Negative integer literals are not supported for unsigned types

[Jurarpereurs]

Aim

Noir throws an error saying that "u32 cannot be used in a unary operation" when directly assigning a negative integer literal to an unsigned integer type.

let a: u32 = -1;

However, if you do some operations on the negative integer literal, the error is not thrown.

let a: u32 = -1 - 1;

Expected Behavior

Noir should not throw an error when directly assigning a negative integer literal to an unsigned integer type.

Bug

Create a new Noir project with the following command:

noir new neg

Save the following code into src/main.noir:

fn main() {
    let a: u32 = -1;
}

Run nargo prove and observe the following error:

warning: unused variable a
  ┌─ /home/Jurarpereurs/neg/src/main.nr:2:9
  │
2 │     let a: u32 = -1;
  │         - unused variable
  │

error: u32 cannot be used in a unary operation
  ┌─ /home/Jurarpereurs/neg/src/main.nr:2:18
  │
2 │     let a: u32 = -1;
  │                  --
  │

Error: Aborting due to 1 previous error

Location:
    crates/nargo_cli/src/cli/mod.rs:79:5

To see that the error is not thrown when doing some operations on the negative integer literal, save the following code into src/main.noir:

fn main() {
    let a: u32 = -1 - 1;
}

Run nargo prove and observe that the error is not thrown.

To Reproduce

1. Create a new Noir project with the following command:

   noir new neg

2. Save the following code into src/main.noir:

   fn main() {
       let a: u32 = -1;
   }

3. Run nargo prove and observe the error.

Installation Method

Binary

Nargo Version

nargo 0.10.5 (git version hash: 9fe4cfd, is dirty: false)

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response

[guipublic]

The issue is not that -1 is forbidden, but rather the fact that -1-1 is allowed. This is because the check we perform is too simple.
We have a similar issue if you assign 300 to u8 vs 300+4 to u8.
Because we are moving toward preventing integer overflows, the solution is to error on overflows also in these cases