x.json2: fix decoding wrong value depending on field order (fix #26503) (made with copilot) (#26571)

Author: esquerbatua

vlib/x/json2/decode.v

@@ -460,7 +460,27 @@ fn (mut decoder Decoder) decode_value[T](mut val T) ! {
 				// field loop
 				for {
 					if current_field_info == unsafe { nil } {
-						decoder.current_node = decoder.current_node.next // skip value
+						// The key doesn't match any field in the struct, skip the entire value
+						// including all nested objects/arrays
+						decoder.current_node = decoder.current_node.next // move to value node
+
+						if decoder.current_node != unsafe { nil } {
+							// Calculate the end position of this value
+							value_end := decoder.current_node.value.position +
+								decoder.current_node.value.length
+
+							// Skip all nodes that belong to this value (nested content)
+							for {
+								if decoder.current_node == unsafe { nil } {
+									break
+								}
+								// Check if current node is still within the value's boundaries
+								if decoder.current_node.value.position >= value_end {
+									break
+								}
+								decoder.current_node = decoder.current_node.next
+							}
+						}
 
 						break
 					}

vlib/x/json2/tests/decode_nested_object_same_kay_test.v

@@ -0,0 +1,69 @@
+import x.json2
+
+// Test for issue #26503: Decoder incorrectly reads nested object fields
+// When a required field appears after unmatched fields with nested objects/arrays,
+// the decoder was incorrectly picking up values from nested structures.
+
+struct Foo {
+	id    string @[required]
+	title string @[required]
+}
+
+fn test_decode_with_nested_objects_field_order() {
+	// Test case 1: required fields appear before the nested array
+	s1 := '{"id":"sss","title":"ttt","thumb":[{ "url":"i1.jpg","id":"000"}]}'
+	f1 := json2.decode[Foo](s1)!
+
+	assert f1.id == 'sss', 'f1.id should be "sss" but got "${f1.id}"'
+	assert f1.title == 'ttt', 'f1.title should be "ttt" but got "${f1.title}"'
+
+	// Test case 2: required fields appear after the nested array
+	s2 := '{"title":"ttt","thumb":[{ "url":"i1.jpg","id":"000"}],"id":"sss"}'
+	f2 := json2.decode[Foo](s2)!
+
+	assert f2.id == 'sss', 'f2.id should be "sss" but got "${f2.id}"'
+	assert f2.title == 'ttt', 'f2.title should be "ttt" but got "${f2.title}"'
+
+	// Test case 3: nested array appears between required fields
+	s3 := '{"id":"sss","thumb":[{ "url":"i1.jpg","id":"000"}],"title":"ttt"}'
+	f3 := json2.decode[Foo](s3)!
+
+	assert f3.id == 'sss', 'f3.id should be "sss" but got "${f3.id}"'
+	assert f3.title == 'ttt', 'f3.title should be "ttt" but got "${f3.title}"'
+}
+
+fn test_decode_with_deeply_nested_objects() {
+	// Test with deeply nested structures to ensure complete skipping
+	s := '{"id":"outer","data":{"nested":{"deep":{"id":"inner","title":"inner_title"}}},"title":"outer_title"}'
+	f := json2.decode[Foo](s)!
+
+	assert f.id == 'outer', 'id should be "outer" but got "${f.id}"'
+	assert f.title == 'outer_title', 'title should be "outer_title" but got "${f.title}"'
+}
+
+fn test_decode_with_multiple_nested_arrays() {
+	// Test with multiple nested arrays to ensure complete skipping
+	s := '{"id":"correct","items":[{"id":"a"},{"id":"b"}],"more":[{"id":"c"}],"title":"correct_title"}'
+	f := json2.decode[Foo](s)!
+
+	assert f.id == 'correct', 'id should be "correct" but got "${f.id}"'
+	assert f.title == 'correct_title', 'title should be "correct_title" but got "${f.title}"'
+}
+
+struct BarWithOptional {
+	id    string @[required]
+	title ?string
+	extra string
+}
+
+fn test_decode_optional_fields_with_nested() {
+	// Test optional fields work correctly with nested structures
+	s := '{"id":"main","nested":{"title":"nested_title"},"extra":"extra_value"}'
+	b := json2.decode[BarWithOptional](s)!
+
+	assert b.id == 'main', 'id should be "main" but got "${b.id}"'
+	assert b.extra == 'extra_value', 'extra should be "extra_value" but got "${b.extra}"'
+	if title := b.title {
+		assert false, 'title should be none but got "${title}"'
+	}
+}