Final by default?

[Walter Bright]

The argument for final by default, as eloquently expressed by Manu, is a good 
one. Even Andrei agrees with it (!).

The trouble, however, was illuminated most recently by the std.json regression 
that broke existing code. The breakage wasn't even intentional; it was a 
mistake. The user fix was also simple, just a tweak here and there to user code, 
and the compiler pointed out where each change needed to be made.

But we nearly lost a major client over it.

We're past the point where we can break everyone's code. It's going to cost us 
far, far more than we'll gain. (And you all know that if we could do massive 
do-overs, I'd get rid of put's auto-decode.)

Instead, one can write:

    class C { final: ... }

as a pattern, and everything in the class will be final. That leaves the "but 
what if I want a single virtual function?" There needs to be a way to locally 
turn off 'final'. Adding 'virtual' is one way to do that, but:

1. there are other attributes we might wish to turn off, like 'pure' and 'nothrow'.

2. it seems excessive to dedicate a keyword just for that.

So, there's the solution that has been proposed before:

    !final
    !pure
    !nothrow
    etc.