Add test to reproduce #2051

pilopt/tests/optimizer.rs

@@ -281,3 +281,24 @@ fn enum_ref_by_trait() {
     let optimized = optimize(analyze_string::<GoldilocksField>(input).unwrap()).to_string();
     assert_eq!(optimized, expectation);
 }
+
+#[test]
+#[should_panic = "Symbol not found: N::x[0]"]
+fn handle_array_references_in_prover_functions() {
+    // Reproduces https://github.com/powdr-labs/powdr/issues/2051
+    let input = r#"namespace N(8);
+    col witness x[1];
+    
+    // non-trivial constraint so that `x[0]` does not get removed.
+    x[0]' = x[0] + 1;
+
+    {
+        let intermediate = x[0] + 1;
+        query |i| {
+            // No-op, but references `x[0]`.
+            let _ = intermediate;
+        }
+    };
+    "#;
+    optimize(analyze_string::<GoldilocksField>(input).unwrap()).to_string();
+}