Hello, The following code fails to compile when using GCC 4.4.6 and boost::optional from Boost 1.49: #include <boost/optional/optional.hpp> #include <boost/type_traits/is_convertible.hpp> struct grill { template<typename U> grill (U) {} }; template<typename T> struct bar { template<typename U> bar (U, typename boost::is_convertible<U,T>::type* = 0) {} }; void foo() { typedef bar<grill> data_type; boost::optional<data_type> opt; boost::optional<data_type> opt2(opt); // this line (void)opt2; } Here are the error messages (I apologize for the long lines, I shortened them as much as possible by truncating the header paths): type_traits/is_convertible.hpp: In instantiation of ‘const bool boost::detail::is_convertible_basic_impl<boost::optional_detail::optional_base<bar<grill> >&, grill>::value’: type_traits/is_convertible.hpp:329: instantiated from ‘const bool boost::detail::is_convertible_impl<boost::optional_detail::optional_base<bar<grill> >, grill>::value’ type_traits/is_convertible.hpp:452: instantiated from ‘boost::is_convertible<boost::optional_detail::optional_base<bar<grill> >, grill>’ optional/optional.hpp:604: instantiated from ‘boost::optional<T>::optional(const boost::optional<T>&) [with T = bar<grill>]’ optional.cpp:25: instantiated from here optional/optional.hpp:285: error: ‘boost::optional_detail::optional_base<T>::optional_base(const boost::optional_detail::optional_base<T>&) [with T = bar<grill>]’ is protected type_traits/is_convertible.hpp:170: error: within this context type_traits/is_convertible.hpp:170: error: initializing argument 1 of ‘grill::grill(U) [with U = boost::optional_detail::optional_base<bar<grill> >]’ optional/optional.hpp:308: error: ‘boost::optional_detail::optional_base<T>::~optional_base() [with T = bar<grill>]’ is protected type_traits/is_convertible.hpp:170: error: within this context This reproduction case is distilled from a real world example (not my code) where bar=boost::fusion::vector1 and grill=boost::function<void()> The code compiles fine using the same compiler and Boost 1.44. Any ideas on the root of the problem? Thanks in advance! Jon