From 0997734e5cfd1ae083d706b88af8d640f227b46f Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Fri, 9 Jun 2017 02:05:34 +0300
Subject: [PATCH] Make emplace constexpr

---
 include/boost/variant2/variant.hpp | 73 +++++++++++++++++---------
 test/Jamfile                       |  2 +
 test/variant_emplace_index_cx.cpp  | 83 ++++++++++++++++++++++++++++++
 test/variant_emplace_type_cx.cpp   | 63 +++++++++++++++++++++++
 4 files changed, 196 insertions(+), 25 deletions(-)
 create mode 100644 test/variant_emplace_index_cx.cpp
 create mode 100644 test/variant_emplace_type_cx.cpp

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index 4a0937c..ee457c9 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -342,12 +342,22 @@ template<class T1, class... T> union variant_storage_impl<mp_true, T1, T...>
     {
     }
 
-    template<class... A> void emplace( mp_size_t<0>, A&&... a ) noexcept
+    template<class... A> void emplace_impl( mp_false, A&&... a ) noexcept
     {
         ::new( &first_ ) T1( std::forward<A>(a)... );
     }
 
-    template<std::size_t I, class... A> void emplace( mp_size_t<I>, A&&... a ) noexcept
+    template<class... A> constexpr void emplace_impl( mp_true, A&&... a ) noexcept
+    {
+        *this = variant_storage_impl( mp_size_t<0>(), std::forward<A>(a)... );
+    }
+
+    template<class... A> constexpr void emplace( mp_size_t<0>, A&&... a ) noexcept
+    {
+        this->emplace_impl( mp_all<std::is_trivially_move_assignable<T1>, std::is_trivially_move_assignable<T>...>(), std::forward<A>(a)... );
+    }
+
+    template<std::size_t I, class... A> constexpr void emplace( mp_size_t<I>, A&&... a ) noexcept
     {
         rest_.emplace( mp_size_t<I-1>(), std::forward<A>(a)... );
     }
@@ -436,20 +446,25 @@ template<class... T> struct variant_base_impl<true, true, T...>
         return st1_.get( mp_size_t<J>() );
     }
 
-    template<std::size_t I, class... A> void emplace( A&&... a )
+    template<std::size_t J, class U, bool B, class... A> constexpr void emplace_impl( mp_true, mp_bool<B>, A&&... a )
     {
-        std::size_t const J = I+1;
+        st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
+        ix_ = J;
+    }
 
+    template<std::size_t J, class U, class... A> constexpr void emplace_impl( mp_false, mp_true, A&&... a )
+    {
+        U tmp( std::forward<A>(a)... );
+
+        st1_.emplace( mp_size_t<J>(), std::move(tmp) );
+        ix_ = J;
+    }
+
+    template<std::size_t J, class U, class... A> void emplace_impl( mp_false, mp_false, A&&... a )
+    {
         std::size_t const K = mp_find_if<mp_list<T...>, std::is_nothrow_constructible>::value;
 
-        using U = mp_at_c<variant<T...>, I>;
-
-        if( std::is_nothrow_constructible<U, A...>::value )
-        {
-            st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
-            ix_ = J;
-        }
-        else if( K < sizeof...(T) ) // have nothrow destructible
+        if( K < sizeof...(T) ) // have nothrow destructible
         {
             try
             {
@@ -458,8 +473,8 @@ template<class... T> struct variant_base_impl<true, true, T...>
             }
             catch( ... )
             {
-                st1_.emplace( mp_size_t<K>() );
-                ix_ = K;
+                st1_.emplace( mp_size_t<K+1>() );
+                ix_ = K+1;
 
                 throw;
             }
@@ -474,6 +489,14 @@ template<class... T> struct variant_base_impl<true, true, T...>
             ix_ = J;
         }
     }
+
+    template<std::size_t I, class... A> constexpr void emplace( A&&... a )
+    {
+        std::size_t const J = I+1;
+        using U = mp_at_c<variant<T...>, I>;
+
+        this->emplace_impl<J, U>( std::is_nothrow_constructible<U, A...>(), mp_all<std::is_trivially_move_constructible<U>, std::is_trivially_move_assignable<T>...>(), std::forward<A>(a)... );
+    }
 };
 
 // trivially destructible, double buffered
@@ -516,7 +539,7 @@ template<class... T> struct variant_base_impl<true, false, T...>
         return ix_ >= 0? st1_.get( j ): st2_.get( j );
     }
 
-    template<std::size_t I, class... A> void emplace( A&&... a )
+    template<std::size_t I, class... A> constexpr void emplace( A&&... a )
     {
         size_t const J = I+1;
 
@@ -830,7 +853,7 @@ public:
     }
 
     template<class E1 = void, class E2 = mp_if<mp_all<std::is_copy_constructible<T>...>, E1>>
-    variant( variant const& r )
+    constexpr variant( variant const& r )
         noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value )
     {
         mp_with_index<sizeof...(T)>( r.index(), [&]( auto I ){
@@ -841,7 +864,7 @@ public:
     }
 
     template<class E1 = void, class E2 = mp_if<mp_all<std::is_move_constructible<T>...>, E1>>
-    variant( variant && r )
+    constexpr variant( variant && r )
         noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value )
     {
         mp_with_index<sizeof...(T)>( r.index(), [&]( auto I ){
@@ -885,7 +908,7 @@ public:
 
     // assignment
     template<class E1 = void, class E2 = mp_if<mp_all<std::is_copy_constructible<T>..., std::is_copy_assignable<T>...>, E1>>
-    variant& operator=( variant const & r )
+    constexpr variant& operator=( variant const & r )
         noexcept( mp_all<std::is_nothrow_copy_constructible<T>..., std::is_nothrow_copy_assignable<T>...>::value )
     {
         mp_with_index<sizeof...(T)>( r.index(), [&]( auto I ){
@@ -905,7 +928,7 @@ public:
     }
 
     template<class E1 = void, class E2 = mp_if<mp_all<std::is_move_constructible<T>..., std::is_move_assignable<T>...>, E1>>
-    variant& operator=( variant && r )
+    constexpr variant& operator=( variant && r )
         noexcept( mp_all<std::is_nothrow_move_constructible<T>..., std::is_nothrow_move_assignable<T>...>::value )
     {
         mp_with_index<sizeof...(T)>( r.index(), [&]( auto I ){
@@ -929,7 +952,7 @@ public:
         class V = variant2::detail::resolve_overload_type<U, T...>,
         class E2 = std::enable_if_t<std::is_assignable<V&, U>::value && std::is_constructible<V, U>::value>
     >
-    variant& operator=( U&& u )
+    constexpr variant& operator=( U&& u )
         noexcept( std::is_nothrow_assignable<V&, U>::value && std::is_nothrow_constructible<V, U>::value )
     {
         std::size_t const I = variant2::detail::resolve_overload_index<U, T...>::value;
@@ -949,28 +972,28 @@ public:
     // modifiers
 
     template<class U, class... A, class I = mp_find<variant<T...>, U>, class E = std::enable_if_t<std::is_constructible<U, A...>::value>>
-    U& emplace( A&&... a )
+    constexpr U& emplace( A&&... a )
     {
         variant_base::template emplace<I::value>( std::forward<A>(a)... );
         return _get_impl( I() );
     }
 
     template<class U, class V, class... A, class I = mp_find<variant<T...>, U>, class E = std::enable_if_t<std::is_constructible<U, std::initializer_list<V>&, A...>::value>>
-    U& emplace( std::initializer_list<V> il, A&&... a )
+    constexpr U& emplace( std::initializer_list<V> il, A&&... a )
     {
         variant_base::template emplace<I::value>( il, std::forward<A>(a)... );
         return _get_impl( I() );
     }
 
     template<std::size_t I, class... A, class E = std::enable_if_t<std::is_constructible<mp_at_c<variant<T...>, I>, A...>::value>>
-    variant_alternative_t<I, variant<T...>>& emplace( A&&... a )
+    constexpr variant_alternative_t<I, variant<T...>>& emplace( A&&... a )
     {
         variant_base::template emplace<I>( std::forward<A>(a)... );
         return _get_impl( mp_size_t<I>() );
     }
 
     template<std::size_t I, class V, class... A, class E = std::enable_if_t<std::is_constructible<mp_at_c<variant<T...>, I>, std::initializer_list<V>&, A...>::value>>
-    variant_alternative_t<I, variant<T...>>& emplace( std::initializer_list<V> il, A&&... a )
+    constexpr variant_alternative_t<I, variant<T...>>& emplace( std::initializer_list<V> il, A&&... a )
     {
         variant_base::template emplace<I>( il, std::forward<A>(a)... );
         return _get_impl( mp_size_t<I>() );
@@ -1044,7 +1067,7 @@ public:
 
 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 )
+    template<class... U, class V, std::size_t J, class E = std::enable_if_t<J != sizeof...(U)>> static constexpr variant<U...> _subset_impl( mp_size_t<J>, V && v )
     {
         return variant<U...>( in_place_index<J>, std::forward<V>(v) );
     }
diff --git a/test/Jamfile b/test/Jamfile
index c11eef5..b9ad29b 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -33,7 +33,9 @@ run variant_copy_assign.cpp : : : $(REQ) ;
 run variant_move_assign.cpp : : : $(REQ) ;
 run variant_value_assign.cpp : : : $(REQ) ;
 run variant_emplace_index.cpp : : : $(REQ) ;
+compile variant_emplace_index_cx.cpp : : : $(REQ) ;
 run variant_emplace_type.cpp : : : $(REQ) ;
+compile variant_emplace_type_cx.cpp : : : $(REQ) ;
 run variant_swap.cpp : : : $(REQ) ;
 run variant_eq_ne.cpp : : : $(REQ) ;
 run variant_destroy.cpp : : : $(REQ) ;
diff --git a/test/variant_emplace_index_cx.cpp b/test/variant_emplace_index_cx.cpp
new file mode 100644
index 0000000..3a5d73d
--- /dev/null
+++ b/test/variant_emplace_index_cx.cpp
@@ -0,0 +1,83 @@
+
+// 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
+
+#include <boost/variant2/variant.hpp>
+
+using namespace boost::variant2;
+
+struct X
+{
+    int v;
+    constexpr X(): v( 0 ) {}
+    constexpr explicit X( int v ): v( v ) {}
+	constexpr operator int() const { return v; }
+};
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+template<class V, std::size_t I, class A> constexpr A test( A const& a )
+{
+    V v;
+
+    v.template emplace<I>( a );
+
+    return get<I>(v);
+}
+
+int main()
+{
+    {
+        constexpr auto w = test<variant<int>, 0>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<X>, 0>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, float>, 0>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, float>, 1>( 3.0f );
+        STATIC_ASSERT( w == 3.0f );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X, X>, 0>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X, X>, 1>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X, X>, 2>( 2.0f );
+        STATIC_ASSERT( w == 2.0f );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X, X>, 3>( 3.0f );
+        STATIC_ASSERT( w == 3.0f );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X, X>, 4>( 4 );
+        STATIC_ASSERT( w == 4 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X, X>, 5>( 5 );
+        STATIC_ASSERT( w == 5 );
+    }
+}
diff --git a/test/variant_emplace_type_cx.cpp b/test/variant_emplace_type_cx.cpp
new file mode 100644
index 0000000..4162484
--- /dev/null
+++ b/test/variant_emplace_type_cx.cpp
@@ -0,0 +1,63 @@
+
+// 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
+
+#include <boost/variant2/variant.hpp>
+
+using namespace boost::variant2;
+
+struct X
+{
+    int v;
+    constexpr X(): v( 0 ) {}
+    constexpr explicit X( int v ): v( v ) {}
+    constexpr operator int() const { return v; }
+};
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+template<class V, class T, class A> constexpr A test( A const& a )
+{
+    V v;
+
+    v.template emplace<T>( a );
+
+    return get<T>(v);
+}
+
+int main()
+{
+    {
+        constexpr auto w = test<variant<int>, int>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<X>, X>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, float>, int>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+
+    {
+        constexpr auto w = test<variant<int, float>, float>( 3.0f );
+        STATIC_ASSERT( w == 3.0f );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float>, float>( 3.0f );
+        STATIC_ASSERT( w == 3.0f );
+    }
+
+    {
+        constexpr auto w = test<variant<int, int, float, float, X>, X>( 1 );
+        STATIC_ASSERT( w == 1 );
+    }
+}