[Boost-bugs] [Boost C++ Libraries] #11570: Boost 1.59.0 breaks lexical_cast to a move-only type (from a string)

Subject: [Boost-bugs] [Boost C++ Libraries] #11570: Boost 1.59.0 breaks lexical_cast to a move-only type (from a string)
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-08-23 15:57:59

#11570: Boost 1.59.0 breaks lexical_cast to a move-only type (from a string)
-------------------------------------------------+-------------------------
 Reporter:  Tony Lewis <tonyelewis@…>            |      Owner:  apolukhin
     Type:  Bugs                                 |     Status:  new
Milestone:  To Be Determined                     |  Component:
  Version:  Boost 1.59.0                         |  lexical_cast
 Keywords:  lexical_cast,move-only,1.59.0,copy   |   Severity:  Regression
  constructor,regression                         |
-------------------------------------------------+-------------------------
 I think lexical_cast 1.59.0 introduces a regression over 1.58.0 because
 1.58.0 allows lexical_cast from string to a move-only type but this now
 fails to compile.

 C++11 example:

 {{{
 #!cpp
 #include <boost/lexical_cast.hpp>

 #include <iostream>
 #include <string>

 using boost::lexical_cast;

 using namespace std;

 struct my_move_only final {
         string data;

         my_move_only() = default;
         my_move_only(my_move_only &&) = default;
         my_move_only(const my_move_only &) = delete;
 };

 istream & operator>>(istream &arg_is, my_move_only &arg_mot) {
         arg_is >> arg_mot.data;
         return arg_is;
 }

 int main() {
         cerr << "from string gives : " << lexical_cast<my_move_only>(
 string{ "mr_carbohdrate" } ).data << endl;
 }
 }}}

 This works against Boost 1.58.0 under both `g++ -std=c++11` and `clang++
 -std=c++11 -stdlib=libc++` but fails against Boost 1.59.0 under both.

 The key compile error message appears to be:

 {{{
 boost/lexical_cast.hpp:46:26: error: use of deleted function
 ‘my_move_only::my_move_only(const my_move_only&)’
          return get(result);
 }}}

 (ie, you're trying to copy-construct this move-only type).

 It looks to me like this functionality broke with commit
 [https://github.com/boostorg/lexical_cast/commit/bdc355759e3259209f1107ae21babee6dcf8d80b

 bdc355759e].

 Is it possible to keep that commit's benefits whilst restoring support for
 move-only types? Perhaps with some `std::move()` magic?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11570>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.