Disable variant_issue_55.cpp on clang-cl 32 bit

Add variant_gcc_false_positive.cpp to reproduce GCC 12+ spurious
-Wmaybe-uninitialized warnings when copying/moving variant containing
non-trivially-copyable types like std::exception_ptr and std::string.

Tests use boost::system::result which internally uses variant2, and
include coroutine-based tests when C++20 coroutine support is available.

The test uses -O3 optimization to trigger the aggressive inlining that
exposes the false positive where GCC's dataflow analysis cannot prove
the discriminator ensures only initialized alternatives are accessed.

See https://github.com/boostorg/variant2/issues/33

.drone.jsonnet

@@ -34,7 +34,6 @@ local linux_pipeline(name, image, environment, packages = "", sources = [], arch
                 'set -e',
                 'uname -a',
                 'echo $DRONE_STAGE_MACHINE',
-                'wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -',
             ] +
             (if sources != [] then [ ('apt-add-repository "' + source + '"') for source in sources ] else []) +
             (if packages != "" then [ 'apt-get update', 'apt-get -y install ' + packages ] else []) +
@@ -213,6 +212,13 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         "g++-14-multilib",
     ),
 
+    linux_pipeline(
+        "Linux 25.04 GCC 15 32/64",
+        "cppalliance/droneubuntu2504:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-15', CXXSTD: '11,14,17,20,23,2c', ADDRMD: '32,64' },
+        "g++-15-multilib",
+    ),
+
     linux_pipeline(
         "Linux 16.04 Clang 3.5",
         "cppalliance/droneubuntu1604:1",
@@ -340,40 +346,47 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
     ),
 
     linux_pipeline(
-        "Linux 24.04 Clang 17 UBSAN",
+        "Linux 24.04 Clang 17",
         "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' } + ubsan,
+        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' },
         "clang-17",
     ),
 
     linux_pipeline(
-        "Linux 24.04 Clang 17 ASAN",
+        "Linux 24.04 Clang 18",
         "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' } + asan,
-        "clang-17",
-    ),
-
-    linux_pipeline(
-        "Linux 24.04 Clang 18 UBSAN",
-        "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' } + ubsan,
+        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' },
         "clang-18",
     ),
 
     linux_pipeline(
-        "Linux 24.04 Clang 18 ASAN",
+        "Linux 24.04 Clang 19",
         "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' } + asan,
-        "clang-18",
-    ),
-
-    linux_pipeline(
-        "Linux 24.10 Clang 19",
-        "cppalliance/droneubuntu2410:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-19', CXXSTD: '11,14,17,20,2b' } + asan,
+        { TOOLSET: 'clang', COMPILER: 'clang++-19', CXXSTD: '11,14,17,20,2b' },
         "clang-19",
     ),
 
+    linux_pipeline(
+        "Linux 24.04 Clang 20 UBSAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '11,14,17,20,23,2c' } + ubsan,
+        "clang-20",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 20 ASAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '11,14,17,20,23,2c' } + asan,
+        "clang-20",
+    ),
+
+    linux_pipeline(
+        "Linux 25.10 Clang 21",
+        "cppalliance/droneubuntu2510:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-21', CXXSTD: '11,14,17,20,23,2c' },
+        "clang-21",
+    ),
+
     macos_pipeline(
         "MacOS 10.15 Xcode 12.2 UBSAN",
         { TOOLSET: 'clang', COMPILER: 'clang++', CXXSTD: '11,14,1z' } + ubsan,
@@ -437,4 +450,16 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         "cppalliance/dronevs2022:1",
         { TOOLSET: 'msvc-14.3', CXXSTD: '14,17', CXXFLAGS: '/permissive-' },
     ),
+
+    windows_pipeline(
+        "Windows VS2026 msvc-14.5",
+        "cppalliance/dronevs2026:1",
+        { TOOLSET: 'msvc-14.5', CXXSTD: '14,17,20,latest' },
+    ),
+
+    windows_pipeline(
+        "Windows VS2026 msvc-14.5 permissive-",
+        "cppalliance/dronevs2026:1",
+        { TOOLSET: 'msvc-14.5', CXXSTD: '14,17', CXXFLAGS: '/permissive-' },
+    ),
 ]

include/boost/variant2/variant.hpp

@@ -93,7 +93,7 @@ struct monostate
 {
 };
 
-#if !BOOST_WORKAROUND(BOOST_MSVC, < 1950)
+#if !BOOST_WORKAROUND(BOOST_MSVC, < 1960)
 
 constexpr bool operator<(monostate, monostate) noexcept { return false; }
 constexpr bool operator>(monostate, monostate) noexcept { return false; }
@@ -558,10 +558,20 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_false, T1, T.
     T1 first_;
     variant_storage<T...> rest_;
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 12)
+// false positive, see https://github.com/boostorg/variant2/issues/55
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
     template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
     {
     }
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 12)
+# pragma GCC diagnostic pop
+#endif
+
     template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
     {
     }
@@ -704,6 +714,7 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_true, T1, T..
 # pragma GCC diagnostic ignored "-Wuninitialized"
 #endif
 #endif
+
         *this = variant_storage_impl( mp11::mp_size_t<I>(), std::forward<A>(a)... );
 
 #if defined(BOOST_GCC) && (__GNUC__ >= 7)
@@ -1071,7 +1082,18 @@ template<class... T> struct variant_base_impl<false, true, T...>
         template<class I> BOOST_CXX14_CONSTEXPR void operator()( I ) const noexcept
         {
             using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
+
+#if defined(BOOST_GCC) && (__GNUC__ >= 12)
+// false positive, see https://github.com/boostorg/variant2/issues/55
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
             this_->st_.get( I() ).~U();
+
+#if defined(BOOST_GCC) && (__GNUC__ >= 12)
+# pragma GCC diagnostic pop
+#endif
         }
     };
 
@@ -1119,8 +1141,18 @@ template<class... T> struct variant_base_impl<false, true, T...>
 
         static_assert( std::is_nothrow_move_constructible<U>::value, "Logic error: U must be nothrow move constructible" );
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 12)
+// false positive, see https://github.com/boostorg/variant2/issues/55
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
         U tmp( std::forward<A>(a)... );
 
+#if defined(BOOST_GCC) && (__GNUC__ >= 12)
+# pragma GCC diagnostic pop
+#endif
+
         _destroy();
 
         st_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
@@ -1689,7 +1721,7 @@ public:
         class Ud = typename std::decay<U>::type,
         class E1 = typename std::enable_if< !std::is_same<Ud, variant>::value && !std::is_base_of<variant, Ud>::value && !detail::is_in_place_index<Ud>::value && !detail::is_in_place_type<Ud>::value >::type,
 
-#if BOOST_WORKAROUND(BOOST_MSVC, < 1950)
+#if BOOST_WORKAROUND(BOOST_MSVC, < 1960)
 
         class V = mp11::mp_apply_q< mp11::mp_bind_front<detail::resolve_overload_type, U&&>, variant >,
 

test/CMakeLists.txt

@@ -6,6 +6,6 @@ include(BoostTestJamfile OPTIONAL RESULT_VARIABLE HAVE_BOOST_TEST)
 
 if(HAVE_BOOST_TEST)
 
-boost_test_jamfile(FILE Jamfile LINK_LIBRARIES Boost::variant2 Boost::core Boost::container_hash)
+boost_test_jamfile(FILE Jamfile LINK_LIBRARIES Boost::variant2 Boost::core Boost::container_hash Boost::system)
 
 endif()

test/Jamfile

@@ -172,3 +172,12 @@ compile variant_default_construct_cx_5.cpp ;
 compile variant_value_construct_cx_2.cpp ;
 compile variant_value_construct_cx_3.cpp ;
 compile variant_value_construct_cx_4.cpp ;
+
+# GCC 12+ false positive -Wmaybe-uninitialized with non-trivially-copyable types
+run variant_issue_55.cpp
+  : : :
+    <library>/boost/system//boost_system
+
+    # clang-cl 32 bit fails with an assertion in mp_with_index, likely due to a codegen bug
+    "<toolset>clang-win,<address-model>32:<build>no"
+  ;

test/variant_default_construct_cx_5.cpp

@@ -10,9 +10,9 @@
 BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because __cpp_constexpr_dynamic_alloc < 201907L")
 int main() {}
 
-#elif defined(BOOST_MSVC) && BOOST_MSVC < 1950
+#elif defined(BOOST_MSVC) && BOOST_MSVC < 1960
 
-BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because BOOST_MSVC < 1950")
+BOOST_PRAGMA_MESSAGE("Skipping constexpr destructor test because BOOST_MSVC < 1960")
 int main() {}
 
 #elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000

test/variant_issue_55.cpp

@@ -0,0 +1,194 @@
+// Copyright 2025 Peter Dimov
+// Copyright 2025 Vinnie Falco
+//
+// 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
+
+// GCC 12+ -Wmaybe-uninitialized false positive tests
+// https://github.com/boostorg/variant2/issues/55
+//
+// GCC 12+'s improved dataflow analysis sees code paths for all alternatives
+// in mp_with_index and warns that members may be uninitialized, even though
+// the variant's discriminator guarantees only initialized alternatives are
+// accessed.
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config.hpp>
+#include <exception>
+#include <string>
+
+// Check for C++17 std::optional support
+#if BOOST_CXX_VERSION >= 201703L
+# include <optional>
+# define BOOST_VARIANT2_TEST_HAS_OPTIONAL 1
+#endif
+
+// Check for C++20 coroutine support
+#if defined(__cpp_impl_coroutine) && __cpp_impl_coroutine >= 201902L
+# include <coroutine>
+# define BOOST_VARIANT2_TEST_HAS_CORO 1
+#endif
+
+using result_void = boost::system::result<void, std::exception_ptr>;
+using result_string = boost::system::result<std::string, std::exception_ptr>;
+
+void testGccUninitialized()
+{
+    // Test 1: Simple copy construction
+    {
+        result_void r1;
+        result_void r2(r1);
+        (void)r2;
+    }
+
+    // Test 2: Copy assignment
+    {
+        result_void r1;
+        result_void r2;
+        r2 = r1;
+        (void)r2;
+    }
+
+#ifdef BOOST_VARIANT2_TEST_HAS_OPTIONAL
+    // Test 3: std::optional assignment (matches spawn pattern)
+    {
+        std::optional<result_void> opt;
+        opt = result_void{};
+        (void)opt;
+    }
+#endif
+
+    // Test 4: Pass to function via copy
+    {
+        auto fn = [](result_void r) { (void)r; };
+        fn(result_void{});
+    }
+
+#ifdef BOOST_VARIANT2_TEST_HAS_OPTIONAL
+    // Test 5: Lambda capture + optional (closest to spawn)
+    {
+        auto fn = [](result_void r) {
+            std::optional<result_void> opt;
+            opt = r;
+            return opt.has_value();
+        };
+        (void)fn(result_void{});
+    }
+#endif
+
+    // Test 6: Non-void result with string (triggers string warning)
+    {
+        result_string r1;
+        result_string r2(r1);
+        (void)r2;
+    }
+
+    // Test 7: Assign exception to result holding value
+    {
+        result_string r1{"hello"};
+        r1 = std::make_exception_ptr(std::runtime_error("test"));
+        (void)r1;
+    }
+
+#ifdef BOOST_VARIANT2_TEST_HAS_OPTIONAL
+    // Test 8: Optional with string result
+    {
+        std::optional<result_string> opt;
+        opt = result_string{};
+        (void)opt;
+    }
+#endif
+
+#ifdef BOOST_VARIANT2_TEST_HAS_CORO
+    // Minimal fire-and-forget coroutine for testing
+    struct fire_and_forget
+    {
+        struct promise_type
+        {
+            fire_and_forget get_return_object() { return {}; }
+            std::suspend_never initial_suspend() noexcept { return {}; }
+            std::suspend_never final_suspend() noexcept { return {}; }
+            void return_void() {}
+            void unhandled_exception() { std::terminate(); }
+        };
+    };
+
+    // Test 9: Coroutine returning result (mimics spawn)
+    {
+        auto coro = []() -> fire_and_forget {
+            result_void r{};
+            (void)r;
+            co_return;
+        };
+        coro();
+    }
+
+    // Test 10: Coroutine with handler call (closest to actual spawn)
+    {
+        std::optional<result_void> received;
+        auto handler = [&](result_void r) {
+            received = r;
+        };
+        auto coro = [&]() -> fire_and_forget {
+            handler(result_void{});
+            co_return;
+        };
+        coro();
+        (void)received;
+    }
+
+    // Test 11: Coroutine with try/catch like spawn
+    {
+        std::optional<result_void> received;
+        auto handler = [&](result_void r) {
+            received = r;
+        };
+        auto coro = [&]() -> fire_and_forget {
+            try
+            {
+                handler(result_void{});
+            }
+            catch (...)
+            {
+                handler(result_void{std::current_exception()});
+            }
+            co_return;
+        };
+        coro();
+        (void)received;
+    }
+
+    // Test 12: Coroutine with string result
+    {
+        std::optional<result_string> received;
+        auto handler = [&](result_string r) {
+            received = r;
+        };
+        auto coro = [&]() -> fire_and_forget {
+            try
+            {
+                handler(result_string{"test"});
+            }
+            catch (...)
+            {
+                handler(result_string{std::current_exception()});
+            }
+            co_return;
+        };
+        coro();
+        (void)received;
+    }
+#endif
+}
+
+int main()
+{
+    boost::core::lwt_init();
+
+    testGccUninitialized();
+
+    return boost::report_errors();
+}

test/variant_value_construct_cx_4.cpp

@@ -10,9 +10,9 @@
 BOOST_PRAGMA_MESSAGE("Skipping test because __cpp_constexpr_dynamic_alloc < 201907L")
 int main() {}
 
-#elif defined(BOOST_MSVC) && BOOST_MSVC < 1950
+#elif defined(BOOST_MSVC) && BOOST_MSVC < 1960
 
-BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_MSVC < 1950")
+BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_MSVC < 1960")
 int main() {}
 
 #elif defined(BOOST_CLANG) && BOOST_CLANG_VERSION < 180000