Parsing lvalue subs returning a reference to a scalar

[p5pRT]

Migrated from rt.perl.org#133240 (status was 'open')

Searchable as RT133240$

[p5pRT]

From @philiprbrenan

Reply-To​: philiprbrenan@​gmail.com
To​: perlbug@​perl.org
Subject​: Parsing lvalue subs returning a reference to a scalar
Message-Id​: <5.22.1_20124_1527899987@​pousadouros>
From​: philiprbrenan@​gmail.com

This is a bug report for perl generated with the help of perlbug 1.40
running
under perl 5.22.1.

#!/usr/bin/perl
require v5.16;

=pod

Lvalue subs returning a reference to a scalar are not always recognized in
the
same way that a scalar would be in a similar syntactic context.

=cut

my $aaa; # Scalar - all OK
$aaa = undef;
$aaa += 0;
$aaa //= undef;

sub aaa​:lvalue {\my $r} # Scalar returned via Lvalue sub
aaa = undef;
aaa += 0; # Syntax error near "+="
aaa //= undef; # Can't modify pattern match (m//) in scalar
assignment

---

Flags​:
  category=library
  severity=none
  module=

---

Site configuration information for perl 5.22.1​:

Configured by Debian Project at Thu Apr 5 12​:48​:47 UTC 2018.

Summary of my perl5 (revision 5 version 22 subversion 1) configuration​:

  Platform​:
  osname=linux, osvers=3.16.0, archname=x86_64-linux-gnu-thread-multi
  uname='linux localhost 3.16.0 #1 smp debian 3.16.0 x86_64 gnulinux '
  config_args='-Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc
-Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN
-Wdate-time -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/5.22
-Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.22 -Dvendorprefix=/usr
-Dvendorlib=/usr/share/perl5
-Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.22 -Dsiteprefix=/usr/local
-Dsitelib=/usr/local/share/perl/5.22.1
-Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.22.1
-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 -dEs -Duseshrplib -Dlibperl=libperl.so.5.22.1'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=define, usemultiplicity=define
  use64bitint=define, use64bitall=define, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='x86_64-linux-gnu-gcc', 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='5.4.0 20160609', gccosandvers=''
  intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678,
doublekind=3
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16,
longdblkind=3
  ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
  alignbytes=8, prototype=define
  Linker and Libraries​:
  ld='x86_64-linux-gnu-gcc', ldflags =' -fstack-protector-strong
-L/usr/local/lib'
  libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/5/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-2.23.so, so=so, useshrplib=true, libperl=libperl.so.5.22
  gnulibc_version='2.23'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
  cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib
-fstack-protector-strong'

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/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/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 5.22.1-9ubuntu0.3 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​:debian/makemaker-pasthru - http​://bugs.debian.org/758471 Pass LD
settings through to subdirectories
  DEBPKG​:fixes/pod_man_reproducible_date - http​://bugs.debian.org/759405
Support POD_MAN_DATE in Pod​::Man for the left-hand footer
  DEBPKG​:debian/locale-robustness - http​://bugs.debian.org/782068 [perl
#124310] Make t/run/locale.t survive missing locales masked by LC_ALL
  DEBPKG​:fixes/podman-utc - http​://bugs.debian.org/780259 Make the
embedded date from Pod​::Man reproducible
  DEBPKG​:fixes/podman-utc-docs - http​://bugs.debian.org/780259
Documentation and test suite updates for UTC fix
  DEBPKG​:fixes/podman-empty-date - http​://bugs.debian.org/780259 Support
an empty POD_MAN_DATE environment variable
  DEBPKG​:fixes/podman-pipe - http​://bugs.debian.org/777405 Better errors
for man pages from standard input
  DEBPKG​:debian/pod2man-customized - Update porting/customized.dat for
pod2man modifications
  DEBPKG​:debian/makemaker-manext - http​://bugs.debian.org/247370 Make
EU​::MakeMaker honour MANnEXT settings in generated manpage headers
  DEBPKG​:debian/makemaker_customized - Update t/porting/customized.dat
for files patched in Debian
  DEBPKG​:debian/do-not-record-build-date - [6baa8db]
http​://bugs.debian.org/774422 [perl #125830] Allow overriding the compile
time in "perl -V" output
  DEBPKG​:fixes/podman-source-date-epoch - http​://bugs.debian.org/801621
Make Pod​::Man honor the SOURCE_DATE_EPOCH environment variable
  DEBPKG​:fixes/podman-source-date-epoch-cleanups -
http​://bugs.debian.org/801621 Coding style and documentation for
SOURCE_EPOCH_DATE
  DEBPKG​:fixes/podman-source-date-epoch-testfix -
http​://bugs.debian.org/807086 Guard for building with SOURCE_DATE_EPOCH or
POD_MAN_DATE set
  DEBPKG​:debian/devel-ppport-reproducibility -
http​://bugs.debian.org/801523 Sort the list of XS code files when
generating RealPPPort.xs
  DEBPKG​:fixes/encode-unicode-bom - http​://bugs.debian.org/798727 [
rt.cpan.org #107043] Address
https://rt.cpan.org/Public/Bug/Display.html?id=107043
  DEBPKG​:debian/encode-unicode-bom-doc - http​://bugs.debian.org/798727
Document Debian backport of Encode​::Unicode fix
  DEBPKG​:debian/kfreebsd-softupdates - http​://bugs.debian.org/796798 Work
around Debian Bug#796798
  DEBPKG​:fixes/autodie-scope - http​://bugs.debian.org/798096 Fix a
scoping issue with "no autodie" and the "system" sub
  DEBPKG​:debian/debugperl-compat-fix - [perl #127212]
http​://bugs.debian.org/810326 Disable PERL_TRACK_MEMPOOL for debugging
builds
  DEBPKG​:fixes/CVE-2015-8607_file_spec_taint_fix -
http​://bugs.debian.org/810719 [perl #126862] ensure File​::Spec​::canonpath()
preserves taint
  DEBPKG​:fixes/mkstemp-umask - http​://bugs.debian.org/810924 [perl
#127322] [e57270b] Fix umask for mkstemp(3) calls
  DEBPKG​:fixes/crosscompile-no-targethost - [perl #127234] Fix the
Configure escape with usecrosscompile but no targethost
  DEBPKG​:fixes/podlators-no-encode - [rt.cpan.org #111156] Degrade
gracefully if utf8 is requested but Encode is not available
  DEBPKG​:debian/cross-time-hires - [rt.cpan.org #111391] Add an
environment variable to skip running configuration probes
  DEBPKG​:fixes/encode-unicode-pod - Unicode.pm​: Fix POD error
  DEBPKG​:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in
Memoize
  DEBPKG​:fixes/ok-pod - Added encoding for pod.
  DEBPKG​:fixes/CVE-2016-2381_duplicate_env - remove duplicate environment
variables from environ
  DEBPKG​:fixes/CVE-2017-12837.patch - [PATCH] regcomp [perl #131582]
  DEBPKG​:fixes/CVE-2017-12883.patch - [PATCH] PATCH​: [perl #131598]
  DEBPKG​:fixes/CVE-2016-6185.patch - [PATCH]
=?utf8?q?Don=E2=80=99t=20let=20XSLoader=20load=20relative?=
=?utf8?q?=20paths?=
  DEBPKG​:fixes/CVE-2017-6512-pre.patch - [PATCH] Correct the order of
tests of chmod(). (#294)
  DEBPKG​:fixes/CVE-2017-6512.patch - http​://bugs.debian.org/863870 [
rt.cpan.org #121951] Prevent directory chmod race attack.
  DEBPKG​:fixes/CVE-2018-6797.patch - (perl #132227) restart a node if we
change to uni rules within the node and encounter a sharp S
  DEBPKG​:fixes/CVE-2018-6798-1.patch - [perl #132063]​: Heap buffer
overflow
  DEBPKG​:fixes/CVE-2018-6798-2.patch - v5.24.3​: fix TRIE_READ_CHAR and
DECL_TRIE_TYPE to account for non-utf8 target
  DEBPKG​:fixes/CVE-2018-6798-3.patch - (perl #132063) we should no longer
warn for this code
  DEBPKG​:fixes/CVE-2018-6913.patch - (perl #131844) fix various space
calculation issues in pp_pack.c

---

@​INC for perl 5.22.1​:
  /etc/perl
  /usr/local/lib/x86_64-linux-gnu/perl/5.22.1
  /usr/local/share/perl/5.22.1
  /usr/lib/x86_64-linux-gnu/perl5/5.22
  /usr/share/perl5
  /usr/lib/x86_64-linux-gnu/perl/5.22
  /usr/share/perl/5.22
  /usr/local/lib/site_perl
  /usr/lib/x86_64-linux-gnu/perl-base
  .

---

Environment for perl 5.22.1​:
  HOME=/home/phil
  LANG=en_US.UTF-8
  LANGUAGE=en_US
  LC_ADDRESS=en_US.UTF-8
  LC_ALL=en_US.UTF-8
  LC_CTYPE=en_US.UTF-8
  LC_IDENTIFICATION=en_US.UTF-8
  LC_MEASUREMENT=en_US.UTF-8
  LC_MONETARY=en_US.UTF-8
  LC_NAME=en_US.UTF-8
  LC_NUMERIC=en_US.UTF-8
  LC_PAPER=en_US.UTF-8
  LC_TELEPHONE=en_US.UTF-8
  LC_TIME=en_US.UTF-8
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)

PATH=/usr/local/sbin​:/usr/local/bin​:/usr/sbin​:/usr/bin​:/sbin​:/bin​:/usr/games​:/usr/local/games​:/snap/bin​:/home/phil/Android/sdk/tools/​:/home/phil/Android/sdk/platform-tools/​:/home/phil/.local/bin​:/home/phil/Android/sdk/tools/​:/home/phil/Android/sdk/platform-tools/​:/home/phil/.local/bin
  PERL_BADLANG (unset)
  SHELL=/bin/bash

[p5pRT]

From @cpansprout

On Fri, 01 Jun 2018 17​:47​:28 -0700, philiprbrenan@​gmail.com wrote​:

Lvalue subs returning a reference to a scalar are not always
recognized in
the
same way that a scalar would be in a similar syntactic context.

=cut

my $aaa; # Scalar - all OK
$aaa = undef;
$aaa += 0;
$aaa //= undef;

sub aaa​:lvalue {\my $r} # Scalar returned via Lvalue sub

I think you want to drop the backslash.

aaa = undef;
aaa += 0; # Syntax error near "+="
aaa //= undef; # Can't modify pattern match (m//) in scalar
assignment

Although, conceivably these should do aliasing, just like \$r = \....

--

Father Chrysostomos

[p5pRT]

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

[p5pRT]

From @cpansprout

On Fri, 01 Jun 2018 17​:47​:28 -0700, philiprbrenan@​gmail.com wrote​:

Lvalue subs returning a reference to a scalar are not always
recognized in
the
same way that a scalar would be in a similar syntactic context.

=cut

my $aaa; # Scalar - all OK
$aaa = undef;
$aaa += 0;
$aaa //= undef;

sub aaa​:lvalue {\my $r} # Scalar returned via Lvalue sub
aaa = undef;
aaa += 0; # Syntax error near "+="
aaa //= undef; # Can't modify pattern match (m//) in scalar
assignment

I’ve just looked at it more closely. You are getting those syntax errors because aaa() has list precedence. If you declare it like this​:

sub aaa()​:lvalue {\my $r}

there is no output, so it compiles without incident. But that it runs without incident I think is a bug. It looks like a side effect of refaliasing.

This​:

use experimental 'refaliasing';
our $r;
sub aaa()​:lvalue {\$r}
aaa = \our $s;
warn \$r;
warn \$s;

gives me​:

SCALAR(0x7fc64a82b768) at - line 4.
SCALAR(0x7fc64a82b858) at - line 5.

which shows that the assignment did nothing. It should probably have produced this​:

SCALAR(0x7fc64a82b858) at - line 4.
SCALAR(0x7fc64a82b858) at - line 5.

--

Father Chrysostomos