id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
10839	dereferencing of rvalue optional<T&> fails due to invalid assignment of rvalue to non-const ref	stefan.bund@…	akrzemi1	"Compiling the following code using `g++-4.9.2 -std=gnu++1y`
{{{
#include <boost/optional.hpp>

int main(int, char const **)
{
    int i (0);
    boost::optional<int &> opt (i);
    // we use std::move here to force opt to be an rvalue. The same error
    // is triggered if the optional is returned from a function call
    return * std::move(opt);
}
}}}

results in the error message

> lib/boost/install/include/boost/optional/optional.hpp:1010:89: error: invalid initialization of non-const reference of type 'boost::optional<int&>::reference_type_of_temporary_wrapper {aka int&}' from an rvalue of type 'boost::move_detail::remove_reference<int&>::type {aka int}'

Looking at the source code in `optional.hpp`, the culprit seems to be:

{{{
template <class T>
class optional : public optional_detail::optional_base<T>
{
    // ...

    reference_type_of_temporary_wrapper operator *() &&
    { return boost::move(this->get()) ; }

     // ...
};
}}}

`reference_type_of_termporary_wrapper` is typedefed to `int &` as is to be expected. If I am not mistaken, the `std::move` call however is not correct in this case: it converts it's `int &` argument (the result of `this->get()`) to `int &&` which is ''not'' implicitly convertible to `int &` (the return type).

This problem only occurs with `optional<T&>` not with `optional<T const &>` since for the latter, `reference_type_of_temporary_wrapper` is `int const &` and `int &&` is convertible to `int const &`.

Possible resolution: If the type of the optional is a reference type, don't apply `std::move` (e.g. add a static `move` member function to `types_when_isnt_ref` and `types_when_is_ref` and use that member instead of `boost::move` here)."	Bugs	closed	Boost 1.58.0	optional	Boost 1.57.0	Problem	fixed