Do not trivially copy/move assign when not trivially copy/move constructible

test/variant_move_assign_throw.cpp

@@ -0,0 +1,51 @@
+
+// Copyright 2019 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>
+#include <boost/core/lightweight_test.hpp>
+#include <stdexcept>
+
+using namespace boost::variant2;
+
+struct Y1
+{
+    Y1(){} // =default fails on msvc-14.0
+
+    Y1(Y1&&)
+    {
+        throw std::runtime_error( "Y1(Y1&&)" );
+    }
+
+    Y1& operator=(Y1&&) = default;
+};
+
+struct Y2
+{
+    Y2(){}
+
+    Y2(Y2&&)
+    {
+        throw std::runtime_error( "Y2(Y2&&)" );
+    }
+
+    Y2& operator=(Y2&&) = default;
+};
+
+void test()
+{
+    variant<Y1, Y2> v1( in_place_type_t<Y1>{} );
+    variant<Y1, Y2> v2( in_place_type_t<Y2>{} );
+
+    BOOST_TEST_THROWS( v1 = std::move( v2 ), std::runtime_error )
+}
+
+int main()
+{
+    test();
+    return boost::report_errors();
+}