From 55194b9d81dc23f65c3907ae289491086e429b63 Mon Sep 17 00:00:00 2001
From: Jolse Maginnis <doolse@gmail.com>
Date: Tue, 17 Mar 2026 22:59:12 +1100
Subject: [PATCH] Fix ?? operator returning wrong result when LHS has array
 predicate

The ?? infix handler reused the same `left` AST node reference for both
the $exists() condition and the `then` branch. During AST post-processing,
filter stages were pushed onto the shared node twice, turning e.g.
`array[1]` into `array[1][1]`.

Deep-clone `left` for the condition argument so the two branches have
independent AST nodes.

Fixes #773
---
 src/parser.js                                           | 2 +-
 test/test-suite/groups/coalescing-operator/case013.json | 7 +++++++
 test/test-suite/groups/coalescing-operator/case014.json | 7 +++++++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 test/test-suite/groups/coalescing-operator/case013.json
 create mode 100644 test/test-suite/groups/coalescing-operator/case014.json

diff --git a/src/parser.js b/src/parser.js
index a6d4223e..34906054 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -585,7 +585,7 @@ const parser = (() => {
                 type: 'function',
                 value: '(',
                 procedure: { type: 'variable', value: 'exists' },
-                arguments: [left]
+                arguments: [JSON.parse(JSON.stringify(left))]
             };
             this.then = left;
             this.else = expression(0);
diff --git a/test/test-suite/groups/coalescing-operator/case013.json b/test/test-suite/groups/coalescing-operator/case013.json
new file mode 100644
index 00000000..0f32ae7f
--- /dev/null
+++ b/test/test-suite/groups/coalescing-operator/case013.json
@@ -0,0 +1,7 @@
+{
+    "description": "array predicate on lhs exists, should return lhs value",
+    "expr": "foo.blah[0] ?? 42",
+    "dataset": "dataset0",
+    "bindings": {},
+    "result": {"baz": {"fud": "hello"}}
+}
diff --git a/test/test-suite/groups/coalescing-operator/case014.json b/test/test-suite/groups/coalescing-operator/case014.json
new file mode 100644
index 00000000..c9b1f463
--- /dev/null
+++ b/test/test-suite/groups/coalescing-operator/case014.json
@@ -0,0 +1,7 @@
+{
+    "description": "array predicate on lhs does not exist, should return rhs value",
+    "expr": "foo.blah[5] ?? 42",
+    "dataset": "dataset0",
+    "bindings": {},
+    "result": 42
+}