[Bug c++/46004] New: [C++0x] template constructor used to copy object

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46004

           Summary: [C++0x] template constructor used to copy object
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: r...@gcc.gnu.org
                CC: ja...@gcc.gnu.org


[class.copy]p7 says "A member function template is never instantiated to
perform the copy of a class object to an object of its class type."


struct B
{
    int i;

    template<typename I>
        explicit
        B(I&& ii) : i(ii) { }

    B(const B&) = default;
};

int main()
{
    B b(0);
    B bb(b);
}

cc.cc: In constructor 'B::B(I&&) [with I = B&]':
cc.cc:15:11:   instantiated from here
cc.cc:7:25: error: cannot convert 'B' to 'int' in initialization

Even with an explicitly declared copy constructor the template is still chosen.

This only happens if the template constructor parameter type is an
rvalue-reference, so only affects C++0x mode.

In http://gcc.gnu.org/viewcvs?view=revision&revision=165144 I had to workaround
this behaviour by replacing the template constructor by a pair of overloads,
one for lvalues and one for rvalues:

    explicit B(const int& ii) : i(ii) { }
    explicit B(int&& ii) : i(ii) { }