[Bug libstdc++/71500] regex::icase only works on first character in a range

["timshen at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71500

--- Comment #6 from Tim Shen <timshen at gcc dot gnu.org> ---
(In reply to mwd from comment #5)
> All of the ECMAScript engines I have found work with this , and the
> ECMAScript specs seem to imply that this should work as well.

I think you are right.

According to ECMAScript 262 6th edition [21.2.2.8.1]:
5. If invert is false, then
    a. If there does not exist a member a of set A such that Canonicalize(a) is
cc, return failure.

, when creating the CharSet A, ignorecase is not taken into consideration at
all; rather, when matching a character against a range, both the character and
the candidate from the range is "Canonialized" and compared.

Ideally I would implement this as (which is similar to your suggestion):
  auto toggle_case(auto c) {
    if (isupper(c)) return tolower(c);
    if (islower(c)) return toupper(c);
    return c;
  }

  if (range.first <= c && c < range.second) { ... }
  else if (icase && (range.first <= toggle_case(c) && toggle_case(c) <
range.second)) { ... }

The problem is I'm not sure how to implement this, since:
When collate is off, locale related function like tolower and toupper should
not be called; when collate is on, everything should go through
regex_traits::transform, which doesn't care about icase.

libc++ 3.8.0 accepts the regex, but prints:
aaa : Nope
AAA : Nope
fff : Nope
FFF : Nope
ttt : Nope
TTT : Nope
uuu : Nope
UUU : Nope
ggg : Nope
GGG : Nope

which is similar to what I did previously.

I think this is a design flaw in regex_traits, and the Boost behavior is
slightly better (although all of us are broken :P).