Value Range Propigation Spec

[Shammah Chancellor via Digitalmars-d]

A couple of us working on SDC are trying to get ValueRange propigation 
implemented.   I was wonder if someone could offer some insight as to 
how VRP works in DMD.   If for example, trying to get the value range 
of a global, what is the expected behavior?

It seems as though VRP is a language feature, and not a compiler 
feature -- since this allows some code to compile and not others.   Is 
there a specification for how it should work somewhere?  If not, it's 
hard to implement other compilers that will not generate errors in the 
same circumstances as DMD.