From sullr@cpan.org

Created by sullr@cpan.org

The following statement gets successfully parsed and executed by perl.
Note that at the end of line 3 there is a '.' instead of a ';', i.e.
the author of the code made obviously an error (based on real bug).

  1 my $x = 1;
  2 my $y = 'a';
  3 warn "foobar $y " . "foo " .
  4 $x = 0;

  > 0 at x.pl line 3.

If the code is modified so that the first concationation is skipped
the behavior is completly different but actually the expected behavior (i.e.
error)​:

  1 my $x = 1;
  2 my $y = 'a';
  3 warn "foobar $y foo " .
  4 $x = 0;

  > Can't modify concatenation (.) or string in scalar assignment at x.pl line 4, near "0;"
  > Execution of x.pl aborted due to compilation errors.

The same behavior can be seen with assignments, i.e.

  my $z = "foobar $y " . "foo " . $x = 0; # works, $z is 0
  my $z = "foobar $y foo " . $x = 0; # error

It also works if the variable is outside the quotes or even if no quotes
at all are used but an additional variable.

  my $z = $x = 0; # works as expected
  my $z = $y.$x = 0; # fails as expected
  my $z = "".$y.$x = 0; # works unexpectedly
  my $z = $y."".$x = 0; # works unexpectedly
  my $z = $v.$y.$x = 0; # works unexpectedly

This behavior exists in a wide range of perl versions. I found it in perl
5.8.1 and it is still in the current blead perl 5.25.5.

Flags:
    category=core
    severity=medium

Site configuration information for perl 5.25.5:

Configured by work at Thu Sep  8 20:16:29 CEST 2016.

Summary of my perl5 (revision 5 version 25 subversion 5) configuration:
  Snapshot of: 49fc490652d8b428d67872fae3acb10f0b43cff7
  Platform:
    osname=linux
    osvers=4.4.0-34-generic
    archname=x86_64-linux
    uname='linux rincewind 4.4.0-34-generic #53~14.04.1-ubuntu smp wed jul 27 16:56:40 utc 2016 x86_64 x86_64 x86_64 gnulinux '
    config_args='-de -Dprefix=/home/work-public/perl/perls/perl-blead -Dusedevel -Aeval:scriptdir=/home/work-public/perl/perls/perl-blead/bin'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    bincompat5005=undef
  Compiler:
    cc='cc'
    ccflags ='-fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2'
    cppflags='-fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion=''
    gccversion='4.8.4'
    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='cc'
    ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.8/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=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=
    so=so
    useshrplib=false
    libperl=libperl.a
    gnulibc_version='2.19'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector'



@INC for perl 5.25.5:
    /home/work-public/perl/perls/perl-blead/lib/site_perl/5.25.5/x86_64-linux
    /home/work-public/perl/perls/perl-blead/lib/site_perl/5.25.5
    /home/work-public/perl/perls/perl-blead/lib/5.25.5/x86_64-linux
    /home/work-public/perl/perls/perl-blead/lib/5.25.5


Environment for perl 5.25.5:
    HOME=/home/work
    LANG=en_US.UTF-8
    LANGUAGE=en_US:en
    LC_ADDRESS=de_DE.UTF-8
    LC_IDENTIFICATION=de_DE.UTF-8
    LC_MEASUREMENT=de_DE.UTF-8
    LC_MONETARY=de_DE.UTF-8
    LC_NAME=de_DE.UTF-8
    LC_NUMERIC=de_DE.UTF-8
    LC_PAPER=de_DE.UTF-8
    LC_TELEPHONE=de_DE.UTF-8
    LC_TIME=de_DE.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/work-public/perl/bin:/home/work-public/perl/perls/perl-blead/bin:/home/Shared/anaconda2/bin:/home/Shared/anaconda3/bin:/home/work/bin/:/home/work/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
    PERLBREW_BASHRC_VERSION=0.64
    PERLBREW_HOME=/home/work/.perlbrew
    PERLBREW_MANPATH=/home/work-public/perl/perls/perl-blead/man
    PERLBREW_PATH=/home/work-public/perl/bin:/home/work-public/perl/perls/perl-blead/bin
    PERLBREW_PERL=perl-blead
    PERLBREW_ROOT=/home/work-public/perl
    PERLBREW_VERSION=0.66
    PERL_BADLANG (unset)
    SHELL=/bin/bash

From @iabyn

On Thu, Sep 08, 2016 at 12​:24​:52PM -0700, sullr@​cpan.org wrote​:

The same behavior can be seen with assignments, i.e.

my $z = "foobar $y " . "foo " . $x = 0; # works, $z is 0
my $z = "foobar $y foo " . $x = 0; # error

It can be reduced to the following difference​:

  (($x . "a") . "b") = 0; # compiles (bug)
  (($x + 1 ) + 2 ) = 0; # error (correct behaviour)

--
"Strange women lying in ponds distributing swords is no basis for a system
of government. Supreme executive power derives from a mandate from the
masses, not from some farcical aquatic ceremony."
  -- Dennis, "Monty Python and the Holy Grail"

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

From zefram@fysh.org

Dave Mitchell wrote​:

It can be reduced to the following difference​:

(($x . "a") . "b") = 0; # compiles (bug)
(($x + 1 ) + 2 ) = 0; # error (correct behaviour)

Where a concat op has a lhs that is another concat op, ck_concat sets
the OPf_STACKED flag on the outer concat. Addition doesn't get that
treatment. It seems that the flag is intended as a signal to the peephole
optimiser, where it suppresses an optimisation that I can't make sense
of. But op_lvalue_flags() also looks at OPf_STACKED, for concat, add,
and many other non-lvalue op types, and quite explicitly suppresses the
"can't modify" error if it's set. Presumably there's something else that
I haven't identified that sets the flag on these ops in some situation
that makes them an acceptable lvalue. The two uses of the flag are
interfering with each other. Neither is the standard meaning of the flag,
and neither is described in the comment in op.h that describes the flag.

-zefram

From @cpansprout

On Fri Sep 09 03​:47​:58 2016, zefram@​fysh.org wrote​:

Dave Mitchell wrote​:

It can be reduced to the following difference​:

(($x . "a") . "b") = 0; # compiles (bug)
(($x + 1 ) + 2 ) = 0; # error (correct behaviour)

Where a concat op has a lhs that is another concat op, ck_concat sets
the OPf_STACKED flag on the outer concat. Addition doesn't get that
treatment.

$a.$b.$c is optimised to ($a.$b).=$c to avoid having to copy the value returned by $a.$b to a new temporary scalar to be returned by $c.

It seems that the flag is intended as a signal to the peephole
optimiser, where it suppresses an optimisation that I can't make sense
of.

It seems that the ($a.$b).=$c optimisation can interfere in some cases with the removal of the stringify op in "$a$b$c" in some circumstances involving lexical scalar assignment. I don’t understand why though. That checking of the flag in the peephole optimizer is not directly related to this bug.

But op_lvalue_flags() also looks at OPf_STACKED, for concat, add,
and many other non-lvalue op types, and quite explicitly suppresses the
"can't modify" error if it's set. Presumably there's something else that
I haven't identified that sets the flag on these ops in some situation
that makes them an acceptable lvalue.

All the assignment versions of binary ops, .= += -= etc., have the OPf_STACKED flag set.

--

Father Chrysostomos