The following code, compiled with 

-Wall -std=c++11 -pedantic

is rejected by gcc 4.7.2 and gcc 4.8.0 20130106 (experimental):

//----------------
struct mine
{
  mine& operator=(mine rhs) { return *this; }
  mine& operator=(mine&& rhs) noexcept { return *this; }
};

int main()
{
  mine a;
  mine b;
  a = b;
}
//----------------

"In function 'int main()':|
12|error: use of deleted function 'constexpr mine::mine(const mine&)'|
1|note: 'constexpr mine::mine(const mine&)' is implicitly declared as deleted because 'mine' declares a move constructor or move assignment operator|
3|error:   initializing argument 1 of 'mine& mine::operator=(mine)'|
"

According to 12.8 p17:

"A user-declared copy assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&."

and p18:

"If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy
assignment operator is defined as deleted;"

Therefore class mine provides a user-declared copy-assignment operator, but gcc doesn't recognize that.

This code should be accepted.

I reduced the severity, because this kind of code is expected to be a rather unusual edge-case. Every other form of the copy-assignment operator using references is correctly detected.