fix(jsonpath): report unsupported, not no-match, for negative index on-demand

include/sonic/internal/arch/simd_skip.h

@@ -677,6 +677,16 @@ class SkipScanner2 {
       JsonGeneratorInterface<serializeFlags> *jsonGenerator,
       const JsonGeneratorFactory<serializeFlags> &jsonGeneratorFactory,
       const int64_t idx) {
+    // On-demand scanning is forward-only, so a negative index cannot be
+    // resolved without knowing the array length up front. Report that
+    // explicitly: the loop below compares `cur_idx == idx` with cur_idx >= 0,
+    // which a negative idx never satisfies, so without this check the scan
+    // walks the whole array and the caller receives kErrorNoneNoMatch -- the
+    // same result as a key that genuinely is not there.
+    if (idx < 0) {
+      setError(SonicError::kUnsupportedJsonPath);
+      return false;
+    }
     RETURN_FALSE_IF_PARSE_ERROR(consume('['));
     int64_t cur_idx = 0;
     bool dirty = false;

tests/jsonpath_test.cpp

@@ -779,6 +779,33 @@ TEST(JsonPathDom, NegativeIndexSupportedButOnDemandUnsupported) {
   EXPECT_EQ(std::get<0>(ondemand_got), "");
 }
 
+TEST(JsonPathOnDemand, NegativeIndexReportsUnsupportedNotNoMatch) {
+  // A negative index is a feature this backend does not implement, which is
+  // not the same thing as the path being absent from the document. Reporting
+  // both as kErrorNoneNoMatch leaves callers unable to tell "sonic cannot
+  // evaluate this path" from "the document has no such member", so a path the
+  // DOM backend resolves fine looks merely missing here.
+  for (const char* path : {"$[-1]", "$[-3]", "$[-9]"}) {
+    auto got = GetByJsonPathOnDemand<kSerializeJavaStyleFlag>("[0,1,2]", path);
+    EXPECT_EQ(std::get<1>(got), kUnsupportedJsonPath) << "path: " << path;
+    EXPECT_EQ(std::get<0>(got), "") << "path: " << path;
+  }
+
+  // Nested arrays reach the same code through a different call site.
+  auto nested = GetByJsonPathOnDemand<kSerializeJavaStyleFlag>(
+      R"({"a":[10,11,12]})", "$.a[-1]");
+  EXPECT_EQ(std::get<1>(nested), kUnsupportedJsonPath);
+
+  // A genuinely absent member must keep reporting no-match, so the two
+  // outcomes stay distinguishable.
+  auto absent_key =
+      GetByJsonPathOnDemand<kSerializeJavaStyleFlag>(R"({"a":1})", "$.zz");
+  EXPECT_EQ(std::get<1>(absent_key), kErrorNoneNoMatch);
+  auto absent_index =
+      GetByJsonPathOnDemand<kSerializeJavaStyleFlag>("[0,1,2]", "$[9]");
+  EXPECT_EQ(std::get<1>(absent_index), kErrorNoneNoMatch);
+}
+
 TEST(JsonPathDump, SerializeCoversEmptySingleAndMulti) {
   Document doc;
   doc.Parse(R"({"a":null,"s":"abc","arr":[null,"x",1]})");