By allowing constructors in mixins?
https://d.puremagic.com/issues/show_bug.cgi?id=8228
It's also trivial to implement with string mixins, although it's bad style ;-)
```d
enum simpleException(string name) =
"
class " ~ name ~ " : Exception
{
    @safe pure nothrow
    this(string msg, string file = __FILE__, size_t line = __LINE__)
    {
        super(msg, file, line);
    }
}
";
mixin(simpleException!"GetOptException");

void main()
{
	throw new GetOptException("blah");
}
```
Or did you think of a more generic/powerful abstraction which could also assign member variables and so on?