Arithmetic conversion -- warnings missing?

[Q. Schroll via Digitalmars-d-learn]

import std.stdio;

void main()
{
     short  ss = -1;
     ushort us =  0;
     int    si = -1;
     uint   ui =  0;
     if (ss < us)
         writeln("a");
     else
         writeln("b");

     if (si < ui)
         writeln("A");
     else
         writeln("B");
}

prints "aB" due to integral promotion and arithmetic conversion 
rules. I know it's this kind of thing: If it compiles and looks 
like c, it behaves the same.
But why does it compile and why does it so without any warning?
Is there any rationale to make this even work?