Should implicitly generated assignment operators be & ref-qualified?

Question

The following code compiles without problem on gcc 4.8.1:

#include 

struct foo
{
};

int main()
{
    foo bar;

    foo() = bar;
    foo() =          


        

Answer 1

It seems like your question is more "Why would assignment to an rvalue ever be useful?" rather than "Why doesn't the standard ref-qualify auto generated constructors?"

The reason assignment to an rvalue is allowed is because there are some cases where it is useful.

One example usage is with std::tie (link):

#include 
#include 

int main()
{
    std::set s;

    std::set::iterator iter;
    bool inserted;

    // unpacks the return value of insert into iter and inserted
    std::tie(iter, inserted) = s.insert(7);
}

Example borrowed from cppreference.com then modified