[Boost-bugs] [Boost C++ Libraries] #11251: boost::static_visitor<T&> causes a compilation error in C++14 mode

Subject: [Boost-bugs] [Boost C++ Libraries] #11251: boost::static_visitor<T&> causes a compilation error in C++14 mode
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-04-29 20:49:13

#11251: boost::static_visitor<T&> causes a compilation error in C++14 mode
----------------------------------------------+------------------------
 Reporter:  bogdan <inadgob@…>                |      Owner:  ebf
     Type:  Bugs                              |     Status:  new
Milestone:  To Be Determined                  |  Component:  variant
  Version:  Boost 1.58.0                      |   Severity:  Regression
 Keywords:  variant visitor reference return  |
----------------------------------------------+------------------------
 Consider the following code:

 {{{#!cpp
 #include <iostream>
 #include "boost/variant.hpp"

 struct test_vis : boost::static_visitor<int&>
 {
    int& operator()(int& i) const { return i; }
 };

 int main()
 {
    boost::variant<int> dummy;
    ++boost::apply_visitor(test_vis(), dummy);
    std::cout << dummy << '\n';
 }
 }}}

 Clang 3.6.0 compiles the code with no diagnostics in C++11 mode, but in
 C++14 mode it issues the following error:

 {{{
 prog.cc:12:6: error: call to 'apply_visitor' is ambiguous
    ++boost::apply_visitor(test_vis(), dummy);
      ^~~~~~~~~~~~~~~~~~~~
 /boost/variant/detail/apply_visitor_unary.hpp:82:1: note: candidate
 function [with Visitor = test_vis, Visitable = boost::variant<int>]
 apply_visitor(const Visitor& visitor, Visitable& visitable)
 ^
 /boost/variant/detail/apply_visitor_unary.hpp:160:23: note: candidate
 function [with Visitor = test_vis, Visitable = boost::variant<int>]
 inline decltype(auto) apply_visitor(const Visitor& visitor, Visitable&
 visitable,
                       ^
 1 error generated.
 }}}

 The same behaviour occurs in GCC 5.1.0. MSVC12 compiles the code, MSVC14
 issues a similar error.

 This is due to the fact that the `apply_visitor` overloads using automatic
 return type detection are not disabled in this case, because the
 `boost::detail::variant::has_result_type` trait is not working properly
 for reference types. Internally, it uses an overload with a parameter type
 of `typename C::result_type*`, which tries to form a pointer to reference
 type in this case, which causes a deduction failure.

 A quick workaround is to remove the base class for the visitor in C++14
 mode in this case.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11251>
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