[perl #57768] Negative-smartmatching on a regexp produces a Null PMC access in rakudo

[Carl Mäsak]

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r30155:
$ ./perl6 -e '"o" ~~ /o/' # works

$ ./perl6 -e '"o" !~~ /o/' # fails
Null PMC access in set_pmc_keyed()

I added two tests in t/operators/smartmatch.t to exercise the correct behaviour.