[boost] [type_traits] Tests for implicit convertibility?

Subject: [boost] [type_traits] Tests for implicit convertibility?
From: John Maddock (jz.maddock_at_[hidden])
Date: 2016-01-19 14:18:11

Folks, I suspect we may have unintentionally backed ourselves into a 
corner with is_convertible/is_constructible.

While trying to fix https://svn.boost.org/trac/boost/ticket/11922 I 
found I needed to SFINAE-out some conversion operators when the 
conversion already exists (as an implicit constructor in the target 
type).  So far so easy, but consider:

class Int {
public:
    Int(const int& i) {};
    Int(const Int& i) = delete;
};

static_assert(is_convertible<int, Int>::value, "Ooops");

Now you would think this would pass right?  But it doesn't (at least on 
GCC and clang), and this is true for both boost:: and std:: type_traits 
and appears to be an unintended side effect of the way is_convertible is 
defined, that a valid copy constructor is required on the target type.

Of course:

is_constructible<Int, int>::value

is true, but that also catches explicit constructors.

So... have we backed ourselves into a corner or do we still have a way 
to test for implicit-constructibilty that doesn't also require 
copy-constructibility on the target type?

Thanks, John.