RFC: Value range propagation for if-else

["Nordlöw" via Digitalmars-d]

> static if (this.min <= r[0] &&
>                        r[1] <= this.max)

BTW: I believe valueRange also enables specialization of constant 
arguments inside existing functions without having to move them 
to template arguments of specialized functions. Showcase:

auto pow(T)(T arg, uint n)
{
     enum vr = __traits(valueRange, arg);
     static if (vr.min == vr.max) // if arg is constant
         static      if (vr.min == 0)
             return 1;
         else static if (vr.min == 1)
             return arg;
         else static if (vr.min == 2)
             return arg*arg;
         else static if (vr.min == 3)
             return arg*arg*arg;
         // etc...
     // generic case
}

I guess a __trait, named something like has[Fixed|Constant]Value, 
would be useful here.

I bet there are more possibilities with these traits we haven't 
thought of yet.