bug? re named groups in Annex B

[jmdyck]

In the main body, we have (in part):

Atom[U, N] :: ( GroupSpecifier[?U] Disjunction[?U, ?N] )

Annex B doesn't change that production, but it does restrict its use (in Term) to [+U]. For [~U], the corresponding production is

ExtendedAtom[N] :: ( Disjunction[~U, ?N] )

That is, if a web browser (following Annex B) is parsing a regex with [~U], it is supposed to reject named groups.

I'm not sure if this is a bug, but it does seem odd to me.

• A non-browser is required to accept named groups when parsing with [~U], and I thought Annex B was only supposed to add features, not subtract.

• The Annex B production for AtomEscape allows named backreferences under [~U, +N], but what's the point if [~U] doesn't allow named groups in the first place?

This goes back to the introduction of named capture groups in 95ec0c6 (PR #1027).

The final state of the original proposal doesn't mention Annex B, so it's unclear what #1027 was supposed to do re Annex B.

@mathiasbynens and/or @littledan, please comment.