jinja: treat a null left operand of in as a plain lookup (#28620)

Templates that default an optional variable to none and then test its
membership in a map hit an error, while the same expression is a normal
lookup returning false in Jinja. The undefined counterpart of this case
was already handled just above.

common/jinja/runtime.cpp

@@ -167,6 +167,12 @@ value binary_expression::execute_impl(context & ctx) {
         }
         throw std::runtime_error("Cannot perform operation " + op.value + " on undefined values");
     } else if (is_val<value_none>(left_val) || is_val<value_none>(right_val)) {
+        if (!is_val<value_none>(right_val) && (op.value == "in" || op.value == "not in")) {
+            // case: none in {'low': 1}
+            // A null left operand is looked up like any other value.
+            bool member = test_is_in();
+            return mk_val<value_bool>(op.value == "in" ? member : !member);
+        }
         if (op.value == "+" || op.value == "~") {
             value res = mk_val<value_undefined>();
             if (workaround_concat_null_with_str(res)) {

tests/test-jinja.cpp

@@ -374,6 +374,24 @@ static void test_expressions(testing & t) {
         "42"
     );
 
+    test_template(t, "none in object",
+        "{{ x in {'low': 1, 'high': 2} }}",
+        {{"x", nullptr}},
+        "False"
+    );
+
+    test_template(t, "none not in object",
+        "{{ x not in {'low': 1, 'high': 2} }}",
+        {{"x", nullptr}},
+        "True"
+    );
+
+    test_template(t, "none in array",
+        "{{ x in [1, none, 3] }}",
+        {{"x", nullptr}},
+        "True"
+    );
+
     test_template(t, "dot notation",
         "{{ user.name }}",
         {{"user", {{"name", "Bob"}}}},