Make emplace constexpr

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) ;

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 );
+    }
+}

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 );
+    }
+}