upstream: add serialiation for std::optional

src/serialization/optional.h

@@ -0,0 +1,64 @@
+// Copyright (c) 2025, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+//local headers
+#include "serialization.h"
+
+//third party headers
+
+//standard headers
+#include <optional>
+
+//forward declarations
+
+// [De]serialize a std::optional<T> as a heterogenous array: either [0], or [1, x], where x is the contained value
+template <bool W, template <bool> class Archive, typename T>
+bool do_serialize(Archive<W> &ar, std::optional<T> &v)
+{
+    if constexpr (!W)
+        v.reset();
+
+    ar.begin_array();
+
+    uint8_t has_value = v.has_value();
+    ar.serialize_int(has_value);
+    if (has_value > 1)
+        return false; // to prevent malleability
+
+    if (has_value)
+    {
+        ar.delimit_array();
+        if (!do_serialize(ar, v ? *v : v.emplace()))
+            return false;
+    }
+
+    ar.end_array();
+    return ar.good();
+}