Fix handling of None in Num::add (#88)

* Fix value handling in Num::add

* Attempt to fix check-lurk-compiles CI workflow

* Attempt to install deps in CI

* Use correct action path

* Better attempt at fixing CI

.github/workflows/rust.yml

@@ -44,6 +44,13 @@ jobs:
     env:
       RUSTFLAGS: -D warnings
     steps:
+    - uses: actions/checkout@v4
+      with:
+        repository: lurk-lab/ci-workflows
+    - uses: ./.github/actions/ci-env
+    - uses: ./.github/actions/install-deps
+      with:
+        packages: 'pkg-config libssl-dev protobuf-compiler libprotobuf-dev'
     - uses: actions/checkout@v4
       with:
         path: ${{ github.workspace }}/bellpepper
@@ -59,6 +66,7 @@ jobs:
       run: |
         echo "[patch.'https://github.com/lurk-lab/bellpepper']" >> Cargo.toml
         echo "bellpepper = { path='../bellpepper/crates/bellpepper' }" >> Cargo.toml
+        echo "[patch.crates-io]" >> Cargo.toml
         echo "bellpepper-core = { path='../bellpepper/crates/bellpepper-core' }" >> Cargo.toml
     - name: Check Lurk-rs types don't break spectacularly
       working-directory: ${{ github.workspace }}/lurk

crates/bellpepper-core/src/gadgets/num.rs

@@ -520,8 +520,7 @@ impl<Scalar: PrimeField> Num<Scalar> {
                 tmp.add_assign(&v2);
                 Some(tmp)
             }
-            (Some(v), None) | (None, Some(v)) => Some(v),
-            (None, None) => None,
+            _ => None,
         };
 
         Num { value, lc }
@@ -571,6 +570,23 @@ mod test {
         assert!(cs.get("num") == Fr::ONE);
     }
 
+    #[test]
+    fn test_num_partial_addition() {
+        let a = Num::<Fr>::zero();
+        let b = Num::<Fr> {
+            value: None,
+            lc: Default::default(),
+        };
+        let c = a.clone().add(&b);
+        assert!(c.value.is_none());
+        let c = b.clone().add(&a);
+        assert!(c.value.is_none());
+        let c = b.clone().add(&b);
+        assert!(c.value.is_none());
+        let c = a.clone().add(&a);
+        assert!(c.value == Some(Fr::ZERO));
+    }
+
     #[test]
     fn test_num_addition() {
         let mut cs = TestConstraintSystem::<Fr>::new();