[Bug c++/81311] An std::ref argument calls copy constructor instead of template constructor in C++17 mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81311

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-09-14
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
extern "C" int puts(const char*);

struct function
{
    function()
    {
        puts("function()");
    }

    template<class F> function( F )
    {
        puts("function<F>(F)");
    }

    function( function const& )
    {
        puts("function(function const&)");
    }
};

struct ref
{
  ref() = default;

  operator function&() const;
};

int main()
{
    function f2( ref{} );
}

Compiles OK with C++14, printing "function<F>(F)" but fails to link with C++17:

/tmp/ccB3mFFt.o: In function `main':
/tmp/ref.cc:30: undefined reference to `ref::operator function&() const'
collect2: error: ld returned 1 exit status

Presumably caused by the changes for guaranteed elision in C++17.

Interestingly if the conversion operator is defined as deleted then it compiles
OK, choosing the function<F>(F) constructor instead.