Numeric limits tracking

[Manu]

So, here's an issue that constantly drives me nuts, and an elegant solution
seems so do-able.

void func(int x)
{
  x &= 0xFFFF;
  short s = x; // Error! (but we know x is 0 .. 65535)

  if(x < 256)
  {
    byte b = x; // Error! (we also know x is 0 .. 255)
  }
}

It would be really nice if the compiler would carry around the known
possible range of values, and refer to that information when performing
down-casting assignments.
It would also be very useful information for the back end, it can choose
more efficient types if it knows the range, or produce jump tables rather
than branch sequences.

I think the compiler would need a min, max, and mask for any integers, and
update them as it works.

There are many sources of this information.
Comparisons effectively limit the range:
if(x > 0)
{
  // x = 0 .. int.max
}
else
{
  // x = int.min .. -1
}

Masks, obviously:
x &= 15;

Also contracts are a really great source of seeding this information on
function entry.

Has this been discussed? It seems simple, and it's invisible to the
language. It would just reduce some boilerplate (tedious casts), and also
offer some great optimisation opportunities.
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