[enum.copy_options] Poor wording for requirement on elements set in copy_options

[jwakely]

[enum.copy_options] says:

Calling a library function with more than a single constant for an option group results in undefined behavior.

I don't like how this is phrased. Firstly, which library? Does it apply to any function in the standard library, e.g. std::move?

Secondly, I'd prefer if it used the term of art "is set" for bitmask elements set in a bitmask object.

This doesn't exactly roll off the tongue, but I think it's more precise:

"A value of type copy_options that is passed to a function described in subclause 27.10 shall have at most one element set from each option group."

[timsong-cpp]

There are only two functions taking copy_options: copy and copy_file. Both of them have a similarly poorly phrased Requires: clause restating this requirement, e.g.:

Requires: At most one constant from each option group ([enum.copy_options]) is present in options.

I'd suggest dropping the offending sentence entirely from [enum.copy_options] or turning it into a non-normative note, and rewording the two Requires clauses to use the correct terms of art ("element" and "is set").

[jensmaurer]

@timsong-cpp: Sounds like a good idea.