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
@@ -243,15 +244,7 @@ jobs:
         run: |
           cd ../boost-root
           export ADDRMD=${{matrix.address-model}}
-          ./b2 -j3 libs/$LIBRARY/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} ${ADDRMD:+address-model=$ADDRMD} variant=debug,release
-
-      - name: Run examples
-        run: |
-          cd ../boost-root
-          export ADDRMD=${{matrix.address-model}}
-          # The demos use fixed temp filenames, so run them serially (-j1)
-          # to avoid concurrent runs clobbering each other's archive files.
-          ./b2 -j1 libs/$LIBRARY/example toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} ${ADDRMD:+address-model=$ADDRMD} variant=debug,release
+          ./b2 -j3 libs/$LIBRARY/test libs/$LIBRARY/example toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} ${ADDRMD:+address-model=$ADDRMD} variant=debug,release
 
   windows:
     strategy:
@@ -306,15 +299,7 @@ jobs:
         shell: cmd
         run: |
           cd ../boost-root
-          b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
-
-      - name: Run examples
-        shell: cmd
-        run: |
-          cd ../boost-root
-          rem The demos use fixed temp filenames, so run them serially (-j1)
-          rem to avoid concurrent runs clobbering each other's archive files.
-          b2 -j1 libs/%LIBRARY%/example toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
+          b2 -j3 libs/%LIBRARY%/test libs/%LIBRARY%/example toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
 
   posix-cmake-subdir:
     strategy:

example/Jamfile.v2

@@ -11,8 +11,6 @@ project libs/serialization/example
     : requirements <library>../build//boost_serialization
     ;
 
-import testing ;
-
 import ../util/test :
     run-template
     run-invoke

include/boost/serialization/filesystem.hpp

@@ -18,12 +18,8 @@
 
 #include <boost/config.hpp>
 
-// GCC's <filesystem> before version 9 is the initial, incomplete
-// implementation (shipped in a separate libstdc++fs and buggy at runtime),
-// so the feature is disabled there.
 #if defined(__has_include)
-#  if __has_include(<filesystem>) && (BOOST_CXX_VERSION >= 201703L) \
-      && ! (defined(__GNUC__) && ! defined(__clang__) && __GNUC__ < 9)
+#  if __has_include(<filesystem>) && (BOOST_CXX_VERSION >= 201703L)
 #    define BOOST_SERIALIZATION_HAS_STD_FILESYSTEM
 #  endif
 #endif

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_filesystem_path.cpp

@@ -17,11 +17,8 @@
 
 // `std::filesystem::path` is a C++17 feature; when it isn't available, this
 // test has nothing to exercise and simply succeeds.
-// Note the guard for GCC < 9, which matches the one in
-// boost/serialization/filesystem.hpp: see the comment there for the why.
 #if defined(__has_include)
-#  if __has_include(<filesystem>) && (BOOST_CXX_VERSION >= 201703L) \
-      && ! (defined(__GNUC__) && ! defined(__clang__) && __GNUC__ < 9)
+#  if __has_include(<filesystem>) && (BOOST_CXX_VERSION >= 201703L)
 #    define BOOST_SERIALIZATION_TEST_STD_FILESYSTEM
 #  endif
 #endif
@@ -32,11 +29,6 @@
 
 #include <boost/serialization/filesystem.hpp>
 #include <filesystem>
-#include <type_traits>
-
-#ifndef BOOST_NO_STD_WSTREAMBUF
-#include <boost/archive/text_woarchive.hpp>
-#endif
 
 void check_roundtrip(const std::filesystem::path & original){
     const char * testfile = boost::archive::tmpnam(NULL);
@@ -65,21 +57,6 @@ std::filesystem::path from_utf8(const std::string & utf8){
 #endif
 }
 
-// The wide *text* archive is the one archive that cannot round-trip non-ASCII
-// content: it transcodes the narrow UTF-8 std::string through the stream
-// locale's codecvt (codecvt_null), which mangles bytes outside the ASCII
-// range.  Every other archive in the test list preserves them -- narrow text
-// and XML store the bytes verbatim, binary stores them raw, and wide XML
-// installs a UTF-8 codecvt.  So the non-ASCII case is exercised everywhere
-// except that single archive.
-bool archive_round_trips_non_ascii(){
-#ifndef BOOST_NO_STD_WSTREAMBUF
-    return ! std::is_same<test_oarchive, boost::archive::text_woarchive>::value;
-#else
-    return true;
-#endif
-}
-
 int test_main(int /* argc */, char * /* argv */ []){
     check_roundtrip(std::filesystem::path());              // empty
     check_roundtrip("foo/bar/baz.txt");                    // relative
@@ -89,8 +66,7 @@ int test_main(int /* argc */, char * /* argv */ []){
     // The escapes are the UTF-8 encoding of U+00E9, U+00EF and U+00FC
     // (e-acute, i-diaeresis, u-diaeresis); written as `\x` so the source
     // file stays pure ASCII and encoding-independent.
-    if(archive_round_trips_non_ascii())
-        check_roundtrip(from_utf8("caf\xC3\xA9/na\xC3\xAF" "ve/\xC3\xBC.txt"));
+    check_roundtrip(from_utf8("caf\xC3\xA9/na\xC3\xAF" "ve/\xC3\xBC.txt"));
     return EXIT_SUCCESS;
 }
 

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;
 }