Additional note (December, 2008):

The proposed resolution addresses only cv-qualified assignment operators and is silent on ref-qualified versions. However, it would seem that the spirit of the resolution would indicate that a ref-qualified assignment operator would not be considered a copy assignment operator.

There appears to be an emerging idiom that relies on the idea that providing an lvalue-only assignment operator would prevent assignment to rvalues:

    struct A {
      A& operator=(const A&) &; // disable assignemt to rvalue
    };

The resolution should also be reconsidered in light of the use of a const-qualified assignment operator as part of the implementation of a proxy class, where the proxy object itself is constant and should not be changed, but the copy assignment operator would apply to the object to which the proxy object refers.