Added a test to verify that boost::optional copy constructor does not invoke T's template constructors.

test/Jamfile.v2

@@ -76,6 +76,7 @@ import testing ;
     [ run optional_test_static_properties.cpp ]
     [ compile optional_test_maybe_uninitialized_warning.cpp ]
     [ compile optional_test_deleted_default_ctor.cpp ]
+    [ compile optional_test_constructible_from_other.cpp ]
     #[ run optional_xconfig_HACK_TO_LIST_PREDEFINED_MACROS.cpp ]
     [ run optional_xconfig_NO_PROPER_ASSIGN_FROM_CONST_INT_pass.cpp ]
     [ run-fail optional_xconfig_NO_PROPER_ASSIGN_FROM_CONST_INT_fail.cpp ]

test/optional_test_constructible_from_other.cpp

@@ -0,0 +1,57 @@
+// Copyright (c) 2018 Andrey Semashev
+//
+// Use, modification, and distribution is subject to 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)
+
+// The test verifies that Boost.Optional copy constructors do not attempt to invoke
+// the element type initializing constructors from templated arguments
+
+#include <boost/optional/optional.hpp>
+#include <boost/core/enable_if.hpp>
+
+struct no_type
+{
+    char data;
+};
+
+struct yes_type
+{
+    char data[2];
+};
+
+template< unsigned int Size >
+struct size_tag {};
+
+template< typename T, typename U >
+struct is_constructible
+{
+    template< typename T1, typename U1 >
+    static yes_type check_helper(size_tag< sizeof(static_cast< T1 >(U1())) >*);
+    template< typename T1, typename U1 >
+    static no_type check_helper(...);
+
+    static const bool value = sizeof(check_helper< T, U >(0)) == sizeof(yes_type);
+};
+
+template< typename T >
+class wrapper
+{
+public:
+    wrapper() {}
+    wrapper(wrapper const&) {}
+    template< typename U >
+    wrapper(U const&, typename boost::enable_if_c< is_constructible< T, U >::value, int >::type = 0) {}
+};
+
+inline boost::optional< wrapper< int > > foo()
+{
+    return boost::optional< wrapper< int > >();
+}
+
+int main()
+{
+    // Invokes boost::optional copy constructor. Should not invoke wrapper constructor from U.
+    boost::optional< wrapper< int > > res = foo();
+    return 0;
+}