I filed a ticket about a potential improvement of type trait boost::is_constructible: https://svn.boost.org/trac/boost/ticket/12003 I am solving the following problem. I have a one-argument perfect forwarding explicit constructor and want to constrain it only to the cases when T is constructible from U: ``` template <typename T> struct optional { template <typename U> explicit optional(U&& v, ENABLE_IF(is_constructible<T, U&&>)) {...} }; ``` I cannot afford to use the "fallback" implementation of boost::is_constructible (the one that forwards to is_convertible) because I would be constraint my constructor too much: I would rather leave it under-constrained. I do not need the full implementation of is_constructible (that would need variadic templates) because I am only using the two-argument version (and I suppose my problem is not isolated). I would like a partial implementation of is_constructible that works correctly for two arguments only; and a macro that allows me to test if it is available. I suspect that this is the common case that you use only the two-argument version, and implementing only this would handle a quite a big number of all the use cases. Is it something that fits into the library scope, or do is it something I would have to implement myself? Regards, &rzej