[perl #64952] Iterating over @(Match) flattens positional captures

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# New Ticket Created by  Moritz Lenz 
# Please include the string:  [perl #64952]
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Consider this:

00:57 <@moritz_> rakudo: 'ab' ~~ m/(.)+/; say $0.WHAT; say $1.WHAT
00:57 < p6eval> rakudo ad7389: OUTPUT«List␤Failure␤»

A quantified positional captures places a List into $0, and nothing into
$1 - that's correct, in my understanding. But when iterating over @($/)
it runs the body of the loop twice:

00:57 <@moritz_> rakudo: .say for @('ab' ~~ m/(.)+/)
00:57 < p6eval> rakudo ad7389: OUTPUT«a␤b␤»

That should be one iteration, where $_ is a List of two Match'es.

I also believe that this accounts for a bug in Match.caps, even though I
changed the tests to think otherwise... I'll try to think a bit more
about it tomorrow, after a good portion of sleep ;-)

Cheers,
Moritz