[perl #125618] [REGEX] Positional capture of separator on ?% operator causes compiler error <[email protected]>

So I looked at this, and at very best the proposed error message is wrong. It
says that ? is useless when used with %%, but that's not the case:

<AlexDaniel> m: say ‘af’|‘a’|‘f’|‘’ ~~ /a? %% f/
<camelia> rakudo-moar 00af9ce27: OUTPUT: «｢af｣␤｢a｣␤｢f｣␤｢｣␤»
<AlexDaniel> m: say ‘af’|‘a’|‘f’|‘’ ~~ /a?f?/
<camelia> rakudo-moar 00af9ce27: OUTPUT: «｢af｣␤｢a｣␤｢f｣␤｢｣␤»

So yes, while in this case it is equivalent to just `a?f?`, it's not totally
useless. And I think it makes sense to allow this behavior to have all
quantifiers behaving consistently.

And according to this discussion, that particular fix will not go in:
https://irclog.perlgeek.de/perl6-dev/2018-02-11#i_15804761

On 2018-02-11 02:04:06, [email protected] wrote:
> On Wed, 15 Jul 2015 07:31:55 -0700, lue wrote:
> > <ShimmerFairy> m: say "ab" ~~ /^ a +% (b) $/
> > <camelia> rakudo-moar e4077e: OUTPUT«Nil␤»
> > <ShimmerFairy> m: say "ab" ~~ /^ a *% (b) $/
> > <camelia> rakudo-moar e4077e: OUTPUT«Nil␤»
> > <ShimmerFairy> m: say "ab" ~~ /^ a ?% (b) $/
> > <camelia> rakudo-moar e4077e: OUTPUT«===SORRY!===␤QAST::Block with
> > cuid cuid_1_1436969557.11546 has not appeared␤»
> >
> > On JVM:
> >
> > <ShimmerFairy> j: say "ab" ~~ /^ a ?% (b) $/
> > <camelia> rakudo-jvm e4077e: OUTPUT«===SORRY!===␤setcodeobj can only
> > be used with a CodeRef␤»
> >
> > And just to show that it's not just the ?% operator alone
> > contributing
> > to the issue, but the capture too:
> >
> > <ShimmerFairy> m: say "ab" ~~ /^ a ?% [b] $/
> > <camelia> rakudo-moar e4077e: OUTPUT«Nil␤»
>
>
> Made potential fix in branches in
> https://github.com/perl6/nqp/commit/ac3d5307f3 and
> https://github.com/perl6/roast/commit/f2b07afbba
>
> The fix is to throw on `?`/`??` quantifiers used with `%`/`%%`.
>
> It's blocked by 2 lines of 6.c tests that indirectly use this
> combination:
>
https://github.com/perl6/roast/commit/aaa71d077431cd0a86be84bd386e1032a12e8697#diff-
> ae7b47444302486e790af6ee18121fabR16
>
> I delegated it to the release manager whether to merge that stuff or
> to require a fix that leaves these working and then we fix the
> compilation error itself.