diff --git a/src/backend/utils/adt/jsonfuncs.c b/src/backend/utils/adt/jsonfuncs.c
index bc7b556e22e..64c85e9ddd1 100644
--- a/src/backend/utils/adt/jsonfuncs.c
+++ b/src/backend/utils/adt/jsonfuncs.c
@@ -233,7 +233,7 @@ struct RecordIOData
 	Oid			record_type;
 	int32		record_typmod;
 	int			ncolumns;
-	ColumnIOData columns[FLEXIBLE_ARRAY_MEMBER];
+	ColumnIOData columns[FLEXIBLE_ARRAY_MEMBER] pg_attribute_counted_by(ncolumns);
 };
 
 /* per-query cache for populate_record_worker and populate_recordset_worker */
diff --git a/src/include/c.h b/src/include/c.h
index 3b75f0b8610..57d891e52a7 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -297,6 +297,30 @@ extern "C++"
 #define pg_attribute_target(...)
 #endif
 
+/*
+ * pg_attribute_counted_by specifies that a flexible array member is "counted
+ * by" another struct member.  This allows the compiler to improve detection
+ * of object size information and to provide better results in compile-time
+ * diagnostics and run-time features, such as the array bounds sanitizer.
+ *
+ * Using this annotation comes with additional responsibilities:
+ *
+ * - The count must be assigned before the first reference to the array.
+ * - The array must have at least count elements available at all times,
+ *   including after either member is updated.
+ *
+ * The attribute is ignored in C++ due to lack of compiler support.
+ */
+#ifndef __cplusplus
+#if __has_attribute (counted_by)
+#define pg_attribute_counted_by(count) __attribute__((counted_by(count)))
+#else
+#define pg_attribute_counted_by(count)
+#endif
+#else
+#define pg_attribute_counted_by(count)
+#endif
+
 /*
  * Append PG_USED_FOR_ASSERTS_ONLY to definitions of variables that are only
  * used in assert-enabled builds, to avoid compiler warnings about unused