class Foo
{
   void opAssign(int bar)
   {
   }
}

void main()
{
   auto foo = new Foo;
   foo = null;
}

test.d(11): Error: function test.Foo.opAssign (int bar) is not callable using argument types (void*)
test.d(11): Error: cannot implicitly convert expression (null) of type void* to int