Add cxxstd=latest to Appveyor

.github/workflows/ci.yml

@@ -17,10 +17,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - toolset: gcc-4.7
-            cxxstd: "03,11"
-            os: ubuntu-16.04
-            install: g++-4.7
           - toolset: gcc-4.8
             cxxstd: "03,11"
             os: ubuntu-16.04

.travis.yml

@@ -95,7 +95,7 @@ matrix:
     - os: linux
       dist: bionic
       compiler: g++-10
-      env: TOOLSET=gcc COMPILER=g++-10 CXXSTD=11,14,17,2a
+      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-10 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
       addons:
         apt:
           packages:
@@ -103,21 +103,6 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
-    - os: linux
-      compiler: g++-8
-      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-8 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
-      addons:
-        apt:
-          packages:
-            - g++-8
-          sources:
-            - ubuntu-toolchain-r-test
-
-    - os: linux
-      dist: trusty
-      compiler: clang++
-      env: DIST=trusty TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z
-
     - os: linux
       compiler: clang++-3.5
       env: TOOLSET=clang COMPILER=clang++-3.5 CXXSTD=11,14,1z
@@ -248,15 +233,17 @@ matrix:
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - os: linux
-      compiler: clang++-8
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-8 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
+      dist: xenial
+      compiler: clang++-11
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-11 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
       addons:
         apt:
           packages:
-            - clang-8
+            - clang-11
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-xenial-8
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-11 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - os: linux
       dist: trusty
@@ -268,18 +255,14 @@ matrix:
             - libc++-dev
 
     - os: linux
-      dist: trusty
+      dist: bionic
       compiler: clang++-libc++
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=11,14,1z UBSAN_OPTIONS=print_stacktrace=1
+      env: TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=11,14,17,2a
       addons:
         apt:
           packages:
             - libc++-dev
 
-    - os: osx
-      compiler: clang++
-      env: TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z
-
     - os: osx
       compiler: clang++
       env: UBSAN=1 TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z UBSAN_OPTIONS=print_stacktrace=1

appveyor.yml

@@ -20,15 +20,15 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       TOOLSET: msvc-14.1
       ADDRMD: 32,64
-      CXXSTD: 14,17
+      CXXSTD: 14,17,latest
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       TOOLSET: clang-win
       ADDRMD: 64
-      CXXSTD: 14,17
+      CXXSTD: 14,17,latest
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: msvc-14.2
       ADDRMD: 32,64
-      CXXSTD: 14,17
+      CXXSTD: 14,17,latest
 
 install:
   - set BOOST_BRANCH=develop

doc/variant2/changelog.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2019, 2020 Peter Dimov
+Copyright 2019-2021 Peter Dimov
 
 Distributed under the Boost Software License, Version 1.0.
 
@@ -11,6 +11,10 @@ http://www.boost.org/LICENSE_1_0.txt
 # Revision History
 :idprefix: changelog_
 
+## Changes in 1.76.0
+
+* Improved generated code for the double buffered case.
+
 ## Changes in 1.74.0
 
 * Added support for derived types in `visit`

doc/variant2/reference.adoc

@@ -490,8 +490,8 @@ template<size_t I, class... A>
 +
 Requires: :: `I < sizeof...(T)`.
 Effects:  ::
-  Destroys the currently contained value, then initializes a new contained
-  value as if using the expression `Ti(std::forward<A>(a)...)`.
+  Initializes a new contained value as if using the expression
+  `Ti(std::forward<A>(a)...)`, then destroys the currently contained value.
 Ensures:  :: `index() == I`.
 Returns:  :: A reference to the new contained value.
 Throws:   ::
@@ -511,8 +511,8 @@ template<size_t I, class V, class... A>
 +
 Requires: :: `I < sizeof...(T)`.
 Effects:  ::
-  Destroys the currently contained value, then initializes a new contained
-  value as if using the expression `Ti(il, std::forward<A>(a)...)`.
+  Initializes a new contained value as if using the expression
+  `Ti(il, std::forward<A>(a)...)`, then destroys the currently contained value.
 Ensures:  :: `index() == I`.
 Returns:  :: A reference to the new contained value.
 Throws:   ::

include/boost/variant2/variant.hpp

@@ -81,6 +81,8 @@ struct monostate
 {
 };
 
+#if !BOOST_WORKAROUND(BOOST_MSVC, < 1940)
+
 constexpr bool operator<(monostate, monostate) noexcept { return false; }
 constexpr bool operator>(monostate, monostate) noexcept { return false; }
 constexpr bool operator<=(monostate, monostate) noexcept { return true; }
@@ -88,6 +90,17 @@ constexpr bool operator>=(monostate, monostate) noexcept { return true; }
 constexpr bool operator==(monostate, monostate) noexcept { return true; }
 constexpr bool operator!=(monostate, monostate) noexcept { return false; }
 
+#else
+
+constexpr bool operator<(monostate const&, monostate const&) noexcept { return false; }
+constexpr bool operator>(monostate const&, monostate const&) noexcept { return false; }
+constexpr bool operator<=(monostate const&, monostate const&) noexcept { return true; }
+constexpr bool operator>=(monostate const&, monostate const&) noexcept { return true; }
+constexpr bool operator==(monostate const&, monostate const&) noexcept { return true; }
+constexpr bool operator!=(monostate const&, monostate const&) noexcept { return false; }
+
+#endif
+
 // variant forward declaration
 
 template<class... T> class variant;
@@ -520,11 +533,11 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_false, T1, T.
     T1 first_;
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
     {
     }
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
+    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)... )
     {
     }
 
@@ -564,18 +577,18 @@ template<class T0, class T1, class T2, class T3, class T4, class T5, class T6, c
 
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
+    template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
 
     ~variant_storage_impl()
     {
@@ -637,11 +650,11 @@ template<class T1, class... T> union variant_storage_impl<mp11::mp_true, T1, T..
     T1 first_;
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): first_( std::forward<A>(a)... )
     {
     }
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-1>(), std::forward<A>(a)... )
+    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)... )
     {
     }
 
@@ -687,18 +700,18 @@ template<class T0, class T1, class T2, class T3, class T4, class T5, class T6, c
 
     variant_storage<T...> rest_;
 
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
-    template<class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<0>, A&&... a ): t0_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<1>, A&&... a ): t1_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<2>, A&&... a ): t2_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<3>, A&&... a ): t3_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<4>, A&&... a ): t4_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<5>, A&&... a ): t5_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<6>, A&&... a ): t6_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<7>, A&&... a ): t7_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<8>, A&&... a ): t8_( std::forward<A>(a)... ) {}
+    template<class... A> constexpr variant_storage_impl( mp11::mp_size_t<9>, A&&... a ): t9_( std::forward<A>(a)... ) {}
 
-    template<std::size_t I, class... A> constexpr explicit variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
+    template<std::size_t I, class... A> constexpr variant_storage_impl( mp11::mp_size_t<I>, A&&... a ): rest_( mp11::mp_size_t<I-10>(), std::forward<A>(a)... ) {}
 
     template<class... A> void emplace_impl( mp11::mp_false, mp11::mp_size_t<0>, A&&... a ) { ::new( &t0_ ) T0( std::forward<A>(a)... ); }
     template<class... A> void emplace_impl( mp11::mp_false, mp11::mp_size_t<1>, A&&... a ) { ::new( &t1_ ) T1( std::forward<A>(a)... ); }
@@ -814,21 +827,21 @@ struct none {};
 // trivially destructible, single buffered
 template<class... T> struct variant_base_impl<true, true, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
+    unsigned ix_;
+    variant_storage<none, T...> st_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): ix_( 0 ), st_( mp11::mp_size_t<0>() )
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
     {
     }
 
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ::new( &st_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
         ix_ = I::value + 1;
     }
 
@@ -843,7 +856,7 @@ template<class... T> struct variant_base_impl<true, true, T...>
 
         assert( ix_ == J );
 
-        return st1_.get( mp11::mp_size_t<J>() );
+        return st_.get( mp11::mp_size_t<J>() );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
@@ -851,14 +864,14 @@ template<class... T> struct variant_base_impl<true, true, T...>
         // size_t const J = I+1;
         // assert( ix_ == I+1 );
 
-        return st1_.get( mp11::mp_size_t<I+1>() );
+        return st_.get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t J, class U, class... A> BOOST_CXX14_CONSTEXPR void emplace_impl( mp11::mp_true, A&&... a )
     {
         static_assert( std::is_nothrow_constructible<U, A&&...>::value, "Logic error: U must be nothrow constructible from A&&..." );
 
-        st1_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
+        st_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
         ix_ = J;
     }
 
@@ -868,7 +881,7 @@ template<class... T> struct variant_base_impl<true, true, T...>
 
         U tmp( std::forward<A>(a)... );
 
-        st1_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
+        st_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
         ix_ = J;
     }
 
@@ -884,84 +897,78 @@ template<class... T> struct variant_base_impl<true, true, T...>
 // trivially destructible, double buffered
 template<class... T> struct variant_base_impl<true, false, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
-    variant_storage<none, T...> st2_;
+    unsigned ix_;
+    variant_storage<none, T...> st_[ 2 ];
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() ), st2_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): ix_( 0 ), st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( ( I::value + 1 ) * 2 ), st_{ { mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... }, { mp11::mp_size_t<0>() } }
     {
     }
 
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
-        ix_ = I::value + 1;
+        ::new( &st_[ 0 ] ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ix_ = ( I::value + 1 ) * 2;
     }
 
     constexpr std::size_t index() const noexcept
     {
-        return ix_ >= 0? ix_ - 1: -ix_ - 1;
+        return ix_ / 2 - 1;
     }
 
     template<std::size_t I> BOOST_CXX14_CONSTEXPR mp11::mp_at_c<variant<T...>, I>& _get_impl( mp11::mp_size_t<I> ) noexcept
     {
+        assert( index() == I );
+
         size_t const J = I+1;
 
-        assert( ix_ == J || -ix_ == J );
-
         constexpr mp11::mp_size_t<J> j{};
-        return ix_ >= 0? st1_.get( j ): st2_.get( j );
+        return st_[ ix_ & 1 ].get( j );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
     {
+        // assert( index() == I );
         // size_t const J = I+1;
-        // assert( ix_ == J || -ix_ == J );
         // constexpr mp_size_t<J> j{};
 
-        return ix_ >= 0? st1_.get( mp11::mp_size_t<I+1>() ): st2_.get( mp11::mp_size_t<I+1>() );
+        return st_[ ix_ & 1 ].get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t I, class... A> BOOST_CXX14_CONSTEXPR void emplace( A&&... a )
     {
         size_t const J = I+1;
 
-        if( ix_ >= 0 )
-        {
-            st2_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            ix_ = -static_cast<int>( J );
-        }
-        else
-        {
-            st1_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            ix_ = J;
-        }
+        unsigned i2 = 1 - ( ix_ & 1 );
+
+        st_[ i2 ].emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
+
+        ix_ = J * 2 + i2;
     }
 };
 
 // not trivially destructible, single buffered
 template<class... T> struct variant_base_impl<false, true, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
+    unsigned ix_;
+    variant_storage<none, T...> st_;
 
-    constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() )
+    constexpr variant_base_impl(): ix_( 0 ), st_( mp11::mp_size_t<0>() )
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... )
     {
     }
 
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ::new( &st_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
         ix_ = I::value + 1;
     }
 
@@ -977,7 +984,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
         template<class I> void operator()( I ) const noexcept
         {
             using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
-            this_->st1_.get( I() ).~U();
+            this_->st_.get( I() ).~U();
         }
     };
 
@@ -1005,7 +1012,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
 
         assert( ix_ == J );
 
-        return st1_.get( mp11::mp_size_t<J>() );
+        return st_.get( mp11::mp_size_t<J>() );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
@@ -1013,7 +1020,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
         // size_t const J = I+1;
         // assert( ix_ == J );
 
-        return st1_.get( mp11::mp_size_t<I+1>() );
+        return st_.get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t I, class... A> void emplace( A&&... a )
@@ -1028,7 +1035,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
 
         _destroy();
 
-        st1_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
+        st_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
         ix_ = J;
     }
 };
@@ -1036,23 +1043,61 @@ template<class... T> struct variant_base_impl<false, true, T...>
 // not trivially destructible, double buffered
 template<class... T> struct variant_base_impl<false, false, T...>
 {
-    int ix_;
-    variant_storage<none, T...> st1_;
-    variant_storage<none, T...> st2_;
+    unsigned ix_;
+
+#if defined(__GNUC__) && __GNUC__ < 11 && !defined(__clang__) && !defined(__INTEL_COMPILER)
+
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63707 :-(
+
+    variant_storage<none, T...> st1_, st2_;
 
     constexpr variant_base_impl(): ix_( 0 ), st1_( mp11::mp_size_t<0>() ), st2_( mp11::mp_size_t<0>() )
     {
     }
 
-    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( I::value + 1 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() )
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( ( I::value + 1 ) * 2 ), st1_( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... ), st2_( mp11::mp_size_t<0>() )
     {
     }
 
+    BOOST_CXX14_CONSTEXPR variant_storage<none, T...>& storage( unsigned i2 ) noexcept
+    {
+        return i2 == 0? st1_: st2_;
+    }
+
+    constexpr variant_storage<none, T...> const& storage( unsigned i2 ) const noexcept
+    {
+        return i2 == 0? st1_: st2_;
+    }
+
+#else
+
+    variant_storage<none, T...> st_[ 2 ];
+
+    constexpr variant_base_impl(): ix_( 0 ), st_{ { mp11::mp_size_t<0>() }, { mp11::mp_size_t<0>() } }
+    {
+    }
+
+    template<class I, class... A> constexpr explicit variant_base_impl( I, A&&... a ): ix_( ( I::value + 1 ) * 2 ), st_{ { mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... }, { mp11::mp_size_t<0>() } }
+    {
+    }
+
+    BOOST_CXX14_CONSTEXPR variant_storage<none, T...>& storage( unsigned i2 ) noexcept
+    {
+        return st_[ i2 ];
+    }
+
+    constexpr variant_storage<none, T...> const& storage( unsigned i2 ) const noexcept
+    {
+        return st_[ i2 ];
+    }
+
+#endif
+
     // requires: ix_ == 0
     template<class I, class... A> void _replace( I, A&&... a )
     {
-        ::new( &st1_ ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
-        ix_ = I::value + 1;
+        ::new( &storage( 0 ) ) variant_storage<none, T...>( mp11::mp_size_t<I::value + 1>(), std::forward<A>(a)... );
+        ix_ = ( I::value + 1 ) * 2;
     }
 
     //[&]( auto I ){
@@ -1063,35 +1108,18 @@ template<class... T> struct variant_base_impl<false, false, T...>
     struct _destroy_L1
     {
         variant_base_impl * this_;
+        unsigned i2_;
 
         template<class I> void operator()( I ) const noexcept
         {
             using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
-            this_->st1_.get( I() ).~U();
-        }
-    };
-
-    struct _destroy_L2
-    {
-        variant_base_impl * this_;
-
-        template<class I> void operator()( I ) const noexcept
-        {
-            using U = mp11::mp_at<mp11::mp_list<none, T...>, I>;
-            this_->st2_.get( I() ).~U();
+            this_->storage( i2_ ).get( I() ).~U();
         }
     };
 
     void _destroy() noexcept
     {
-        if( ix_ > 0 )
-        {
-            mp11::mp_with_index<1 + sizeof...(T)>( ix_, _destroy_L1{ this } );
-        }
-        else if( ix_ < 0 )
-        {
-            mp11::mp_with_index<1 + sizeof...(T)>( -ix_, _destroy_L2{ this } );
-        }
+        mp11::mp_with_index<1 + sizeof...(T)>( ix_ / 2, _destroy_L1{ this, ix_ & 1 } );
     }
 
     ~variant_base_impl() noexcept
@@ -1101,46 +1129,38 @@ template<class... T> struct variant_base_impl<false, false, T...>
 
     constexpr std::size_t index() const noexcept
     {
-        return ix_ >= 0? ix_ - 1: -ix_ - 1;
+        return ix_ / 2 - 1;
     }
 
     template<std::size_t I> BOOST_CXX14_CONSTEXPR mp11::mp_at_c<variant<T...>, I>& _get_impl( mp11::mp_size_t<I> ) noexcept
     {
+        assert( index() == I );
+
         size_t const J = I+1;
 
-        assert( ix_ == J || -ix_ == J );
-
         constexpr mp11::mp_size_t<J> j{};
-        return ix_ >= 0? st1_.get( j ): st2_.get( j );
+        return storage( ix_ & 1 ).get( j );
     }
 
     template<std::size_t I> constexpr mp11::mp_at_c<variant<T...>, I> const& _get_impl( mp11::mp_size_t<I> ) const noexcept
     {
+        // assert( index() == I );
         // size_t const J = I+1;
-        // assert( ix_ == J || -ix_ == J );
         // constexpr mp_size_t<J> j{};
 
-        return ix_ >= 0? st1_.get( mp11::mp_size_t<I+1>() ): st2_.get( mp11::mp_size_t<I+1>() );
+        return storage( ix_ & 1 ).get( mp11::mp_size_t<I+1>() );
     }
 
     template<std::size_t I, class... A> void emplace( A&&... a )
     {
         size_t const J = I+1;
 
-        if( ix_ >= 0 )
-        {
-            st2_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            _destroy();
+        unsigned i2 = 1 - ( ix_ & 1 );
 
-            ix_ = -static_cast<int>( J );
-        }
-        else
-        {
-            st1_.emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
-            _destroy();
+        storage( i2 ).emplace( mp11::mp_size_t<J>(), std::forward<A>(a)... );
+        _destroy();
 
-            ix_ = J;
-        }
+        ix_ = J * 2 + i2;
     }
 };
 
@@ -2085,7 +2105,7 @@ template<class R, class F, class V1, class V2> struct visit_L2
     template<class I> auto operator()( I ) const -> Vret<R, F, V1, V2>
     {
         auto f2 = bind_front( std::forward<F>(f), unsafe_get<I::value>( std::forward<V1>(v1) ) );
-		return visit<R>( f2, std::forward<V2>(v2) );
+        return visit<R>( f2, std::forward<V2>(v2) );
     }
 };
 

test/variant_eq_ne.cpp

@@ -88,5 +88,12 @@ int main()
         BOOST_TEST_NOT( v1 != v2 );
     }
 
+    {
+        variant<monostate> v1, v2;
+
+        BOOST_TEST( v1 == v2 );
+        BOOST_TEST_NOT( v1 != v2 );
+    }
+
     return boost::report_errors();
 }

test/variant_eq_ne_cx.cpp

@@ -93,6 +93,13 @@ int main()
         STATIC_ASSERT( !(v1 == v2) );
         STATIC_ASSERT( !(v1 != v2) );
     }
+
+    {
+        constexpr variant<monostate> v1, v2;
+
+        STATIC_ASSERT( v1 == v2 );
+        STATIC_ASSERT( !(v1 != v2) );
+    }
 }
 
 #endif

test/variant_in_place_index_construct_cx.cpp

@@ -105,7 +105,7 @@ int main()
         STATIC_ASSERT( v.index() == 4 );
     }
 
-#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 100200)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 110000)
 
     // no idea why this fails on g++ 10
 

test/variant_in_place_type_construct_cx.cpp

@@ -102,7 +102,7 @@ int main()
         STATIC_ASSERT( holds_alternative<X>(v) );
     }
 
-#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 100200)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 100000 && BOOST_GCC < 110000)
 
     // no idea why this fails on g++ 10
 

test/variant_lt_gt.cpp

@@ -81,5 +81,14 @@ int main()
         BOOST_TEST_NOT( v1 >= v2 );
     }
 
+    {
+        variant<monostate> v1, v2;
+
+        BOOST_TEST_NOT( v1 < v2 );
+        BOOST_TEST_NOT( v1 > v2 );
+        BOOST_TEST( v1 <= v2 );
+        BOOST_TEST( v1 >= v2 );
+    }
+
     return boost::report_errors();
 }

test/variant_lt_gt_cx.cpp

@@ -86,6 +86,15 @@ int main()
         STATIC_ASSERT( !(v1 <= v2) );
         STATIC_ASSERT( !(v1 >= v2) );
     }
+
+    {
+        constexpr variant<monostate> v1, v2;
+
+        STATIC_ASSERT( !(v1 < v2) );
+        STATIC_ASSERT( !(v1 > v2) );
+        STATIC_ASSERT( v1 <= v2 );
+        STATIC_ASSERT( v1 >= v2 );
+    }
 }
 
 #endif