Re: Strange behavior with List::Util qw{first} on perl >= 5.20

Hmmmm... Perl v5.20:

$ perl -MList::Util=first -le 'print first {lc eq "abc"}
    keys %{{ABC=>"ABC"}}'
abc
$ perl -MList::Util=first -le 'print first {uc eq "ABC"}
    keys %{{ABC=>"ABC"}}'
ABC
$ perl -MList::Util=first -le 'print first {/abc/i}
    keys %{{ABC=>"ABC"}}'
ABC
$ perl -MList::Util=first -le 'print first {lc eq "abc"}
    values %{{ABC=>"ABC"}}'
ABC
$ perl -MList::Util=first -le 'print first {lc eq "abc"}
    "ABC"'
ABC
$ perl -MList::Util=first -le 'print first {lc eq "abc"}
    values @{["ABC"]}'
ABC
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On v5.10 they all (except for the last one) give "ABC". The inverse case, first {uc eq "ABC"} keys %{{abc=>"abc"}}, also gives the "unexpected" result "ABC". Smells like a bug... could this be rt://96343?

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Re^2: Strange behavior with List::Util qw{first} on perl >= 5.20 by Anonymous Monk on May 26, 2015 at 23:09 UTC
A `git bisect` appears to confirm that. The bug also still appears to be present in the latest v5.22.0-RC2.	[reply] [d/l]
Re^3: Strange behavior with List::Util qw{first} on perl >= 5.20 by ikegami (Patriarch) on May 27, 2015 at 14:42 UTC
There's something fishy about `$_`. It's a TEMP with REFCNT=1. I believe that means it's a mortal. If so, that means noone claims a reference to `$_`, but `first` still uses it after the callback returns. This points to a bug in List::Util. That said, the inplace `lc($_)`^[1] should probably only happen in void context. This is the first I've ever heard about this!	[reply] [d/l] [select]