[Boost-bugs] [Boost C++ Libraries] #11285: Variant: apply_visitor broken with C ++14 and const references

Subject: [Boost-bugs] [Boost C++ Libraries] #11285: Variant: apply_visitor broken with C ++14 and const references
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-05-11 19:56:54

#11285: Variant: apply_visitor broken with C ++14 and const references
------------------------------------+------------------------
 Reporter:  Roger Leigh <rleigh@…>  |      Owner:  ebf
     Type:  Bugs                    |     Status:  new
Milestone:  To Be Determined        |  Component:  variant
  Version:  Boost 1.58.0            |   Severity:  Regression
 Keywords:                          |
------------------------------------+------------------------
 This is broken with C++14 with clang++, and Boost 1.58.  It was working a
 week back with Boost 1.57 prior to homebrew upgrading Boost to 1.58; I'll
 double-check this wasn't due to XCode also being upgraded, though I don't
 think that's a problem--I'd been using -std=c++14 routinely for some
 months.  Hence I've put this down as a regression, but there's a little
 uncertainly there.

 Sample minimal testcase:

 {{{
 #include <boost/variant/apply_visitor.hpp>
 #include <boost/variant/variant.hpp>

 #include <iostream>
 #include <string>

 struct a
 {
   const std::string& type() const
   {
     static const std::string v("a");
     return v;
   }
 };

 struct b
 {
   const std::string& type() const
   {
     static const std::string v("b");
     return v;
   }
 };

 typedef boost::variant<a,b> ab;

 struct TestVisitor : public boost::static_visitor<const std::string&>
   {
     template <typename T>
     const std::string&
     operator() (const T& v)
     {
       return v.type();
     }
   };

 int
 main()
 {
   ab val = a();
   TestVisitor v;
   std::cout << boost::apply_visitor(v, val) << std::endl;
 }
 }}}

 With clang++ on MacOS 10.10:

 {{{
 % clang++ --version
 Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
 Target: x86_64-apple-darwin14.3.0
 Thread model: posix

 # Success for C++98:
 % clang++ -o test test.cpp
 % ./test
 a

 # Success for C++11
 % clang++ -std=c++11 -o test test.cpp
 % ./test
 a

 # Failure for C++14
 % clang++ -std=c++14 -o test test.cpp
 test.cpp:42:16: error: call to 'apply_visitor' is ambiguous
   std::cout << boost::apply_visitor(v, val) << std::endl;
                ^~~~~~~~~~~~~~~~~~~~
 /usr/local/include/boost/variant/detail/apply_visitor_unary.hpp:68:1:
 note:
       candidate function [with Visitor = TestVisitor, Visitable =
 boost::variant<a,
       b>]
 apply_visitor(Visitor& visitor, Visitable& visitable)
 ^
 /usr/local/include/boost/variant/detail/apply_visitor_unary.hpp:150:23:
 note:
       candidate function [with Visitor = TestVisitor, Visitable =
 boost::variant<a,
       b>]
 inline decltype(auto) apply_visitor(Visitor& visitor, Visitable&
 visitable,
                       ^
 /usr/local/include/boost/variant/detail/apply_visitor_unary.hpp:82:1:
 note:
       candidate function [with Visitor = TestVisitor, Visitable =
 boost::variant<a,
       b>]
 apply_visitor(const Visitor& visitor, Visitable& visitable)
 ^
 /usr/local/include/boost/variant/detail/apply_visitor_unary.hpp:160:23:
 note:
       candidate function [with Visitor = TestVisitor, Visitable =
 boost::variant<a,
       b>]
 inline decltype(auto) apply_visitor(const Visitor& visitor, Visitable&
 visitable,
                       ^
 1 error generated.
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11285>
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