2017-10-30 13:02 GMT+01:00 Peter Dimov via Boost <boost@lists.boost.org>:
John Maddock wrote:
I'm mildly against that in Boost.Config: the issue seems to be VC12, ...
No. The suggested addition in https://github.com/boostorg/ty pe_traits/pull/52/commits/9779157a787620d163308afa45cb94ef42391b32 is wrong; there's nothing type_traits can do about it because instantiating the constructor is not an immediate context and not subject to SFINAE.
The problem is in has_trivial_constructor<pair<...>> and is caused by the following logic in boost/type_traits/has_trivial_constructor.hpp:
#if (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__ >= 409)) || defined(BOOST_CLANG) || (defined(__SUNPRO_CC) && defined(BOOST_HAS_TRIVIAL_CONSTRUCTOR)) #include <boost/type_traits/is_default_constructible.hpp> #define BOOST_TT_TRIVIAL_CONSTRUCT_FIX && is_default_constructible<T>::v alue #else
which is then used in
template <typename T> struct has_trivial_constructor #ifdef BOOST_HAS_TRIVIAL_CONSTRUCTOR : public integral_constant <bool, ((::boost::is_pod<T>::value || BOOST_HAS_TRIVIAL_CONSTRUCTOR(T)) BOOST_TT_TRIVIAL_CONSTRUCT_FIX)>{}; #else
That is, the issue only occurs when (a) the compiler is gcc >= 4.9, clang, Sun, (b) the compiler has intrinsic support for __has_trivial_constructor, (c) instantiating the type's default constructor is a hard error.
Is the above a conjunction of (a) and (b) and (c)?
The latter is the case for pair<deleted-constructor, int> before the change in the standard that made its default constructor SFINAE-friendly.
This sounds like the trait works fine and it is just implementations of `std::pair` that are not SFINAE friendly.
So in practice, this manifests on Travis on gcc 4.9 and clang using libstdc++ before 5.
You mean version of libstdc++ is before 5? Regards, &rzej;