From ead417f01ce9769249e26fd1aaf700de83003a5c Mon Sep 17 00:00:00 2001
From: "Zhihao \"Zephyr\" Yao" <zeph1912@users.noreply.github.com>
Date: Mon, 30 Mar 2026 14:08:46 -0400
Subject: [PATCH] jinja : handle empty expressions correctly (#20913)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* Reject empty computed member expressions before returning slices[0] from parse_member_expression_arguments().

* Treat empty computed member expressions with Jinja2 undefined semantics

Treat empty computed member expressions like `a[]` as undefined instead of
raising a parser error, to match Jinja2 behavior.

- return a noop expression for empty computed member arguments
- return undefined when a computed member key evaluates to undefined
- add Jinja tests covering `a[]|default('fallback')` and `a[] is undefined`

* Handle undefined computed member properties

Move undefined-property handling to the common member access path, and add a test covering `a[undefined] is undefined`.

* Use default undefined value in member access

Initialize val and then return it when property is undefined.

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

* empty statement parses to blank_expression instead of noop_statement

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
---
 common/jinja/parser.cpp  |  3 +++
 common/jinja/runtime.cpp |  9 +++++++--
 common/jinja/runtime.h   |  8 ++++++++
 tests/test-jinja.cpp     | 18 ++++++++++++++++++
 4 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/common/jinja/parser.cpp b/common/jinja/parser.cpp
index 4ae4477445..2b25654a7a 100644
--- a/common/jinja/parser.cpp
+++ b/common/jinja/parser.cpp
@@ -539,6 +539,9 @@ private:
             statement_ptr step = slices.size() > 2 ? std::move(slices[2]) : nullptr;
             return mk_stmt<slice_expression>(start_pos, std::move(start), std::move(stop), std::move(step));
         }
+        if (slices.empty()) {
+            return mk_stmt<blank_expression>(start_pos);
+        }
         return std::move(slices[0]);
     }
 
diff --git a/common/jinja/runtime.cpp b/common/jinja/runtime.cpp
index dce5bbae30..2232790c31 100644
--- a/common/jinja/runtime.cpp
+++ b/common/jinja/runtime.cpp
@@ -771,10 +771,15 @@ value member_expression::execute_impl(context & ctx) {
     }
 
     JJ_DEBUG("Member expression on object type %s, property type %s", object->type().c_str(), property->type().c_str());
-    ensure_key_type_allowed(property);
-
     value val = mk_val<value_undefined>("object_property");
 
+    if (property->is_undefined()) {
+        JJ_DEBUG("%s", "Member expression property is undefined, returning undefined");
+        return val;
+    }
+
+    ensure_key_type_allowed(property);
+
     if (is_val<value_undefined>(object)) {
         JJ_DEBUG("%s", "Accessing property on undefined object, returning undefined");
         return val;
diff --git a/common/jinja/runtime.h b/common/jinja/runtime.h
index 17a6dff5aa..3ca5f1754f 100644
--- a/common/jinja/runtime.h
+++ b/common/jinja/runtime.h
@@ -263,6 +263,14 @@ struct comment_statement : public statement {
 
 // Expressions
 
+// Represents an omitted expression in a computed member, e.g. `a[]`.
+struct blank_expression : public expression {
+    std::string type() const override { return "BlankExpression"; }
+    value execute_impl(context &) override {
+        return mk_val<value_undefined>();
+    }
+};
+
 struct member_expression : public expression {
     statement_ptr object;
     statement_ptr property;
diff --git a/tests/test-jinja.cpp b/tests/test-jinja.cpp
index 2cac38f02a..5d4b2806ac 100644
--- a/tests/test-jinja.cpp
+++ b/tests/test-jinja.cpp
@@ -387,6 +387,24 @@ static void test_expressions(testing & t) {
         "Bob"
     );
 
+    test_template(t, "empty computed member defaults to undefined",
+        "{{ a[]|default('fallback') }}",
+        {{"a", {{"name", "Bob"}}}},
+        "fallback"
+    );
+
+    test_template(t, "empty computed member is undefined",
+        "{{ a[] is undefined }}",
+        {{"a", {{"name", "Bob"}}}},
+        "True"
+    );
+
+    test_template(t, "undefined computed member is undefined",
+        "{{ a[undefined] is undefined }}",
+        {{"a", {{"name", "Bob"}}}},
+        "True"
+    );
+
     test_template(t, "array access",
         "{{ items[1] }}",
         {{"items", json::array({"a", "b", "c"})}},