The value of 'my \@foo' is not '\@foo'

[djerius]

Description

My expectation is that $ref in these two expressions should be be an arrayref:

$ref = my $bar = [];
$ref = my \@bar = [];

However, in the second expression, $ref is undef.

Steps to Reproduce

perl -Mexperimental=declared_refs -E 'say my \@bar = []'

perl -Mexperimental=declared_refs -E 'say my $bar = []'
ARRAY(0x56169c3f9648)

Perl configuration

Summary of my perl5 (revision 5 version 40 subversion 3) configuration:
   
  Platform:
    osname=linux
    osvers=6.12.33+deb12-amd64
    archname=x86_64-linux
    uname='linux leafhopper 6.12.33+deb12-amd64 #1 smp preempt_dynamic debian 6.12.33-1~bpo12+1 (2025-07-09) x86_64 gnulinux '
    config_args='-Dprefix=/home/dj/.plenv/versions/5.40.3 -de -Dversiononly -A'eval:scriptdir=/home/dj/.plenv/versions/5.40.3/bin''
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cc'
    ccflags ='-fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2'
    optimize='-O2'
    cppflags='-fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='12.2.0'
    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-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/x86_64-linux-gnu /usr/lib /usr/lib64
    libs=-lpthread -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=/lib/x86_64-linux-gnu/libc.so.6
    so=so
    useshrplib=false
    libperl=libperl.a
    gnulibc_version='2.36'
  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-strong'


Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_LONG_DOUBLE
    HAS_STRTOLD
    HAS_TIMES
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_HASH_FUNC_SIPHASH13
    PERL_HASH_USE_SBOX32
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    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 linux
  Compiled at Aug  6 2025 22:51:11
  %ENV:
    PERL_CPANM_OPT="--cascade-search --mirror-only  --mirror /home/dj/Work/darkpan --mirror http://www.cpan.org"
    PERL_CPAN_MIRROR_TINY_BASE="/home/dj/Work/darkpan"
    PERL_DARKPAN="/home/dj/Work/darkpan"
  @INC:
    /home/dj/.plenv/versions/5.40.3/lib/perl5/site_perl/5.40.3/x86_64-linux
    /home/dj/.plenv/versions/5.40.3/lib/perl5/site_perl/5.40.3
    /home/dj/.plenv/versions/5.40.3/lib/perl5/5.40.3/x86_64-linux
    /home/dj/.plenv/versions/5.40.3/lib/perl5/5.40.3

[djerius]

Looks like this only happens in 5.40.3. The following versions return the expected result:

5.26, 5.34, 5.36, 5.38, 5.40, 5.42

(which makes in hind-sight makes sense, as I discovered this in tests which started failing when I moved from 5.38 to 5.40)

[ap]

The following versions return the expected result:

Can you add the minor version of each passing perl?

[jkeenan]

I have this phenomenon happening in 5.40, but not in 5.38 or 5.42!

$ perlbrew use perl-5.38.2
$ perl -v | head -2 | tail -1
This is perl 5, version 38, subversion 2 (v5.38.2) built for x86_64-linux
$ perl -Mexperimental=declared_refs -E 'say my \@bar = []'
ARRAY(0x6153ed6db5e8)
$ perl -Mexperimental=declared_refs -E 'say my $bar = []'
ARRAY(0x57899cd085e8)

$ perlbrew use perl-5.40.1
$ perl -v | head -2 | tail -1
This is perl 5, version 40, subversion 1 (v5.40.1) built for x86_64-linux
$ perl -Mexperimental=declared_refs -E 'say my \@bar = []'

$ perl -Mexperimental=declared_refs -E 'say my $bar = []'
ARRAY(0x5eb674e85618)

$ perlbrew use perl-5.42.0
$ perl -v | head -2 | tail -1
This is perl 5, version 42, subversion 0 (v5.42.0) built for x86_64-linux
$ perl -Mexperimental=declared_refs -E 'say my \@bar = []'
ARRAY(0x5d1e9f2db618)
$ perl -Mexperimental=declared_refs -E 'say my $bar = []'
ARRAY(0x6470a2d36618)

[jkeenan]

There are two transitions to be investigated here.

Bisecting with this invocation:

perl Porting/bisect.pl \
--start=v5.38.0 \
--end=v5.40.0 \
-- ./perl -Ilib -Mexperimental=declared_refs -e 'my $ref = my \@bar = []; die unless defined $ref'

... points to v5.39.5-56-g604c0355d2.

604c0355d21cd2bdfa4adbc4cc6e2014acb8760a is the first bad commit
commit 604c0355d21cd2bdfa4adbc4cc6e2014acb8760a
Author: David Mitchell <davem@iabyn.com>
Date:   Sun Sep 24 22:11:21 2023 +0100
Commit:     David Mitchell <davem@iabyn.nospamdeletethisbit.com>
CommitDate: Mon Dec 4 16:42:35 2023 +0000

    make RC-stack-aware: unwrap pp_refassign()

    Remove the temporary wrapper from pp_refassign()

For the second transition, bisecting with this invocation:

perl Porting/bisect.pl \
--start=v5.40.0 \
--end=v5.42.0 \
--expect-fail \
-- ./perl -Ilib -Mexperimental=declared_refs -e 'my $ref = my \@bar = []; die unless defined $ref'

... points to v5.41.8-15-g53119a6d7b:

53119a6d7b7c6f74ccc25456c793f7dfcad0b893 is the first bad commit
commit 53119a6d7b7c6f74ccc25456c793f7dfcad0b893
Author: David Mitchell <davem@iabyn.com>
Date:   Mon Jan 20 11:03:02 2025 +0000
Commit:     David Mitchell <davem@iabyn.nospamdeletethisbit.com>
CommitDate: Wed Jan 22 12:34:07 2025 +0000

    fix refassign in non-void context
    
    GH #22710
    
    The experimental refassign feature, \@a = $aref, which allows an
    array/hash etc to be aliased rather than copied, got broken in rvalue
    non-void context by v5.39.5-56-g604c0355d2. So in something like

        my $x = (\@a = $aref);

    the stack was underflowing. This is because previously pp_refassign()
    would leave its RH argument on the stack when returning, while my commit
    above made it always pop the argument off the stack.

    This commit makes it only pop the arg in void context.

So, yes, there was a bug in 5.40 that is fixed in 5.42. (Would 53119a6 be eligible for back-porting to the next perl-5.40.x maintenance release?)

@iabyn, any comments?

[djerius]

The following versions return the expected result:

Can you add the minor version of each passing perl?

5.26.3: pass
5.34.1: pass
5.36.1: pass
5.38.2: pass
5.38.5: pass
5.40.3: fail
5.42.0: pass

[haarg]

Fixed by #22938

[iabyn]

On Sun, Oct 05, 2025 at 06:18:47PM -0700, James E Keenan wrote: So, yes, there was a bug in 5.40 that is fixed in 5.42. (Would 53119a6 be eligible for back-porting to the next perl-5.40.x maintenance release?)

Yes

[jkeenan]

On Sun, Oct 05, 2025 at 06:18:47PM -0700, James E Keenan wrote:
So, yes, there was a bug in 5.40 that is fixed in 5.42. (Would
53119a6 be eligible for back-porting to the next perl-5.40.x
maintenance release?)
Yes

Added here: 59788c4

[jkeenan]

Fixed by #22938

Accordingly, closing this ticket.