Add variant<>::uses_double_storage(). Refs #37.

test/variant_uses_double_storage.cpp

@@ -0,0 +1,50 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <type_traits>
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X1
+{
+};
+
+STATIC_ASSERT( std::is_nothrow_move_constructible<X1>::value );
+STATIC_ASSERT( std::is_trivially_destructible<X1>::value );
+
+struct X2
+{
+    ~X2() {}
+};
+
+STATIC_ASSERT( std::is_nothrow_move_constructible<X2>::value );
+STATIC_ASSERT( !std::is_trivially_destructible<X2>::value );
+
+struct X3
+{
+    X3( X3&& ) {}
+};
+
+STATIC_ASSERT( !std::is_nothrow_move_constructible<X3>::value );
+STATIC_ASSERT( std::is_trivially_destructible<X3>::value );
+
+struct X4
+{
+    ~X4() {}
+    X4( X4&& ) {}
+};
+
+STATIC_ASSERT( !std::is_nothrow_move_constructible<X4>::value );
+STATIC_ASSERT( !std::is_trivially_destructible<X4>::value );
+
+//
+
+STATIC_ASSERT( !variant<int, float>::uses_double_storage() );
+STATIC_ASSERT( !variant<int, float, X1>::uses_double_storage() );
+STATIC_ASSERT( !variant<int, float, X2>::uses_double_storage() );
+STATIC_ASSERT( variant<int, float, X3>::uses_double_storage() );
+STATIC_ASSERT( variant<int, float, X4>::uses_double_storage() );