When you said above that quid here is equivalent to "what", and that it functions as a relative more than an indefinite, I think that you meant it was compound relative, instead of a simple relative. I think that you used the word "indefinite relative", but I didn't know what you meant there. The simple relative would be "which".
It's an indefinite relative because it means "what(ever)" rather than a specific "what".
Compare:
1) Take what you like = take whatever you like (I don't know what you'll like, but whatever it is, take it).
2) I like what you said yesterday = I like the specific thing which you said yesterday.
In both sentences, "what" is a relative pronoun.
You can also say that in both sentences, "what" is a compound relative pronoun (I didn't know that term before) since in both cases it stands for "that which".
But the difference is that the relative pronoun in 1) is indefinite while that in 2) is definite.
In the case of quidlibet, it seems that we agree that both the indefinite and the relative are required for it to be grammatically correct (aliquid quod libet) , as it were.
No, we don't agree on that. All you need for it to be grammatically correct is
quidlibet.
But the question now appears to be which which one is explicit, and which one is implied. Since L&S defines quislibet as in indefinite, and A&G likewise, I think that the indefinite is the explicit and the relative is the implied, and so I would like to call quislibet a compound indefinite substantive pronoun. Maybe you think that I'm splitting hairs here, but I think this is interesting.
We have to separate etymology and usage.
Etymologically,
quidlibet =
quid, used as an indefinite (and compound) relative pronoun + the verb
libet. So, literally, "what pleases."
Now that combination came to be used simply as an indefinitive pronoun:
quidlibet = anything.
Note that the masculine nominative singular form is
quilibet.
the relative is the implied
Relatives are never implied in Latin. In English you can say things like "the thing I saw" for "the thing
that I saw". But in Latin the relative must always be expressed.
Ex nihilo fit aliquid quod libet = "out of nothing is made something that pleases." (And it frankly sounds wrong due to how
libet is normally used, but let's not get into that right now.)
Ex nihilo fit quidlibet = "out of nothing, anything (or whatever you like) is made."