final switch traps and improvements

[bearophile]

Recently Ada 2012 was accepted as ISO standard, and on the good 
Lambda the Ultimate blog there is a small thread about it, with 
an interesting post:

http://lambda-the-ultimate.org/node/4661#comment-73731

The post makes two simple valid points, worth considering.

Currently in D this code compiles with no errors:


void main() {
     bool b;
     final switch (b) {
         case true:
             break;
     }
}


And gives at run-time:
core.exception.SwitchError at test(3): No appropriate switch clause 
found

It's one of my top fifteen bug reports (it was mislabelled as 
enhancement request):
http://d.puremagic.com/issues/show_bug.cgi?id=5713

In my opinion It's a topic worth discussing and fixing.

The simplest solution is to statically disallow "final switch" to 
operate on anything bug enums.

A better solution is to make it work correctly where it can, and 
statically disallow the other situations. This means a final 
switch on bool must require both true and false cases. If the 
final switching is on a "uint % 3" expression then the compiler 
must statically require all the 0,1,2 cases and nothing else.


If part of all of this enhancement request gets accepted, then 
other useful final switch situations are worth supporting:
http://d.puremagic.com/issues/show_bug.cgi?id=596

One common use case is with Nullable, but syntactically speaking 
this is not good enough:

import std.typecons: Nullable;
void main() {
     auto n = Nullable!int(5);
     final switch (n) {
         case Nullable(false, _): ...
         case Nullable(true, _): ...
     }
}

Bye,
bearophile