How to sort a range

[rcorre via Digitalmars-d-learn]

I was in a situation where I wanted to remove duplicates from an 
OnlyResult.
To do this with uniq, I needed to sort it. OnlyResult doesn't 
satisfy the template constraints of sort, but this seems easy 
enough to fix. I made front, back, and opIndex return by ref. 
With this, the following compiles:

     assert(only(3,1,2).sort.equal(only(1,2,3)));

However, it fails with:

     core.exception.AssertError at std/algorithm/sorting.d(1052): 
Failed to sort range of type OnlyResult!(int, 3LU)

So, if you have a range for which sort compiles, what does it 
take to make sorting actually work?

For reference, my two attempts were:

https://github.com/rcorre/phobos/commit/d89b3cfab7a0938e178a506b4ceb8faae6ecbfe2

https://github.com/rcorre/phobos/commit/512d9b8db6f311db6a9b6ccb077a691cec66ce70