+/**
+ * Nullable indicates that a return value or a parameter may be a `nullptr`,
+ * e.g.
+ *
+ * int* FOLLY_NULLABLE foo(int* a, int* FOLLY_NULLABLE b) {
+ * if (*a > 0) { // safe dereference
+ * return nullptr;
+ * }
+ * if (*b < 0) { // unsafe dereference
+ * return *a;
+ * }
+ * if (b != nullptr && *b == 1) { // safe checked dereference
+ * return new int(1);
+ * }
+ * return nullptr;
+ * }
+ */
+#if FOLLY_HAS_EXTENSION(nullability)
+#define FOLLY_NULLABLE _Nullable
+#else
+#define FOLLY_NULLABLE
+#endif
+