default implemented opAssign purity

[Dan]

Is the default implemented opAssign really pure and therefore 
should be considered such.

The code below works. If you turn off the static if, it will fail 
with:

Error: pure function 'opAssign.foo' cannot call impure function 
'opAssign.S.opAssign'.

Also, in case the answer is no and default implemented opAssign 
must remain impure, is there a standard way, in a member function 
to do like:

auto ref opAssign(S other) pure {
    blit other into this
    call this(this)
}

Thanks
Dan

-------------
import std.stdio;
import std.algorithm;

struct S {
   char[] c;
   this(this) pure { c = c.dup; }
   static if(1) {
     auto ref opAssign(S other) pure { c = other.c.dup; return 
this; }
   }
}

S foo(S s) pure {
   S other;
   other = s;
   return other;
}

void main() {
   S s;
   S another = foo(s);
}