Re: [Boost-bugs] [Boost C++ Libraries] #7251: is_convertible works incorrectly for rvalue source types

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #7251: is_convertible works incorrectly for rvalue source types
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-08-31 01:12:11

#7251: is_convertible works incorrectly for rvalue source types
--------------------------------------+-------------------------------------
  Reporter:  michel                   |       Owner:  johnmaddock
      Type:  Patches                  |      Status:  new        
 Milestone:  To Be Determined         |   Component:  type_traits
   Version:  Boost Development Trunk  |    Severity:  Problem    
Resolution:                           |    Keywords:             
--------------------------------------+-------------------------------------

Comment (by michel):

 I have some questions.

 First, here is the description of `is_convertible` from the C++11
 Standard:

   Given the following function prototype:
 {{{
 template <class T>
 typename add_rvalue_reference<T>::type create();
 }}}
   the predicate condition for a template specialization
 `is_convertible<From, To>` shall be satisfied if and only if the following
 code would be well-formed, including any implicit conversions to the
 return type of the function:
 {{{
 To test() {
     return create<From>();
 }
 }}}
 [[BR]]

 In C++11, for a function type `Func`, `is_convertible<Func, Func&>`
 returns true_type. But, on gcc 4.5-4.6, `Func&&` cannot be convertible to
 `Func&`.
 My question is "What should `is_convertible<Func, Func&>` return on gcc
 4.5-4.6?"
 [[BR]][[BR]]

 Also, in C++03, what should `is_convertible<Func, Func*>` return?
 It would be reasonable to return true_type, since `is_convertible<Func,
 Func*>` returns true_type in C++11.
 But we can make another interpretation. Boost's interpretation of C++03
 `add_rvalue_reference<T>` is just returning `T`. Accordingly,
 `declval<T>()` returns a value of `T`. So,
 {{{
 template <class T>
 typename add_rvalue_reference<T>::type create();

 To test() {
     return create<From>();
 }
 }}}
 is ill-formed when `From` is a function type. With this interpretation,
 `is_convertible<Func, Func*>` should return false_type.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7251#comment:6>
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