith("BadHappened")

Please note that to.be.revertedWith(myString) searches for myString inside the error, so all these check pass:

await expect(rpContract.test3()).to.be.revertedWith("Happened")
await expect(rpContract.test3()).to.be.revertedWith("Bad")
await expect(rpContract.test3()).to.be.revertedWith("B")