Ternary expression assigned by rvalue doesn't use move asssignment operator

Question

Anyone knows why the following code doesn't compile:

[hidden]$ g++ -v |& tail -n 1
gcc version 4.8.1 20130603 (Red Hat 4.8.1-1) (GCC)
[hidden]$ cat c.cpp
struct X {
  X() = default;
  X(const X&) = default;
  X(X&&) = default;
  X& operator=(const X&) = delete;
  X& operator=(X&&) = default;
};

void f(bool t) {
  X a, b;
  (t ? a : b) = X();
}

[hidden]$ g++ -std=c++11 -c c.cpp
c.cpp: In function ‘void f(bool)’:
c.cpp:11:15: error: use of deleted function ‘X& X::operator=(const X&)’
   (t ? a : b) = X();
               ^
c.cpp:5:6: error: declared here
   X& operator=(const X&) = delete;
      ^
c.cpp:11:15: error: use of deleted function ‘X& X::operator=(const X&)’
   (t ? a : b) = X();
               ^
c.cpp:5:6: error: declared here
   X& operator=(const X&) = delete;
      ^

Isn't that X() rvalue so the move assignment operator should be called in this case? What section in C++11 standard talks about the case ternary expression assigned by rvalue?

Note: the ternary expression is lvalue in this case because if I change the = delete to = default, it compiles.

Answer 1

This a bug in the compiler, see Bogus overload resolution for the assignment operator in assignment to a conditional, it still fails in gcc 4.9.

The problem is not with the ternary operator but that the overload resolution ends up at the wrong assignment operator.