From f4f949fc2f73c78c594b92f3dfd4a868db7fc9c3 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Fri, 2 Jun 2017 05:14:40 +0300
Subject: [PATCH 1/4] Add converting constructors, subset<U...>()

---
 include/boost/variant2/variant.hpp | 103 ++++++++++++++++++++++++++++-
 test/variant_copy_construct.cpp    |   5 ++
 test/variant_move_construct.cpp    |   5 ++
 3 files changed, 111 insertions(+), 2 deletions(-)

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index 0df00df..cf41d4a 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -376,9 +376,10 @@ template<class T1, class... T> struct overload<T1, T...>: overload<T...>
 
 #if BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
 
-template<class U, class... T> struct resolve_overload_type_impl
+template<class U, class... T> using resolve_overload_type_ = decltype( overload<T...>()(std::declval<U>()) );
+
+template<class U, class... T> struct resolve_overload_type_impl: mp_defer< resolve_overload_type_, U, T... >
 {
-    using type = decltype( overload<T...>()(std::declval<U>()) );
 };
 
 template<class U, class... T> using resolve_overload_type = typename resolve_overload_type_impl<U, T...>::type::type;
@@ -991,6 +992,104 @@ public:
     }
 
     using variant_base::_get_impl;
+
+    // converting constructors (extension)
+
+    template<class... U,
+        class E2 = mp_if<mp_all<std::is_copy_constructible<U>..., mp_contains<mp_list<T...>, U>...>, void> >
+    variant( variant<U...> const& r )
+        noexcept( mp_all<std::is_nothrow_copy_constructible<U>...>::value )
+    {
+        mp_for_index<sizeof...(U)>( r.index(), [&]( auto I ){
+
+            using J = mp_find<mp_list<T...>, mp_at_c<mp_list<U...>, I>>;
+
+            ::new( static_cast<variant_base*>(this) ) variant_base( J{}, r._get_impl( I ) );
+
+        });
+    }
+
+    template<class... U,
+        class E2 = mp_if<mp_all<std::is_move_constructible<U>..., mp_contains<mp_list<T...>, U>...>, void> >
+    variant( variant<U...> && r )
+        noexcept( mp_all<std::is_nothrow_move_constructible<U>...>::value )
+    {
+        mp_for_index<sizeof...(U)>( r.index(), [&]( auto I ){
+
+            using J = mp_find<mp_list<T...>, mp_at_c<mp_list<U...>, I>>;
+
+            ::new( static_cast<variant_base*>(this) ) variant_base( J{}, std::move( r._get_impl( I ) ) );
+
+        });
+    }
+
+    // subset (extension)
+
+private:
+
+    template<class... U, class V, std::size_t J, class E = std::enable_if_t<J != sizeof...(U)>> static variant<U...> _subset_impl( mp_size_t<J>, V && v )
+    {
+        return variant<U...>( in_place_index<J>, std::forward<V>(v) );
+    }
+
+    template<class... U, class V> static variant<U...> _subset_impl( mp_size_t<sizeof...(U)>, V && /*v*/ )
+    {
+        throw bad_variant_access();
+    }
+
+public:
+
+    template<class... U,
+        class E2 = mp_if<mp_all<std::is_copy_constructible<U>..., mp_contains<mp_list<T...>, U>...>, void> >
+    constexpr variant<U...> subset() &
+    {
+        return mp_for_index<sizeof...(T)>( index(), [&]( auto I ){
+
+            using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
+
+            return _subset_impl<U...>( J{}, get<I>( *this ) );
+
+        });
+    }
+
+    template<class... U,
+        class E2 = mp_if<mp_all<std::is_copy_constructible<U>..., mp_contains<mp_list<T...>, U>...>, void> >
+    constexpr variant<U...> subset() const&
+    {
+        return mp_for_index<sizeof...(T)>( index(), [&]( auto I ){
+
+            using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
+
+            return _subset_impl<U...>( J{}, get<I>( *this ) );
+
+        });
+    }
+
+    template<class... U,
+        class E2 = mp_if<mp_all<std::is_copy_constructible<U>..., mp_contains<mp_list<T...>, U>...>, void> >
+    constexpr variant<U...> subset() &&
+    {
+        return mp_for_index<sizeof...(T)>( index(), [&]( auto I ){
+
+            using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
+
+            return _subset_impl<U...>( J{}, get<I>( std::move(*this) ) );
+
+        });
+    }
+
+    template<class... U,
+        class E2 = mp_if<mp_all<std::is_copy_constructible<U>..., mp_contains<mp_list<T...>, U>...>, void> >
+    constexpr variant<U...> subset() const&&
+    {
+        return mp_for_index<sizeof...(T)>( index(), [&]( auto I ){
+
+            using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
+
+            return _subset_impl<U...>( J{}, get<I>( std::move(*this) ) );
+
+        });
+    }
 };
 
 // relational operators
diff --git a/test/variant_copy_construct.cpp b/test/variant_copy_construct.cpp
index 473d8a2..6762776 100644
--- a/test/variant_copy_construct.cpp
+++ b/test/variant_copy_construct.cpp
@@ -121,7 +121,12 @@ int main()
         BOOST_TEST_TRAIT_FALSE((std::is_nothrow_copy_constructible<variant<X1, X2, int, int>>));
 
         BOOST_TEST_TRAIT_TRUE((std::is_copy_constructible<variant<X1, X2>>));
+
+#if !BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
+
         BOOST_TEST_TRAIT_FALSE((std::is_copy_constructible<variant<int, float, Y>>));
+
+#endif
     }
 
     return boost::report_errors();
diff --git a/test/variant_move_construct.cpp b/test/variant_move_construct.cpp
index e3d8e98..4561ebd 100644
--- a/test/variant_move_construct.cpp
+++ b/test/variant_move_construct.cpp
@@ -122,7 +122,12 @@ int main()
         BOOST_TEST_TRAIT_FALSE((std::is_nothrow_move_constructible<variant<X1, X2, int, int>>));
 
         BOOST_TEST_TRAIT_TRUE((std::is_move_constructible<variant<X1, X2>>));
+
+#if !BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
+
         BOOST_TEST_TRAIT_FALSE((std::is_move_constructible<variant<int, float, Y>>));
+
+#endif
     }
 
     return boost::report_errors();

From 2a6c157fcc98fdb74e3acb2210496a2160693b80 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Fri, 2 Jun 2017 05:15:51 +0300
Subject: [PATCH 2/4] Add preliminary expected<T, E...> implementation

---
 include/boost/variant2/expected.hpp | 367 ++++++++++++++++++++++++++++
 1 file changed, 367 insertions(+)
 create mode 100644 include/boost/variant2/expected.hpp

diff --git a/include/boost/variant2/expected.hpp b/include/boost/variant2/expected.hpp
new file mode 100644
index 0000000..f6651f3
--- /dev/null
+++ b/include/boost/variant2/expected.hpp
@@ -0,0 +1,367 @@
+#ifndef BOOST_VARIANT2_EXPECTED_HPP_INCLUDED
+#define BOOST_VARIANT2_EXPECTED_HPP_INCLUDED
+
+//  Copyright 2017 Peter Dimov.
+//
+//  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
+
+#ifndef BOOST_VARIANT2_VARIANT_HPP_INCLUDED
+#include <boost/variant2/variant.hpp>
+#endif
+#include <boost/core/demangle.hpp>
+#include <system_error>
+#include <type_traits>
+#include <typeinfo>
+#include <cassert>
+
+//
+
+namespace boost
+{
+namespace variant2
+{
+
+// unexpected_
+
+template<class... E> using unexpected_ = variant<E...>;
+
+// bad_expected_access
+
+class bad_expected_access: public std::exception
+{
+private:
+
+    std::string msg_;
+
+public:
+
+    bad_expected_access() noexcept
+    {
+    }
+
+    template<class E> explicit bad_expected_access( mp_identity<E> ) noexcept: msg_( "bad_expected_access: " + boost::core::demangle( typeid(E).name() ) )
+    {
+    }
+
+    char const * what() const noexcept
+    {
+        return msg_.empty()? "bad_expected_access": msg_.c_str();
+    }
+};
+
+// expected
+
+template<class E> void throw_on_unexpected( E const & /*e*/ )
+{
+    throw bad_expected_access( mp_identity<E>() );
+}
+
+void throw_on_unexpected( std::error_code const & e )
+{
+    throw std::system_error( e );
+}
+
+void throw_on_unexpected( std::exception_ptr const & e )
+{
+    if( e )
+    {
+        std::rethrow_exception( e );
+    }
+    else
+    {
+        throw bad_expected_access( mp_identity<std::exception_ptr>() );
+    }
+}
+
+template<class T, class... E> class expected
+{
+private:
+
+    variant<T, E...> v_;
+
+private:
+
+    void _bad_access() const
+    {
+        mp_for_index<mp_size<expected>>( v_.index(), [&]( auto I )
+        {
+            if( I == 0 )
+            {
+                throw bad_expected_access( mp_identity<T>() );
+            }
+            else
+            {
+                throw_on_unexpected( get<I>(v_) );
+            }
+        });
+    }
+
+public:
+
+    // value constructors
+
+    constexpr expected() noexcept( std::is_nothrow_default_constructible<T>::value )
+    {
+    }
+
+    constexpr expected( T const& t ) noexcept( std::is_nothrow_copy_constructible<T>::value ): v_( in_place_index<0>, t )
+    {
+    }
+
+    constexpr expected( T && t ) noexcept( std::is_nothrow_move_constructible<T>::value ): v_( in_place_index<0>, std::move(t) )
+    {
+    }
+
+    // template<class U> constexpr expected( U && u ); where U in E...?
+
+    // in-place constructor?
+
+    // unexpected constructor
+
+    template<class... E2,
+        class En = mp_if<mp_all<std::is_copy_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
+    constexpr expected( unexpected_<E2...> const & x ): v_( x )
+    {
+    }
+
+    template<class... E2,
+        class En = mp_if<mp_all<std::is_copy_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
+    constexpr expected( unexpected_<E2...> && x ): v_( std::move(x) )
+    {
+    }
+
+    // conversion constructor
+
+    template<class... E2,
+        class En = mp_if<mp_all<std::is_copy_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
+    constexpr expected( expected<T, E2...> const & x ): v_( x.v_ )
+    {
+    }
+
+    template<class... E2,
+        class En = mp_if<mp_all<std::is_move_constructible<E2>..., mp_contains<mp_list<E...>, E2>...>, void>>
+    constexpr expected( expected<T, E2...> && x ): v_( std::move(x.v_) )
+    {
+    }
+
+    // emplace
+
+    template<class... A> void emplace( A&&... a )
+    {
+        v_.emplace( std::forward<A>(a)... );
+    }
+
+    template<class V, class... A> void emplace( std::initializer_list<V> il, A&&... a )
+    {
+        v_.emplace( il, std::forward<A>(a)... );
+    }
+
+    // swap
+
+    void swap( expected & r ) noexcept( noexcept( v_.swap( r.v_ ) ) )
+    {
+        v_.swap( r.v_ );
+    }
+
+    // value queries
+
+    constexpr bool has_value() const noexcept
+    {
+        return v_.index() == 0;
+    }
+
+    constexpr explicit operator bool() const noexcept
+    {
+        return v_.index() == 0;
+    }
+
+    // checked value access
+
+    constexpr T& value() &
+    {
+        if( !has_value() )
+        {
+            _bad_access();
+        }
+
+        return *get_if<0>(&v_);
+    }
+
+    constexpr T const& value() const&
+    {
+        if( !has_value() )
+        {
+            _bad_access();
+        }
+
+        return *get_if<0>(&v_);
+    }
+
+    constexpr T&& value() &&
+    {
+        return std::move( value() );
+    }
+
+    constexpr T const&& value() const&&
+    {
+        return std::move( value() );
+    }
+
+    // unchecked value access
+
+    T* operator->() noexcept
+    {
+        return get_if<0>(&v_);
+    }
+
+    T const* operator->() const noexcept
+    {
+        return get_if<0>(&v_);
+    }
+
+    T& operator*() & noexcept
+    {
+        T* p = get_if<0>(&v_);
+
+        assert( p != 0 );
+
+        return *p;
+    }
+
+    T const& operator*() const & noexcept
+    {
+        T const* p = get_if<0>(&v_);
+
+        assert( p != 0 );
+
+        return *p;
+    }
+
+    T&& operator*() && noexcept
+    {
+        return std::move(**this);
+    }
+
+    T const&& operator*() const && noexcept
+    {
+        return std::move(**this);
+    }
+
+    // error queries
+
+    template<class E2> constexpr bool has_error() const noexcept
+    {
+        using I = mp_find<expected, E2>;
+        return v_.index() == I::value;
+    }
+
+    constexpr bool has_error() const noexcept
+    {
+        static_assert( sizeof...(E) == 1, "has_error() is only valid when there is a single E" );
+        return has_error<mp_first<expected>>();
+    }
+
+    // error access
+
+    unexpected_<E...> unexpected() const
+    {
+        if( has_value() )
+        {
+            _bad_access();
+        }
+
+        return v_.template subset<E...>();
+    }
+
+    template<class E2> constexpr E2 error() const noexcept
+    {
+        using I = mp_find<expected, E2>;
+
+        if( v_.index() != I::value )
+        {
+            _bad_access();
+        }
+
+        return get<I>( v_ );
+    }
+
+    constexpr mp_first<expected> error() const noexcept
+    {
+        static_assert( sizeof...(E) == 1, "error() is only valid when there is a single E" );
+        return error<mp_first<expected>>();
+    }
+
+    // error mapping
+
+private:
+
+    template<class F> struct Qret
+    {
+        template<class... A> using fn = decltype( std::declval<F>()( std::declval<A>()... ) );
+    };
+
+    template<class F> using remapped = mp_append<expected<T>, mp_unique<mp_transform_q<Qret<F>, mp_list<E...>>>>;
+
+    template<class R, std::size_t I, class F, class V> static R _remap_error( mp_size_t<I>, F && f, V && v )
+    {
+        // return R( std::forward<F>(f)( std::forward<V>(v) ) );
+
+        auto e = std::forward<F>(f)( std::forward<V>(v) );
+
+        return unexpected_<decltype(e)>{ e };
+    }
+
+    template<class R, class F, class V> static R _remap_error( mp_size_t<0>, F && /*f*/, V && v )
+    {
+        return R( std::forward<V>(v) );
+    }
+
+public:
+
+    template<class F> remapped<F> remap_errors( F && f ) const
+    {
+        using R = remapped<F>;
+
+        return mp_for_index<mp_size<expected>>( v_.index(), [&]( auto I ) {
+
+            return _remap_error<R>( I, f, get<I>(v_) );
+
+        });
+    }
+
+    expected<T, std::error_code> remap_errors()
+    {
+        using R = expected<T, std::error_code>;
+
+        auto f = []( auto const& e ){ return make_error_code(e); };
+
+        return mp_for_index<mp_size<expected>>( v_.index(), [&]( auto I ) {
+
+            return _remap_error<R>( I, f, get<I>(v_) );
+
+        });
+    }
+};
+
+template<class T, class... E> inline constexpr bool operator==( expected<T, E...> const & x1, expected<T, E...> const & x2 )
+{
+    return x1.v_ == x2.v_;
+}
+
+template<class T, class... E> inline constexpr bool operator!=( expected<T, E...> const & x1, expected<T, E...> const & x2 )
+{
+    return x1.v_ != x2.v_;
+}
+
+template<class T, class... E> inline void swap( expected<T, E...> & x1, expected<T, E...> & x2 ) noexcept( noexcept( x1.swap( x2 ) ) )
+{
+    x1.swap( x2 );
+}
+
+} // namespace variant2
+} // namespace boost
+
+#endif // #ifndef BOOST_VARIANT2_EXPECTED_HPP_INCLUDED

From c99bf2205cc3f23e5310ea97f18db6eb711e7410 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Fri, 2 Jun 2017 05:41:13 +0300
Subject: [PATCH 3/4] Move index() into variant_base to avoid needless ix_ >= 0
 checks when single buffered

---
 include/boost/variant2/variant.hpp | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index cf41d4a..9f3a980 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -413,6 +413,11 @@ template<class... T> struct variant_base_impl<true, true, T...>
     {
     }
 
+    constexpr std::size_t index() const noexcept
+    {
+        return ix_ - 1;
+    }
+
     template<std::size_t I> constexpr mp_at_c<variant<T...>, I>& _get_impl( mp_size_t<I> ) noexcept
     {
         size_t const J = I+1;
@@ -486,6 +491,11 @@ template<class... T> struct variant_base_impl<true, false, T...>
     {
     }
 
+    constexpr std::size_t index() const noexcept
+    {
+        return ix_ >= 0? ix_ - 1: -ix_ - 1;
+    }
+
     template<std::size_t I> constexpr mp_at_c<variant<T...>, I>& _get_impl( mp_size_t<I> ) noexcept
     {
         size_t const J = I+1;
@@ -555,6 +565,11 @@ template<class... T> struct variant_base_impl<false, true, T...>
         _destroy();
     }
 
+    constexpr std::size_t index() const noexcept
+    {
+        return ix_ - 1;
+    }
+
     template<std::size_t I> constexpr mp_at_c<variant<T...>, I>& _get_impl( mp_size_t<I> ) noexcept
     {
         size_t const J = I+1;
@@ -661,6 +676,11 @@ template<class... T> struct variant_base_impl<false, false, T...>
         _destroy();
     }
 
+    constexpr std::size_t index() const noexcept
+    {
+        return ix_ >= 0? ix_ - 1: -ix_ - 1;
+    }
+
     template<std::size_t I> constexpr mp_at_c<variant<T...>, I>& _get_impl( mp_size_t<I> ) noexcept
     {
         size_t const J = I+1;
@@ -958,10 +978,7 @@ public:
 
     // value status
 
-    constexpr size_t index() const noexcept
-    {
-        return this->ix_ >= 0? this->ix_ - 1 : -this->ix_ - 1;
-    }
+    using variant_base::index;
 
     // swap
 

From d55db1dbc6c59c9f17314810d7c0d34b67f5bada Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Fri, 2 Jun 2017 06:02:50 +0300
Subject: [PATCH 4/4] Replace use of get<> with ._get_impl for better codegen

---
 include/boost/variant2/variant.hpp | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index 9f3a980..9c6b47d 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -892,11 +892,11 @@ public:
 
             if( this->index() == I )
             {
-                get<I>(*this) = get<I>(r);
+                this->_get_impl( I ) = r._get_impl( I );
             }
             else
             {
-                this->variant_base::template emplace<I>( get<I>(r) );
+                this->variant_base::template emplace<I>( r._get_impl( I ) );
             }
 
         });
@@ -912,11 +912,11 @@ public:
 
             if( this->index() == I )
             {
-                get<I>(*this) = get<I>(std::move(r));
+                this->_get_impl( I ) = std::move( r._get_impl( I ) );
             }
             else
             {
-                this->variant_base::template emplace<I>( get<I>(std::move(r)) );
+                this->variant_base::template emplace<I>( std::move( r._get_impl( I ) ) );
             }
 
         });
@@ -989,7 +989,7 @@ public:
             mp_for_index<sizeof...(T)>( index(), [&]( auto I ){
 
                 using std::swap;
-                swap( get<I>(*this), get<I>(r) );
+                swap( this->_get_impl( I ), r._get_impl( I ) );
 
             });
         }
@@ -1064,7 +1064,7 @@ public:
 
             using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
 
-            return _subset_impl<U...>( J{}, get<I>( *this ) );
+            return _subset_impl<U...>( J{}, this->_get_impl( I ) );
 
         });
     }
@@ -1077,7 +1077,7 @@ public:
 
             using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
 
-            return _subset_impl<U...>( J{}, get<I>( *this ) );
+            return _subset_impl<U...>( J{}, this->_get_impl( I ) );
 
         });
     }
@@ -1090,7 +1090,7 @@ public:
 
             using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
 
-            return _subset_impl<U...>( J{}, get<I>( std::move(*this) ) );
+            return _subset_impl<U...>( J{}, std::move( this->_get_impl( I ) ) );
 
         });
     }
@@ -1103,7 +1103,7 @@ public:
 
             using J = mp_find<mp_list<U...>, mp_at_c<mp_list<T...>, I>>;
 
-            return _subset_impl<U...>( J{}, get<I>( std::move(*this) ) );
+            return _subset_impl<U...>( J{}, std::move( this->_get_impl( I ) ) );
 
         });
     }
@@ -1116,7 +1116,7 @@ template<class... T> constexpr bool operator==( variant<T...> const & v, variant
 
     return mp_for_index<sizeof...(T)>( v.index(), [&]( auto I ){
 
-        return get<I>(v) == get<I>(w);
+        return v._get_impl( I ) == w._get_impl( I );
 
     });
 }
@@ -1127,7 +1127,7 @@ template<class... T> constexpr bool operator!=( variant<T...> const & v, variant
 
     return mp_for_index<sizeof...(T)>( v.index(), [&]( auto I ){
 
-        return get<I>(v) != get<I>(w);
+        return v._get_impl( I ) != w._get_impl( I );
 
     });
 }
@@ -1139,7 +1139,7 @@ template<class... T> constexpr bool operator<( variant<T...> const & v, variant<
 
     return mp_for_index<sizeof...(T)>( v.index(), [&]( auto I ){
 
-        return get<I>(v) < get<I>(w);
+        return v._get_impl( I ) < w._get_impl( I );
 
     });
 }
@@ -1151,7 +1151,7 @@ template<class... T> constexpr bool operator>(  variant<T...> const & v, variant
 
     return mp_for_index<sizeof...(T)>( v.index(), [&]( auto I ){
 
-        return get<I>(v) > get<I>(w);
+        return v._get_impl( I ) > w._get_impl( I );
 
     });
 }
@@ -1163,7 +1163,7 @@ template<class... T> constexpr bool operator<=( variant<T...> const & v, variant
 
     return mp_for_index<sizeof...(T)>( v.index(), [&]( auto I ){
 
-        return get<I>(v) <= get<I>(w);
+        return v._get_impl( I ) <= w._get_impl( I );
 
     });
 }
@@ -1175,7 +1175,7 @@ template<class... T> constexpr bool operator>=( variant<T...> const & v, variant
 
     return mp_for_index<sizeof...(T)>( v.index(), [&]( auto I ){
 
-        return get<I>(v) >= get<I>(w);
+        return v._get_impl( I ) >= w._get_impl( I );
 
     });
 }