Compiler does some flow analysis with -O..?

[Jarrett Billingsley]

Try this.

void main()
{
    int x = void;
    int y = 3;

    if(y < 10)
        x += 5;
    else
        x = 2;
}

Notice that x is uninitialized, so "x += 5;" shouldn't work.

If you compile this, even with -w, DMD happily accepts it.  But if you
throw -O, it gives the error:

dtest.d(187): Error: variable x used before set

This seems to be a relatively recent development, and doesn't seem to
be documented.  It's also very surprising that it only happens when -O
is thrown.

I like it a lot.  Could this functionality be formalized and extended
(i.e. to include accesses to variables declared like "int x;", like it
says in the spec)?