From a7a14e57d6a69d8ce94d7d22bdeea9d50a61eb9a Mon Sep 17 00:00:00 2001
From: aviavissar <aviavissar@gmail.com>
Date: Tue, 1 Sep 2026 10:13:28 +0300
Subject: [PATCH] 
 test(CEL-PDP)--collision-precedence-rules-(stdlib-shadowing,-bag-namespace-vs-scalar)-lack-test-coverage-#3

Signed-off-by: aviavissar <aviavissar@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
---
 builtins/pdps/cel/src/activation.rs | 47 +++++++++++++++++++++++++++++
 builtins/pdps/cel/src/resolver.rs   | 37 +++++++++++++++++++++++
 2 files changed, 84 insertions(+)

diff --git a/builtins/pdps/cel/src/activation.rs b/builtins/pdps/cel/src/activation.rs
index 9812e54..f1ec762 100644
--- a/builtins/pdps/cel/src/activation.rs
+++ b/builtins/pdps/cel/src/activation.rs
@@ -382,4 +382,51 @@ mod tests {
         bag.set("delegation.depth", 3_i64);
         assert!(truthy("delegation.depth == 3", &bag));
     }
+
+    /// The sibling of `namespace_wins_on_leaf_collision`: a scalar
+    /// arrives *after* the namespace already exists. `insert` keeps the
+    /// branch and drops the scalar (the warning at the terminal-segment
+    /// arm). Driven through `insert` directly because `AttributeBag` is
+    /// a `HashMap` — `set` order is not `iter` order, so a bag cannot
+    /// pin this arm.
+    #[test]
+    fn namespace_wins_when_scalar_arrives_after_branch() {
+        let mut root = BTreeMap::new();
+        insert(
+            &mut root,
+            "delegation.depth",
+            &["delegation", "depth"],
+            Value::from(3_i64),
+        );
+        insert(
+            &mut root,
+            "delegation",
+            &["delegation"],
+            Value::from("scalar-value".to_owned()),
+        );
+
+        let depth_kept = match root.get("delegation") {
+            Some(Node::Branch(children)) => {
+                matches!(children.get("depth"), Some(Node::Leaf(_)))
+            },
+            _ => false,
+        };
+        assert!(
+            depth_kept,
+            "namespace must win when a scalar arrives after the branch exists; \
+             the scalar must be dropped so delegation.depth still lives under the map",
+        );
+
+        let mut ctx = Context::default();
+        for (name, node) in root {
+            ctx.add_variable_from_value(name, node_to_value(node));
+        }
+        assert!(
+            matches!(
+                run_cel("delegation.depth == 3", &ctx),
+                Ok(Value::Bool(true))
+            ),
+            "CEL must still resolve delegation.depth after the colliding scalar is dropped",
+        );
+    }
 }
diff --git a/builtins/pdps/cel/src/resolver.rs b/builtins/pdps/cel/src/resolver.rs
index 9dc034c..cb0fa83 100644
--- a/builtins/pdps/cel/src/resolver.rs
+++ b/builtins/pdps/cel/src/resolver.rs
@@ -145,6 +145,11 @@ impl CelResolver {
     ///
     /// Composes: calling `with_functions` more than once stacks the
     /// callbacks. Each runs in registration order on every context.
+    /// Later custom setups can shadow earlier custom setups. They do
+    /// **not** replace a CEL standard-library overload of the same
+    /// name (`size`, `has`, `matches`, `double`, …) as of `cel` 0.14
+    /// — the stdlib wins and the colliding registration is ignored
+    /// for that signature. Pick names that cannot collide.
     ///
     /// # Example
     ///
@@ -671,6 +676,38 @@ mod tests {
         );
     }
 
+    /// Pins custom-vs-stdlib collision precedence. A host-registered
+    /// `size` that would return 777 must not replace CEL's built-in
+    /// `size` (5 for `"hello"`). As of `cel` 0.14 the stdlib overload
+    /// wins; this test fails if that flips.
+    #[tokio::test]
+    async fn custom_function_does_not_shadow_stdlib_size() {
+        let r = CelResolver::new().with_functions(|ctx| {
+            ctx.add_function("size", |_s: Arc<String>| -> i64 { 777 });
+        });
+        let bag = bag_with(&[("subject.id", "alice")]);
+
+        let stdlib = r
+            .evaluate(&cel_call("size('hello') == 5"), &bag)
+            .await
+            .unwrap();
+        assert_eq!(
+            stdlib.decision,
+            Decision::Allow,
+            "stdlib size('hello') is 5; if this fails, a custom size shadowed the builtin",
+        );
+
+        let custom = r
+            .evaluate(&cel_call("size('hello') == 777"), &bag)
+            .await
+            .unwrap();
+        assert!(
+            matches!(custom.decision, Decision::Deny { .. }),
+            "custom size returning 777 must not run in place of stdlib size; got {:?}",
+            custom.decision,
+        );
+    }
+
     /// The `regex` cel-feature is explicitly enabled in our Cargo.toml.
     /// Pin that `matches(s, pattern)` actually works through the
     /// resolver so a future feature-set churn breaks loudly here.