From @cpansprout

Oops. I sent this to perlbug-follow the first time.

$ perl5.27.7 -le 'use Scalar​::Util "weaken"; weaken ($a = \$_[0]); @​_ = sort @​_; print $a; print \$_[0]'
SCALAR(0x7f8ad0805480)
SCALAR(0x7f8ad0805480)

The first line of output should be blank ($a should be undefined.)

$ perl5.27.7 -V
Summary of my perl5 (revision 5 version 27 subversion 7) configuration​:
Snapshot of​: 2cb35ee
Platform​:
  osname=darwin
  osvers=12.5.0
  archname=darwin-2level
  uname='darwin pint.local 12.5.0 darwin kernel version 12.5.0​: sun sep 29 13​:33​:47 pdt 2013; root​:xnu-2050.48.12~1release_x86_64 x86_64 '
  config_args='-de -Dusedevel'
  hint=recommended
  useposix=true
  d_sigaction=define
  useithreads=undef
  usemultiplicity=undef
  use64bitint=define
  use64bitall=define
  uselongdouble=undef
  usemymalloc=n
  default_inc_excludes_dot=define
  bincompat5005=undef
Compiler​:
  cc='cc'
  ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.8 -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -I/opt/local/include -DPERL_USE_SAFE_PUTENV'
  optimize='-O3'
  cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.8 -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -I/opt/local/include'
  ccversion=''
  gccversion='4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.27)'
  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 =' -mmacosx-version-min=10.8 -fstack-protector -L/usr/local/lib -L/opt/local/lib'
  libpth=/usr/local/lib /usr/bin/../lib/clang/4.2/lib /usr/lib /opt/local/lib
  libs=-lpthread -ldbm -ldl -lm -lutil -lc
  perllibs=-lpthread -ldl -lm -lutil -lc
  libc=
  so=dylib
  useshrplib=false
  libperl=libperl.a
  gnulibc_version=''
Dynamic Linking​:
  dlsrc=dl_dlopen.xs
  dlext=bundle
  d_dlsymun=undef
  ccdlflags=' '
  cccdlflags=' '
  lddlflags=' -mmacosx-version-min=10.8 -bundle -undefined dynamic_lookup -L/usr/local/lib -L/opt/local/lib -fstack-protector'

Characteristics of this binary (from libperl)​:
Compile-time options​:
  HAS_TIMES
  PERLIO_LAYERS
  PERL_COPY_ON_WRITE
  PERL_DONT_CREATE_GVSV
  PERL_MALLOC_WRAP
  PERL_OP_PARENT
  PERL_PRESERVE_IVUV
  PERL_USE_DEVEL
  PERL_USE_SAFE_PUTENV
  USE_64_BIT_ALL
  USE_64_BIT_INT
  USE_LARGE_FILES
  USE_LOCALE
  USE_LOCALE_COLLATE
  USE_LOCALE_CTYPE
  USE_LOCALE_NUMERIC
  USE_LOCALE_TIME
  USE_PERLIO
  USE_PERL_ATOF
Built under darwin
Compiled at Nov 26 2017 13​:47​:48
@​INC​:
  /usr/local/lib/perl5/site_perl/5.27.7/darwin-2level
  /usr/local/lib/perl5/site_perl/5.27.7
  /usr/local/lib/perl5/5.27.7/darwin-2level
  /usr/local/lib/perl5/5.27.7
  /usr/local/lib/perl5/site_perl

From zefram@fysh.org

Father Chrysostomos wrote​:

$ perl5.27.7 -le 'use Scalar​::Util "weaken"; weaken ($a = \$_[0]); @​_ = sort @​_; print $a; print \$_[0]'

After coming up with blessedness and then tiedness, I was wondering what
other kinds of magic might be applied to an array element, and how sort
would need to treat them. This one confirms that in general sort needs
to take the slow path for any kind of magic. We don't distinguish between
magic that's part of the container and magic that's part of the value, but
we have little of the latter, so wouldn't lose much by not catering to it.

-zefram

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

From @iabyn

On Fri, Dec 15, 2017 at 10​:41​:31PM +0000, Zefram wrote​:

Father Chrysostomos wrote​:

$ perl5.27.7 -le 'use Scalar​::Util "weaken"; weaken ($a = \$_[0]); @​_ = sort @​_; print $a; print \$_[0]'

After coming up with blessedness and then tiedness, I was wondering what
other kinds of magic might be applied to an array element, and how sort
would need to treat them. This one confirms that in general sort needs
to take the slow path for any kind of magic. We don't distinguish between
magic that's part of the container and magic that's part of the value, but
we have little of the latter, so wouldn't lose much by not catering to it.

I think that by default, in-place sort should make a copy of every SV
(thus emulating the action of pp_aassign) *except* for special cases
of SV such as SvREFCNT == 1, no magic etc.

--
I don't want to achieve immortality through my work...
I want to achieve it through not dying.
  -- Woody Allen