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hash dereference of undefined value #14271

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p5pRT opened this issue Nov 24, 2014 · 6 comments
Closed

hash dereference of undefined value #14271

p5pRT opened this issue Nov 24, 2014 · 6 comments

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@p5pRT
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@p5pRT p5pRT commented Nov 24, 2014

Migrated from rt.perl.org#123292 (status was 'rejected')

Searchable as RT123292$

@p5pRT
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@p5pRT p5pRT commented Nov 24, 2014

From perl-bug@ton.iguana.be

Created by perl-bug@ton.iguana.be

perl -wle 'use strict; print if %$a; print $a'
Can't use an undefined value as a HASH reference at -e line 1.

perl -wle 'use strict; print if keys %$a; print $a'
HASH(0x1da8e78)

Until I stumbled into this I thought that both ways of testing if a HASH has elements were equivalent.
I'd prefer it if that thought would become true.

Perl Info

Flags:
    category=core
    severity=wishlist

Site configuration information for perl 5.20.1:

Configured by Debian Project at Fri Oct 10 14:16:26 UTC 2014.

Summary of my perl5 (revision 5 version 20 subversion 1) configuration:
   
  Platform:
    osname=linux, osvers=3.2.0-37-generic, archname=x86_64-linux-gnu-thread-multi
    uname='linux batsu 3.2.0-37-generic #58-ubuntu smp thu jan 24 15:28:10 utc 2013 x86_64 x86_64 x86_64 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.20 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.20 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.20 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.20.1 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.20.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.20.1 -des'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2 -g',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion='', gccversion='4.9.1', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=libc-2.19.so, so=so, useshrplib=true, libperl=libperl.so.5.20
    gnulibc_version='2.19'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib -fstack-protector'

Locally applied patches:
    DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
    DEBPKG:debian/db_file_ver - http://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
    DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
    DEBPKG:debian/enc2xs_inc - http://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories.
    DEBPKG:debian/errno_ver - http://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
    DEBPKG:debian/libperl_embed_doc - http://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
    DEBPKG:fixes/respect_umask - Respect umask during installation
    DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories
    DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib
    DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
    DEBPKG:debian/prefix_changes - Fiddle with *PREFIX and variables written to the makefile
    DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
    DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
    DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
    DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
    DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian
    DEBPKG:debian/module_build_man_extensions - http://bugs.debian.org/479460 Adjust Module::Build manual page extensions for the Debian Perl policy
    DEBPKG:debian/prune_libs - http://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
    DEBPKG:fixes/net_smtp_docs - [rt.cpan.org #36038] http://bugs.debian.org/100195 Document the Net::SMTP 'Port' option
    DEBPKG:debian/perlivp - http://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
    DEBPKG:debian/deprecate-with-apt - http://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules
    DEBPKG:debian/squelch-locale-warnings - http://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
    DEBPKG:debian/skip-upstream-git-tests - Skip tests specific to the upstream Git repository
    DEBPKG:debian/patchlevel - http://bugs.debian.org/567489 List packaged patches for 5.20.1-1 in patchlevel.h
    DEBPKG:debian/skip-kfreebsd-crash - http://bugs.debian.org/628493 [perl #96272] Skip a crashing test case in t/op/threads.t on GNU/kFreeBSD
    DEBPKG:fixes/document_makemaker_ccflags - http://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
    DEBPKG:debian/find_html2text - http://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text
    DEBPKG:debian/perl5db-x-terminal-emulator.patch - http://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
    DEBPKG:debian/cpan-missing-site-dirs - http://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable
    DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] http://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected
    DEBPKG:debian/regen-skip - Skip a regeneration check in unrelated git repositories
    DEBPKG:fixes/regcomp-mips-optim - [perl #122817] http://bugs.debian.org/754054 Downgrade the optimization of regcomp.c on mips and mipsel due to a gcc-4.9 bug
    DEBPKG:debian/makemaker-pasthru - http://bugs.debian.org/758471 Pass LD settings through to subdirectories
    DEBPKG:fixes/perldoc-less-R - [rt.cpan.org #98636] http://bugs.debian.org/758689 Tell the 'less' pager to allow terminal escape sequences
    DEBPKG:fixes/hurd_test_todo_socket.t - [perl #122657] http://bugs.debian.org/758718 Disable failing GNU/Hurd test in t/io/socket.t
    DEBPKG:fixes/pod_man_reproducible_date - http://bugs.debian.org/759405 Support POD_MAN_DATE in Pod::Man for the left-hand footer
    DEBPKG:fixes/data_dump_infinite_recurse - [19be3be] don't recurse infinitely in Data::Dumper


@INC for perl 5.20.1:
    /etc/perl
    /usr/local/lib/x86_64-linux-gnu/perl/5.20.1
    /usr/local/share/perl/5.20.1
    /usr/lib/x86_64-linux-gnu/perl5/5.20
    /usr/share/perl5
    /usr/lib/x86_64-linux-gnu/perl/5.20
    /usr/share/perl/5.20
    /usr/local/lib/site_perl
    .


Environment for perl 5.20.1:
    HOME=/home/ton
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/ton/bin:/home/ton/bin.SampleSetup:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:.:/home/ton/chromiumos/depot_tools:/home/ton/chromiumos/depot_tools
    PERL_BADLANG (unset)
    SHELL=/bin/bash

@p5pRT
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@p5pRT p5pRT commented Nov 25, 2014

From @jkeenan

On Mon Nov 24 05​:22​:38 2014, thospel wrote​:

This is a bug report for perl from perl-bug@​ton.iguana.be,
generated with the help of perlbug 1.40 running under perl 5.20.1.

-----------------------------------------------------------------
[Please describe your issue here]

perl -wle 'use strict; print if %$a; print $a'
Can't use an undefined value as a HASH reference at -e line 1.

perl -wle 'use strict; print if keys %$a; print $a'
HASH(0x1da8e78)

Until I stumbled into this I thought that both ways of testing if a
HASH has elements were equivalent.
I'd prefer it if that thought would become true.

I don't think this is a bug in Perl, nor do I think that it should be approved as a feature request.

In the first place, your example is somewhat confusing in that '$a' (along with '$b') is a special variable in Perl's sort function. If instead of '$a' you had used, say, '$c', you would have gotten this​:

#####
$ perl -wle 'use strict; print if keys %$c; print $c'
Global symbol "$c" requires explicit package name at -e line 1.
Global symbol "$c" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
#####

Since strictures are not the real problem here, let's (for once) drop the 'use strict'. Now we get​:

#####
$ perl -wle 'print if keys %$c; print $c'
HASH(0x905148)
#####

But all this says is that at the conclusion of this statement a hash exists at that location in memory. It does *not* tell us if that hash has any elements in it or not. The hash has been autovivified by using the 'keys' function, but it doesn't have any elements. We can see that with this​:

#####
$ perl -wle 'print if keys %$c; print $c; while (($k,$v) = each(%$c)) { print "$k => $v" }'
HASH(0x14e8148)
#####

No elements in the hash. If your objective is to determine whether a hash has elements or not, you can simply say​:

#####
# Autovivify an empty hash​:
$ perl -wle 'keys %$c and print $c'
# No output

# Ensure that hash has at least 1 element​:
$ perl -wle '$c = { alpha => 42 }; keys %$c and print $c'
HASH(0x107c148)
#####

In a boolean context, 'keys %hash' is the way to determine whether a hash has elements or not.

In contrast, '%$c' does not cause a hash to be autovivified. The warning you got is an accurate description of what is happening.

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags​:
category=core
severity=wishlist
---
Site configuration information for perl 5.20.1​:

Configured by Debian Project at Fri Oct 10 14​:16​:26 UTC 2014.

Summary of my perl5 (revision 5 version 20 subversion 1)
configuration​:

Platform​:
osname=linux, osvers=3.2.0-37-generic, archname=x86_64-linux-gnu-
thread-multi
uname='linux batsu 3.2.0-37-generic #58-ubuntu smp thu jan 24
15​:28​:10 utc 2013 x86_64 x86_64 x86_64 gnulinux '
config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat
-Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-
z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro
-Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr
-Dprivlib=/usr/share/perl/5.20 -Darchlib=/usr/lib/x86_64-linux-
gnu/perl/5.20 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5
-Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.20
-Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.20.1
-Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.20.1
-Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3
-Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3
-Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-
pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil
-Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib
-Dlibperl=libperl.so.5.20.1 -des'
hint=recommended, useposix=true, d_sigaction=define
useithreads=define, usemultiplicity=define
use64bitint=define, use64bitall=define, uselongdouble=undef
usemymalloc=n, bincompat5005=undef
Compiler​:
cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-
strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64',
optimize='-O2 -g',
cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-
aliasing -pipe -I/usr/local/include'
ccversion='', gccversion='4.9.1', gccosandvers=''
intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
alignbytes=8, prototype=define
Linker and Libraries​:
ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include-
fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu
/lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
perllibs=-ldl -lm -lpthread -lc -lcrypt
libc=libc-2.19.so, so=so, useshrplib=true, libperl=libperl.so.5.20
gnulibc_version='2.19'
Dynamic Linking​:
dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib -fstack-
protector'

Locally applied patches​:
DEBPKG​:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS
default for modules installed from CPAN.
DEBPKG​:debian/db_file_ver - http​://bugs.debian.org/340047 Remove
overly restrictive DB_File version check.
DEBPKG​:debian/doc_info - Replace generic man(1) instructions with
Debian-specific information.
DEBPKG​:debian/enc2xs_inc - http​://bugs.debian.org/290336 Tweak
enc2xs to follow symlinks and ignore missing @​INC directories.
DEBPKG​:debian/errno_ver - http​://bugs.debian.org/343351 Remove
Errno version check due to upgrade problems with long-running
processes.
DEBPKG​:debian/libperl_embed_doc - http​://bugs.debian.org/186778
Note that libperl-dev package is required for embedded linking
DEBPKG​:fixes/respect_umask - Respect umask during installation
DEBPKG​:debian/writable_site_dirs - Set umask approproately for
site install directories
DEBPKG​:debian/extutils_set_libperl_path - EU​:MM​: set location of
libperl.a under /usr/lib
DEBPKG​:debian/no_packlist_perllocal - Don't install .packlist or
perllocal.pod for perl or vendor
DEBPKG​:debian/prefix_changes - Fiddle with *PREFIX and variables
written to the makefile
DEBPKG​:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to
the binary targets.
DEBPKG​:debian/instmodsh_doc - Debian policy doesn't install
.packlist files for core or vendor.
DEBPKG​:debian/ld_run_path - Remove standard libs from LD_RUN_PATH
as per Debian policy.
DEBPKG​:debian/libnet_config_path - Set location of libnet.cfg to
/etc/perl/Net as /usr may not be writable.
DEBPKG​:debian/mod_paths - Tweak @​INC ordering for Debian
DEBPKG​:debian/module_build_man_extensions -
http​://bugs.debian.org/479460 Adjust Module​::Build manual page
extensions for the Debian Perl policy
DEBPKG​:debian/prune_libs - http​://bugs.debian.org/128355 Prune the
list of libraries wanted to what we actually need.
DEBPKG​:fixes/net_smtp_docs - [rt.cpan.org #36038]
http​://bugs.debian.org/100195 Document the Net​::SMTP 'Port' option
DEBPKG​:debian/perlivp - http​://bugs.debian.org/510895 Make perlivp
skip include directories in /usr/local
DEBPKG​:debian/deprecate-with-apt - http​://bugs.debian.org/747628
Point users to Debian packages of deprecated core modules
DEBPKG​:debian/squelch-locale-warnings -
http​://bugs.debian.org/508764 Squelch locale warnings in Debian
package maintainer scripts
DEBPKG​:debian/skip-upstream-git-tests - Skip tests specific to the
upstream Git repository
DEBPKG​:debian/patchlevel - http​://bugs.debian.org/567489 List
packaged patches for 5.20.1-1 in patchlevel.h
DEBPKG​:debian/skip-kfreebsd-crash - http​://bugs.debian.org/628493
[perl #96272] Skip a crashing test case in t/op/threads.t on
GNU/kFreeBSD
DEBPKG​:fixes/document_makemaker_ccflags -
http​://bugs.debian.org/628522 [rt.cpan.org #68613] Document that
CCFLAGS should include $Config{ccflags}
DEBPKG​:debian/find_html2text - http​://bugs.debian.org/640479
Configure CPAN​::Distribution with correct name of html2text
DEBPKG​:debian/perl5db-x-terminal-emulator.patch -
http​://bugs.debian.org/668490 Invoke x-terminal-emulator rather than
xterm in perl5db.pl
DEBPKG​:debian/cpan-missing-site-dirs -
http​://bugs.debian.org/688842 Fix CPAN​::FirstTime defaults with
nonexisting site dirs if a parent is writable
DEBPKG​:fixes/memoize_storable_nstore - [rt.cpan.org #77790]
http​://bugs.debian.org/587650 Memoize​::Storable​: respect 'nstore'
option not respected
DEBPKG​:debian/regen-skip - Skip a regeneration check in unrelated
git repositories
DEBPKG​:fixes/regcomp-mips-optim - [perl #122817]
http​://bugs.debian.org/754054 Downgrade the optimization of regcomp.c
on mips and mipsel due to a gcc-4.9 bug
DEBPKG​:debian/makemaker-pasthru - http​://bugs.debian.org/758471
Pass LD settings through to subdirectories
DEBPKG​:fixes/perldoc-less-R - [rt.cpan.org #98636]
http​://bugs.debian.org/758689 Tell the 'less' pager to allow terminal
escape sequences
DEBPKG​:fixes/hurd_test_todo_socket.t - [perl #122657]
http​://bugs.debian.org/758718 Disable failing GNU/Hurd test in
t/io/socket.t
DEBPKG​:fixes/pod_man_reproducible_date -
http​://bugs.debian.org/759405 Support POD_MAN_DATE in Pod​::Man for the
left-hand footer
DEBPKG​:fixes/data_dump_infinite_recurse - [19be3be] don't recurse
infinitely in Data​::Dumper

---
@​INC for perl 5.20.1​:
/etc/perl
/usr/local/lib/x86_64-linux-gnu/perl/5.20.1
/usr/local/share/perl/5.20.1
/usr/lib/x86_64-linux-gnu/perl5/5.20
/usr/share/perl5
/usr/lib/x86_64-linux-gnu/perl/5.20
/usr/share/perl/5.20
/usr/local/lib/site_perl
.

---
Environment for perl 5.20.1​:
HOME=/home/ton
LANG=en_US.UTF-8
LANGUAGE (unset)
LD_LIBRARY_PATH (unset)
LOGDIR (unset)
PATH=/home/ton/bin​:/home/ton/bin.SampleSetup​:/usr/local/bin​:/usr/local/sbin​:/usr/bin​:/usr/sbin​:/bin​:/sbin​:.​:/home/ton/chromiumos/depot_tools​:/home/ton/chromiumos/depot_tools
PERL_BADLANG (unset)
SHELL=/bin/bash

--
James E Keenan (jkeenan@​cpan.org)

@p5pRT
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@p5pRT p5pRT commented Nov 25, 2014

The RT System itself - Status changed from 'new' to 'open'

@p5pRT
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@p5pRT p5pRT commented Nov 25, 2014

From @demerphq

On 25 November 2014 at 04​:12, James E Keenan via RT <
perlbug-followup@​perl.org> wrote​:

On Mon Nov 24 05​:22​:38 2014, thospel wrote​:

This is a bug report for perl from perl-bug@​ton.iguana.be,
generated with the help of perlbug 1.40 running under perl 5.20.1.

-----------------------------------------------------------------
[Please describe your issue here]

perl -wle 'use strict; print if %$a; print $a'
Can't use an undefined value as a HASH reference at -e line 1.

perl -wle 'use strict; print if keys %$a; print $a'
HASH(0x1da8e78)

Until I stumbled into this I thought that both ways of testing if a
HASH has elements were equivalent.
I'd prefer it if that thought would become true.

I don't think this is a bug in Perl, nor do I think that it should be
approved as a feature request.

I agree.

However not for the reasons you explained. I think Ton is getting confused
by the minutae of autovivification of hashrefs, which IMO /is/ a gnarly
subject. Basically telling when a dereference operation like %{$hash} is
considered to be in LVALUE context, and as such supposed to autovivify,
from an RVALUE access where it should die is harder than it should be, and
in fact moderately inconsistent.

A raw %$hash where $hash is undef will not autovivify​:

perl -wle'use strict; my $hash; my $k = %$hash'
Can't use an undefined value as a HASH reference at -e line 1.

However, when one hash-derefs an undef var in the *arguments* to a normal
function it *is* autovivified​:

perl -wle'use strict; sub P {print for @​_}; my $hash; P(%$hash); print
$hash'
HASH(0x1798998)

Which is why keys(%$hash) behaves differently than a simple (%$hash)
dereference.

I am under the impression this is because the arguments to a sub call are
treated as being in lvalue context due to the requirement of aliasing their
contents. In particular this has to work​:

perl -wle'use strict; sub P {$_[1]="P";}; my $hash={a=>0}; P(%$hash); print
$hash->{a}'
P

Which if I understand things correctly is why %$hash is treated differently
than P(%$hash).

So, this is either not-a-bug, or it is a can't-fix/won't-fix due to wide
exploitation. On the other hand the behavior is inconsistent, as it does
not apply to all built-ins, such as print()​:

perl -wle'use strict; my $hash; print %$hash'
Can't use an undefined value as a HASH reference at -e line 1.

Anyway, I think one could argue that the inconsistency is a bug somewhere,
but the outcome of that would probably be best to fix the places where it
doesn't happen, like print, and not the other way around. And I kinda
wonder how much trouble we would cause if we made %$undef_var *not*
autovivify, but also *not* die (assuming it is a reasonable thing to do
implementation wise).

cheers,
Yves

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@p5pRT p5pRT commented Dec 16, 2017

From zefram@fysh.org

We can't change default autovivification behaviour for backcompat reasons.
This ticket should be closed.

-zefram

@p5pRT p5pRT closed this as completed Dec 18, 2017
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@p5pRT p5pRT commented Dec 18, 2017

@iabyn - Status changed from 'open' to 'rejected'

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