DIP33: A standard exception hierarchy

[deadalnix]

On Tuesday, 2 April 2013 at 20:11:31 UTC, Lars T. Kyllingstad 
wrote:
> Phobos/druntime devs can always add to the enums.  Users still 
> have the option of subclassing if strictly necessary.
>

This is fundamentally incompatible with :

> I'm not sure I agree with that rule.  And anyway, D's final 
> switch mitigate some of the problems with classic switch.
>

As adding an entry unto the enum will break every single final 
switch in user code.