assert_max_bit_size() causes panic

[calummoore]

Aim

Verify that the the sum of two fields is less than the bit size, to prevent overflow attack.

Expected Behavior

Constraint is applied.

Bug

The application panicked (crashed).
Message: Non-numeric type variable used in expression expecting a value
Location: compiler/noirc_frontend/src/monomorphization/mod.rs:934

I also note in the docs that this field takes a parameter (field.assert_max_bit_size(32);), but it doesn't seem to in the Noir validation. I get an error if I try to use a parameter.

error: Function expects 1 parameter but 2 were given
   ┌─ src/main.nr:15:5
   │
15 │     output_value.assert_max_bit_size(32);
   │     ------------------------------------
   │

To Reproduce

#[recursive]
fn main(field1: Field, field2: Field) {
    let output_value = field1 + field2;

    // Contrain the output value
    output_value.assert_max_bit_size();
}

#[test]
fn test_main() {
    main(1, 2)
}

Workaround

None

Workaround Description

No response

Additional Context

No response

Project Impact

Blocker

Blocker Context

No response

Nargo Version

nargo version = 0.36.0 noirc version = 0.36.0+801c71880ecf8386a26737a5d8bb5b4cb164b2ab (git version hash: 801c718, is dirty: false)

NoirJS Version

n/a

Proving Backend Tooling & Version

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response

[TomAFrench]

Hey, thanks for opening this. Looks like the example in the docs is stale so I've opened #6840 to address this.

Due to *reasons*, we've moved the argument of this function into a numeric generic so to constrain output_value to fit in 32 bits you would need to modify your example as below.

#[recursive]
fn main(field1: Field, field2: Field) {
    let output_value = field1 + field2;

    // Contrain the output value
    output_value.assert_max_bit_size::<32>(); // See turbofish usage.
}

#[test]
fn test_main() {
    main(1, 2)
}

[calummoore]

Ok, sounds good, it worked with specifying a value using the turbofish syntax.

Should it also work with output_value.assert_max_bit_size(); - i.e. no parameter? Currently that triggers an error.

[TomAFrench]

There's no way for the compiler to know how many bits you want to constrain the field to if you don't use the turbofish syntax.

[calummoore]

Ah okay, I thought it would be the max Field size looking at the code.