So, would you say that the contract described by the Traits promised to keep "THE Book" on the able, and then it was able to be violated, or, are you saying that the promise made was actually to keep "A Book" of similar size, dimensions on the table at all times?
In other word, did these "Trait Games" boil down to promising something different that seemingly promised, or promising one thing and doing another?
Doesn't std:mem:swap require that the things being swapped are type compatible? If so, is Pin really promising the presence of "A Book" instead of "THE Book". "A Book" would not be undefined behavior in my mind because there would still be a valid book there when I went there. It just wouldn't be the book I was expecting. But, if I someone swap "THE Book" with "A Zamboni", and I didn't know how to deal with "A Zamboni", that would be UB. Especially if I just pretended like the "A Zamboni" was "A Book" even if "A Book" wouldn't be correct when I expected "THE Book", I would at least still now how to read from and write to the "Book" even though it isn't "THE Book" I started with?
Is that not right? So, is this a case, not of unsoundness, but, a case of promises kept literally, but, not in Spirit?
Would it / does it make sense to say that all Pin guarantees is there will be a valid T there, not necessarily THE T I was originally given? But, that there can't be a Q there instead. Nor can there be an almost T with some of its invariants compromised.
I think, from what I'm reading, that this problem is a case of the promise of Pin being kept, but, having an idea of what that promise is that is not what is being promised.
So, my question is, Does Pin guarantee "THE T" given originally or "A T" which is a valid "T" to be pinned at that location.
It seems like the notion of "THE T" is suspect because it would be difficult to have any real meaning there.
So, if I promise to put a book cover on the table with a 100 pages, and then, when you aren't looking, I swap it with another 100 page book, have I violated the Promise of Pin?
I would think not. However, if I swap it with a bag of Doritos, I've definitely violated the Promise.
True / False?
EDIT: I guess I'm wondering is Pin really promising the original T or is it just making a promise that whatever is there is Lishkov Substitutable/Compatible T? As I think about it, to not have UB, the latter is sufficient, and the ability to do mem::swap doesn't violate that.