Using std.conv.to with enum that is based on string type

[Uranuz via Digitalmars-d]

When I tried to use std.conv.to to serialize enum values that are 
based on string type I faced with situation that *to* not exactly 
as I expected. As far as I understand it tries to find match 
between passed string value and enum value identifier instead of 
just casting it to enum type.

1. So I want to know if it is expected behaviour?
2. What are the reasons for it?

There is a simple example that illustrates what I'm talking about.

import std.stdio, std.conv;

enum Category: string { first = "I", second = "II", third = "III" 
};

void main()
{
	
	assert( to!Category("first") == Category.first ); //This runs 
succesfully
	//assert( to!Category("I") == Category.first ); //This 
conversion fails

}

As a result I could try to fix it if it's not correct behaviour. 
Another concern about using *cast* is that it doesn't make some 
checks if value is one of enum values declared in code.

I found some topics about enum conversions.

http://forum.dlang.org/thread/bug-4744-3@http.d.puremagic.com%2Fissues%2F
http://forum.dlang.org/thread/bug-9821-3@http.d.puremagic.com%2Fissues%2F
http://forum.dlang.org/thread/fpzhgzhrhaxjfuiydwxh@forum.dlang.org

Problem is that (as I think) that std.conv.to documentation 
doesn't describe how  *to* SHOULD work for different ways of 
coversions so it's not obvious how to *fix* it. The only way of 
using is via trials and errors and it's not garanteed that found 
out with this way will not be changed and your code will not be 
broken. It's because that implementers don't have information 
about how exactly it SHOULD work.

Sorry for some emotions but I have problems with lack of doc 
sometimes and it makes my job harder.