struct Foo (size_t N) {
  void opAssign (size_t NN)(Foo!(NN) f) {/*do nothing*/}
}
struct Bar (string S) {
  void opAssign (string SS)(Bar!(SS) f) {/*do nothing*/}
}
void main() {
  Bar!"BAR1" bar1;
  Bar!"BAR2" bar2;
  bar2 = bar1;			// this compiles fine
  Foo!4 foo1;
  Foo!4 foo2;
  foo2 = foo1;			// compilation error
}


Gives an error saying:
Error: cannot implicitly convert expression (foo1) of type Foo!(4) to Foo!(NN)

Structs with string parameters work just fine.