id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
9910	is_convertible doesn't compile with inaccessible copy constructor	Matei David <matei@…>	John Maddock	"I found a situation where (with `gcc-4.8.2` & `boost-1.55`) I cannot compile a call to `boost::is_convertible`, but I can compile a similar call to `std::is_convertible`. This is a bug, unless it is undefined behaviour to pass `is_convertible` a type with a private or deleted copy constructor. If it is a bug, it is a `boost` bug, not a `gcc` bug (explained below).

Here is the discordant code:
{{{
#include <type_traits>
#include <boost/type_traits/is_convertible.hpp>
struct A {
    A() = default;
    A(const A&) = delete; // or private: A(const A&) = default;
};
struct Ref_A { // proxy reference to A
    operator A& () { return *_ptr; }
    A* _ptr;
};
int main() {
    (void)std::is_convertible< Ref_A, A >::value;
    (void)boost::is_convertible< Ref_A, A >::value;
}
}}}
I compile with:
`{g++|clang++} -std=c++11 -I${BOOST_INCLUDE} -Wall -Wextra -c`

With `gcc-4.8.2`, the call to `boost::is_convertible` fails to compile, complaining that the copy constructor of A is deleted (or private.) The similar call to `std::is_convertible` compiles ok (and returns false, but that is irrelevant).

With `clang-3.4`, both calls compile. The reason for the discrepancy is that `boost/type_traits/intrinsics.hpp` defines the macro `BOOST_IS_CONVERTIBLE(T,U)` for `clang`, but not for `gcc`. Because of that, at the bottom of `boost/type_traits/is_convertible.hpp`, `clang` uses the macro to implement `is_convertible`, whereas `gcc` uses `is_convertible_impl_dispatch` instead (which is the one that fails). That's why I said this is not a `gcc` bug.

A temporary fix I found is to define that macro to defer to `std`:
{{{
#include <type_traits>
#include <boost/type_traits/intrinsics.hpp>
#ifndef BOOST_IS_CONVERTIBLE
#define BOOST_IS_CONVERTIBLE(T, U) (std::is_convertible<T, U>::value)
#endif
#include <boost/type_traits/is_convertible.hpp>
}}}
Which `type_traits` library should we prefer these days, post `c++11`? Intuition says `std` should be more stable/portable, is that wrong? If it is indeed `std`, is there a way (perhaps a macro?) to make `boost` mirror `std` (I'm talking about `type_traits` only). The reason for having such a mirror option is not for new code, but for other `boost` libraries which need `type_traits`. E.g, I came across this issue from `boost::iterator`.
"	Bugs	closed	To Be Determined	type_traits	Boost 1.55.0	Problem	fixed	is_convertible