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: 2012-08-30 16:31:55

#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):

 Hi John,

 Thanks for looking into the problem.
 [[BR]][[BR]]

 > I've applied the patch, but restricted it's application to GCC to
 versions 4.7 and later - earlier versions don't build the test suite with
 the patch applied :-[[BR]]
 > (Versions 4.5.x and 4.6.x fail to build with function types,

 gcc 4.3-4.6 implements C++0x drafts, and there are some inconsistencies
 with C++11.
 For example,
 * gcc 4.3-4.4: rvalue references can be bound to lvalues.
 * gcc 4.5-4.6: rvalue references to function type is not an lvalue. So,
 for a function type `Func`, `Func&` cannot be bound to `Func&&`.
 Accordingly, `is_convertible<Func, Func&>` returns different results than
 C++11 and this causes the test failures.
 [[BR]]

 > while version 4.4.x fails with just about everything.[[BR]][[BR]]
 > It's possible that an extra #if branch for SFINAE-expressions could fix
 the remaining issues, but equally it might just break more stuff :-(

 Since `T` is not a dependent type in`_m_check`, SFINAE is not related.
 Indeed, the errors on gcc 4.3-4.4 can be reproduced without using
 templates. (And note that gcc 4.4 supports SFINAE for expressions.) I
 guess that the reason for the test failures on gcc 4.3-4.4 is a compiler's
 bug related to rvalue references.

 `#if` branch is not needed for `BOOST_NO_SFINAE_EXPR`, but is needed for
 `__GNUC__ == 4 && (__GNUC_MINOR__ == 3 || __GNUC_MINOR__ == 4)`.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7251#comment:4>
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