id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
11835,boost::has_trivial_copy is incorrect on Clang,joseph.thomson@…,John Maddock,"Currently, `boost::has_trivial_copy<T>::value` is false when `T` has a trivial copy constructor, but is uncopyable. For example, the following type, despite having a trivial copy constructor, will report false:

{{{
struct T {
    T(T const&) = delete;
}
}}}

The offending code seems to be:

{{{
template <typename T>
struct has_trivial_copy_impl
{
#ifdef BOOST_HAS_TRIVIAL_COPY
#  ifdef __clang__
   BOOST_STATIC_CONSTANT(bool, value = BOOST_HAS_TRIVIAL_COPY(T) && boost::is_copy_constructible<T>::value);
#  else
   BOOST_STATIC_CONSTANT(bool, value = BOOST_HAS_TRIVIAL_COPY(T));
#  endif
#else
   BOOST_STATIC_CONSTANT(bool, value =
      (::boost::type_traits::ice_and<
         ::boost::is_pod<T>::value,
         ::boost::type_traits::ice_not< ::boost::is_volatile<T>::value >::value
      >::value));
#endif
};
}}}

This fix was applied for Clang for the following ticket:

https://svn.boost.org/trac/boost/ticket/10389

The fix actually broke the behaviour of `boost::has_trivial_copy` in Clang, to make it match the broken behaviour in GCC 4.8 and VC++. The bug was actually in Boost.Container, not Boost.TypeTraits, but that bug has now been fixed (Boost.Container no longer depends on Boost.TypeTraits). GCC actually fixed its intrinsic `__has_trivial_copy` in GCC 4.9, so `boost::has_trivial_copy` works correctly in the latest GCC versions. It's still broken on VC++ because of its faulty implementation of `__has_trivial_copy`, though it seems probable that there is no possible workaround.

But I digress. `__has_trivial_copy` works on Clang, but the ""fix"" in Boost breaks `boost::has_trivial_copy`. Note that `boost::has_trivial_assign`, `boost::has_trivial_move_constructor` and `boost::has_trivial_move_assign` appear to be okay.

And one final thought: `boost::has_trivial_copy` is not really useful for anything except implementing higher-level type-traits like `std::is_trivially_copyable` (which essentially says whether a type can be safely `memcpy`'d -- a copy optimization). `std::has_trivial_copy_constructor` does not exist, probably for this reason. In my opinion, it would be better for `boost::has_trivial_copy` to not exist in the public API, instead replacing it with a `boost::is_trivially_copyable`.",Bugs,closed,To Be Determined,type_traits,Boost 1.59.0,Problem,wontfix,has_trivial_copy clang memcpy is_trivially_copyable,