Move, don't copy, the loaded value into the optional

`load_impl` read the value into a local and copy-assigned it to the
optional. A move-only type then failed to compile: `optional`'s
perfect-forwarding assignment is removed by SFINAE when the value is not
copy-assignable, leaving no viable `operator=`.

So, move the local into the optional instead.

The one-line fix was proposed by Venkat Murty in PR #330; this extends
it with `boost::move` and a regression test.

.github/workflows/ci.yml

@@ -7,6 +7,7 @@ on:
       - master
       - develop
       - feature/**
+  merge_group:
 
 env:
   UBSAN_OPTIONS: print_stacktrace=1

include/boost/serialization/optional.hpp

@@ -20,6 +20,7 @@
 #include <optional>
 #endif
 
+#include <boost/move/utility_core.hpp>
 #include <boost/serialization/item_version_type.hpp>
 #include <boost/serialization/library_version_type.hpp>
 #include <boost/serialization/version.hpp>
@@ -83,7 +84,7 @@ void load_impl(
     }
     typename OT::value_type t;
     ar >> boost::serialization::make_nvp("value",t);
-    ot = t;
+    ot = boost::move(t);
 }
 
 } // detail

test/test_optional.cpp

@@ -2,6 +2,7 @@
 // test_optional.cpp
 
 // (C) Copyright 2004 Pavel Vozenilek
+// Copyright 2026 Gennaro Prota
 // Distributed under the Boost Software License, Version 1.0.
 // (See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
@@ -92,6 +93,59 @@ int test(){
     return EXIT_SUCCESS;
 }
 
+// A move-only value type: deleted copy, defaulted move.  Loading an
+// optional<M> must move the deserialized value into place rather than copy
+// it.
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES) \
+    && !defined(BOOST_NO_CXX11_DELETED_FUNCTIONS) \
+    && !defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS)
+#define BOOST_SERIALIZATION_TEST_OPTIONAL_MOVE_ONLY
+
+struct M {
+    int m_x;
+    M() : m_x(0) {}
+    explicit M(int x) : m_x(x) {}
+    M(const M &) = delete;
+    M & operator=(const M &) = delete;
+    M(M &&) = default;
+    M & operator=(M &&) = default;
+    template<class Archive>
+    void serialize(Archive & ar, const unsigned int /* version */){
+        ar & boost::serialization::make_nvp("x", m_x);
+    }
+};
+
+template<template<class> class Optional>
+int test_move_only(){
+    const char * testfile = boost::archive::tmpnam(NULL);
+    BOOST_REQUIRE(NULL != testfile);
+
+    Optional<M> o_empty;
+    Optional<M> o_value(M(42));
+    {
+        test_ostream os(testfile, TEST_STREAM_FLAGS);
+        test_oarchive oa(os, TEST_ARCHIVE_FLAGS);
+        oa << boost::serialization::make_nvp("o_empty", o_empty);
+        oa << boost::serialization::make_nvp("o_value", o_value);
+    }
+    // start each target in the opposite state, so load must both reset and
+    // assign
+    Optional<M> o_empty_a(M(7));
+    Optional<M> o_value_a;
+    {
+        test_istream is(testfile, TEST_STREAM_FLAGS);
+        test_iarchive ia(is, TEST_ARCHIVE_FLAGS);
+        ia >> boost::serialization::make_nvp("o_empty", o_empty_a);
+        ia >> boost::serialization::make_nvp("o_value", o_value_a);
+    }
+    BOOST_CHECK(! o_empty_a);
+    BOOST_CHECK(static_cast<bool>(o_value_a) && 42 == o_value_a->m_x);
+
+    std::remove(testfile);
+    return EXIT_SUCCESS;
+}
+#endif // move-only support available
+
 #include <boost/serialization/optional.hpp>
 #ifndef BOOST_NO_CXX17_HDR_OPTIONAL
 #include <optional>
@@ -102,5 +156,11 @@ int test_main( int /* argc */, char* /* argv */[] ){
     #ifndef BOOST_NO_CXX17_HDR_OPTIONAL
     test<std::optional>();
     #endif
+    #ifdef BOOST_SERIALIZATION_TEST_OPTIONAL_MOVE_ONLY
+    test_move_only<boost::optional>();
+    #ifndef BOOST_NO_CXX17_HDR_OPTIONAL
+    test_move_only<std::optional>();
+    #endif
+    #endif
     return EXIT_SUCCESS;
 }