Bug in compiler?

[Shachar Shemesh via Digitalmars-d]

Please consider the following program:
import std.exception;

void main()
{
     struct A {
         int a;

         @disable this(this);
         @disable ref A opAssign(const ref A);

         ref A opOpAssign(string op: "~")(int data) {
             a += data;

             return this;
         }
     }

     auto a = A(2);

     a ~= 3;

     assertThrown!Exception(a ~= 3);
}

Compilation (dmd 2.066.1) fails on the assertThrown line:
Error: struct test.main.A is not copyable because it is annotated with 
@disable

What I do not understand is why A should need to be copyable. Where is 
the copy made? I'm guessing this is because of the lazy definition of 
the expression, but still I don't see any reason to create a copy.

Help?
Shachar