Unexpected overflow error while using nargo prove.

[TAdev0]

Aim

While working on a small Noir program, I ran into an issue related to overflow/underflow detection using nargo prove command. This bug doesn't concern compilation, as nargo compile worked well and compiled my program.

Let's consider the following code :

 if (firstX > secondX) {
            diffX = firstX - secondX;
        }

with firstX and secondX being 2 u32. The if statement makes sure firstX is greater than secondX, making sure the substraction firstX - secondX will never encounter underflow issue.

Neverthless, when using nargo prove to generate a proof of my program with valid inputs, i get this error:

error: Assertion failed: 'attempt to subtract with overflow'

This seems odd as there is no situation in which there is an overflow (moreover, subtraction could theoretically only lead to underflow with uint values).

Expected Behavior

The short snippet of code :

 if (firstX > secondX) {
            diffX = firstX - secondX;
        }

should work without additional treatment of firstX and secondX.

Bug

In order to make nargo prove working, the snippet of code needs to be modified as follows:

        if (firstX > secondX) {
            diffX = std::wrapping_sub(firstX, secondX);
        }

std::wrapping_sub is not mentioned in Noir Documentation . I guess this is supposed to compute a modular subtraction, just like in Rust.
Nonetheless, wrapping firstX and secondX shouldn't be necessary, as diffX = firstX - secondX; will never require modular arithmetic in order to be computed successfully.

Installation Method

Noirup

Nargo Version

nargo 0.19.2

Would you like to submit a PR for this Issue?

Yes

[TAdev0]

@jfecher @kevaundray it seems this issue has already been reported, could you confirm this? Thank you.

[akonior]

Duplicate of issue #3493 and should already be fixed by PR #3494. Will be fixed in release 0.19.3.

[TomAFrench]

Yep, this bug should already be fixed in 0.19.3. Updating to this release will fix the issue.

Thanks for your help @akonior :)