What type is "MyInterface.MyCustomError" in Solidity?

Question

I'm toying with the recently introduced custom errors. Here's a mini replica of my set up:

interface MyInterface {
    error MyCustomError();
}

I'm writing my tests in Solidity and I'd like to use the expectRevert cheatcode (part of HEVM) to test that my implementation contract reverts with MyCustomError. Here's what I came up with so far:

vm.expectRevert(abi.encodeWithSignature("MyCustomError()"));

This works since the expectRevert cheatcode is implemented in three different ways:

• expectRevert()
• expectRevert(bytes calldata)
• expectRevert(bytes4)

But using abi.encodeWithSignature feels like going in a roundabout way. I tried to reference the custom error directly:

vm.expectRevert(MyInterface.MyCustomError);

But this doesn't compile, so it follows that MyCustomError is neither bytes nor bytes4 when referenced like this. That's fine. But then, what type is it, and could I cast it to either bytes or bytes4?

Answer 1

I eventually figured out half of the answer on my own by trying to force cast the type to bytes4. It seems like MyInterface.MyCustomError is of type function() pure:

Explicit type conversion not allowed from "function () pure" to "bytes4".

Note that I compile my code with Solidity v0.8.13+commit.abaa5c0e.

The other half was elucidated by @hroussille's tip to use the .selector member to access the bytes4 signature of the MyCustomError custom error. The following works:

vm.expectRevert(MyInterface.MyCustomError.selector);

Side note: I ended making a suggestion in the Solidity forum to add a bespoke type for custom errors, i.e. something like error which would be an alias for bytes24 (just like function).

Update: it looks like this is actually a bug in Solidity. Casting MyInterface.MyCustomError to bytes4 should work. I opened an issue for this.

Answer 2

Actually MyInterface.MyCustomError.selector will return bytes4 but expectRevert expects bytes, so the above would throw an error. But that can be easily fixed using

vm.expectRevert(abi.encodePacked(MyInterface.MyCustomError.selector));