Boost 1.59.0 breaks lexical_cast to a move-only type (from a string)

[Tony Lewis <tonyelewis@…>]

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:

#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 ​bdc355759e.

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

[Antony Polukhin]

​Reverted the commit. Attempted to ​suppress the warnings in this commit.

[lopresti@…]

See ​https://github.com/boostorg/lexical_cast/pull/18 and /19.

[Tony Lewis <tonyelewis@…>]

Still looks good to me. Thanks to all for the work on this.