[Boost-bugs] [Boost C++ Libraries] #11044: boost::rv inherits off union, when such passed as template argument

Subject: [Boost-bugs] [Boost C++ Libraries] #11044: boost::rv inherits off union, when such passed as template argument
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-02-18 00:13:39

#11044: boost::rv inherits off union, when such passed as template argument
---------------------------------------+------------------------
 Reporter:  Petr Machata <pmachata@…>  |      Owner:  igaztanaga
     Type:  Bugs                       |     Status:  new
Milestone:  To Be Determined           |  Component:  move
  Version:  Boost 1.57.0               |   Severity:  Problem
 Keywords:                             |
---------------------------------------+------------------------
 Consider the following minimal reproducer:

 {{{
 template<typename T> struct wrap { typedef T type; };
 template <class T> class rv: public wrap <T>::type {};

 template <class value_type>
 struct circular_buffer
 {
     typedef const value_type& param_value_type;
     typedef rv< value_type >& rvalue_type;

     void push_back(param_value_type item) {}
     void push_back(rvalue_type item) {}
 };

 union U { int i; char c; };

 void f(circular_buffer<U> b, const U& u) { b.push_back(u); }
 }}}

 Certain GCC 5.0's had a bug where rv would be instantiated, even though it
 doesn't have to be (so I am told), which uncovered the latent bug in Boost
 that rv inherits off union when given one.

 Proposed patch:
 {{{
 diff -up ./move/core.hpp~ ./move/core.hpp
 --- boost/move/core.hpp~        2015-02-09 17:33:35.000000000 +0100
 +++ boost/move/core.hpp 2015-02-13 13:54:52.012130813 +0100
 @@ -43,6 +43,7 @@
  #if defined(BOOST_NO_CXX11_RVALUE_REFERENCES) &&
 !defined(BOOST_MOVE_DOXYGEN_INVOKED)

     #include <boost/move/detail/meta_utils.hpp>
 +   #include <boost/type_traits/is_class.hpp>

     //Move emulation rv breaks standard aliasing rules so add workarounds
 for some compilers
     #if defined(__GNUC__) && (__GNUC__ >= 4) && \
 @@ -65,7 +66,7 @@
     template <class T>
     class rv
        : public ::boost::move_detail::if_c
 -         < ::boost::move_detail::is_class_or_union<T>::value
 +         < ::boost::is_class<T>::value
           , T
           , ::boost::move_detail::nat
           >::type
 }}}

 I regtested this on x86_64 using GCC 5.0.0, and noted no regressions.
 However I'm not sure if pulling in type traits like this is admissible--I
 mean, why else reimplement is_class_or_union in Boost.Move?  Historical
 reasons?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11044>
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