<regex> syntax_option_type is not a proper bitmask

[miyuki]

Bugzilla Link	35967
Resolution	FIXED
Resolved on	Mar 28, 2019 10:32
Version	6.0
OS	Linux
CC	@hubert-reinterpretcast,@mclow

Extended Description

According to [re.synopt]:
"The type syntax_option_type is an implementation-defined bitmask type"

[bitmask.types] defines some requirements for such types, specifically:
"Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such elements have distinct, nonzero values"

Contrarily to this requirement, in libc++ std::regex_constants::ECMAScript is 0.

[hubert-reinterpretcast]

As mentioned in http://lists.llvm.org/pipermail/cfe-dev/2016-October/051290.html,
the use of "0" breaks use cases like the following:
bool isEcmaScript(::std::regex_constants::syntax_option_type v) {
return v & ::std::regex_constants::ECMAScript;
}

[mclow]

We've got some ABI-breaking stuff coming for <regex>
When that happens, we'll circle back to this.

[mclow]

Fixed in commit 357190. Note that this requires a new dylib.
(and is an ABI break, so it's off by default)

Also, I looked at 'format_default' and 'match_default', but they're defined as empty bitmasks, so they stayed == 0.