"Possible precedence problem between ! and numeric eq (==)" failure in automake

[IgorTodorovskiIBM]

Module:

Description

When using Perl 5.41.7 or Perl 5.41.8, we are seeing this error message appear when running automake (written in perl).
Perl 5.41.3 did not have this problem.

Possible precedence problem between ! and numeric eq (==) at /data/zopen/usr/local/zopen/automake/automake-1.17.20241029_040611.zos/bin/automake line 6875.

Is this expected? It's preventing us from building autotools based projects.

Here's the reduced snippet of what automake is doing:

# test.pl
use strict;
use warnings;

sub transform_token {
    my ($token, $transform_ref, $key) = @_;
    return exists $transform_ref->{$key} ? $transform_ref->{$key} : '';
}

sub transform {
    my ($token, $transform) = @_;

    if ($token =~ /^%([\w\-]+)%$/) {
        return transform_token($token, $transform, $1);
    }
    elsif ($token =~ /^%\?([\w\-]+)%$/) {
        return transform_token($token, $transform, $1) ? 'TRUE' : 'FALSE';
    }
    elsif ($token =~ /^ \? (!?) ([\w\-]+) \? $/x) {
        my $neg = ($1 eq '!') ? 1 : 0;
        my $val = transform_token($token, $transform, $2);
        return (!!$val == $neg) ? '##%' : '';
    }
    else {
        die "Unknown request format: $token";
    }
}

# Simple test cases
my %transform_data = (
    'KEY1' => 'VALUE1',
    'KEY2' => '',
    'KEY3' => '1',
);

print transform('?KEY3?', \%transform_data) eq '##%' ? "PASS\n" : "FAIL\n";

Steps to Reproduce

perl test.pl

Expected behavior

No "Possible precedence problem between ! and numeric eq (==)" message

Perl configuration

# perl -V output goes here
 perl -V
Summary of my perl5 (revision 5 version 41 subversion 7) configuration:
  Derived from: 3a3761b09845d0d6d68f91eb45c52ae70de89489
  Platform:
    osname=os390
    osvers=27.00
    archname=os390
    uname='os390 vs01 27.00 04 8562 '
    config_args='-Dprefix=/home/itodoro/zopen/usr/local/zopen/perl5/perl5 -Duserelocatableinc -Dusedevel -des -Duse64bitall -Dusedl -D ccflags=-fzos-le-char-mode=ascii -std=gnu11 -mnocsect -fno-short-enums -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -m64 -mzos-target=zosv2r4 -O3  -I/home/itodoro/zopen/usr/local/zopen/curl/curl/include -I/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/include -I/home/itodoro/zopen/usr/local/zopen/zlib/zlib/include -isystem /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include -include /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include/zos-v2r5-symbolfixes.h  -DNSIG=42 -D_XOPEN_SOURCE=600 -D_ALL_SOURCE -D_OPEN_SYS_FILE_EXT=1 -D_AE_BIMODAL=1 -D_ENHANCED_ASCII_EXT=0xFFFFFFFF  -DZOSLIB_OVERRIDE_CLIB=1 -DZOSLIB_OVERRIDE_CLIB_GETENV=1 -DOS390 -DZOS -DNO_LOCALE_MESSAGES -DOEMVS -DMAXSIG=39 -D_EXT=1 -fvisibility=default -D ldflags=-Wl,-bedit=no -m64  -L/home/itodoro/zopen/usr/local/zopen/curl/curl/lib -L/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/lib -L/home/itodoro/zopen/usr/local/zopen/zlib/zlib/lib -L/home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib -D libs=  -lcurl -lncurses -lz -lzoslib /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib/celquopt.s.o -lzoslib-supp /jenkins/workspace/Port-Build/perl/perl5/.zoslib_hooks/zoslib_env_hook.c.o CC=clang CPPFLAGS=-DNSIG=42 -D_XOPEN_SOURCE=600 -D_ALL_SOURCE -D_OPEN_SYS_FILE_EXT=1 -D_AE_BIMODAL=1 -D_ENHANCED_ASCII_EXT=0xFFFFFFFF  -DZOSLIB_OVERRIDE_CLIB=1 -DZOSLIB_OVERRIDE_CLIB_GETENV=1 CFLAGS=-fzos-le-char-mode=ascii -std=gnu11 -mnocsect -fno-short-enums -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -m64 -mzos-target=zosv2r4 -O3  -I/home/itodoro/zopen/usr/local/zopen/curl/curl/include -I/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/include -I/home/itodoro/zopen/usr/local/zopen/zlib/zlib/include -isystem /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include -include /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include/zos-v2r5-symbolfixes.h  CXX=clang++ CXXFLAGS=-fzos-le-char-mode=ascii -mnocsect -fno-short-enums -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -m64 -mzos-target=zosv2r4 -O3     -I/home/itodoro/zopen/usr/local/zopen/curl/curl/include -I/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/include -I/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/include/ncurses -I/home/itodoro/zopen/usr/local/zopen/zlib/zlib/include -isystem /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include -include /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include/zos-v2r5-symbolfixes.h  LDFLAGS=-Wl,-bedit=no -m64  -L/home/itodoro/zopen/usr/local/zopen/curl/curl/lib -L/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/lib -L/home/itodoro/zopen/usr/local/zopen/zlib/zlib/lib -L/home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib LIBS=  -lcurl -lncurses -lz -lzoslib /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib/celquopt.s.o -lzoslib-supp /jenkins/workspace/Port-Build/perl/perl5/.zoslib_hooks/zoslib_env_hook.c.o LDLIBS=  -lcurl -lncurses -lz -lzoslib /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib/celquopt.s.o -lzoslib-supp /jenkins/workspace/Port-Build/perl/perl5/.zoslib_hooks/zoslib_env_hook.c.o'
    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='clang'
    ccflags ='-fzos-le-char-mode=ascii -std=gnu11 -mnocsect -fno-short-enums -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -m64 -mzos-target=zosv2r4 -O3 -I/home/itodoro/zopen/usr/local/zopen/curl/curl/include -I/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/include -I/home/itodoro/zopen/usr/local/zopen/zlib/zlib/include -isystem /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include -include /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include/zos-v2r5-symbolfixes.h -DNSIG=42 -D_XOPEN_SOURCE=600 -D_ALL_SOURCE -D_OPEN_SYS_FILE_EXT=1 -D_AE_BIMODAL=1 -D_ENHANCED_ASCII_EXT=0xFFFFFFFF -DZOSLIB_OVERRIDE_CLIB=1 -DZOSLIB_OVERRIDE_CLIB_GETENV=1 -DOS390 -DZOS -DNO_LOCALE_MESSAGES -DOEMVS -DMAXSIG=39 -D_EXT=1 -fvisibility=default'
    optimize=' '
    cppflags='-fzos-le-char-mode=ascii -std=gnu11 -mnocsect -fno-short-enums -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -m64 -mzos-target=zosv2r4 -O3 -I/home/itodoro/zopen/usr/local/zopen/curl/curl/include -I/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/include -I/home/itodoro/zopen/usr/local/zopen/zlib/zlib/include -isystem /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include -include /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/include/zos-v2r5-symbolfixes.h -DNSIG=42 -D_XOPEN_SOURCE=600 -D_ALL_SOURCE -D_OPEN_SYS_FILE_EXT=1 -D_AE_BIMODAL=1 -D_ENHANCED_ASCII_EXT=0xFFFFFFFF -DZOSLIB_OVERRIDE_CLIB=1 -DZOSLIB_OVERRIDE_CLIB_GETENV=1 -DOS390 -DZOS -DNO_LOCALE_MESSAGES -DOEMVS -DMAXSIG=39 -D_EXT=1 -fvisibility=default'
    ccversion=''
    gccversion=''
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=87654321
    doublekind=4
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=2
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='clang'
    ldflags ='-Wl,-bedit=no -m64 -L/home/itodoro/zopen/usr/local/zopen/curl/curl/lib -L/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/lib -L/home/itodoro/zopen/usr/local/zopen/zlib/zlib/lib -L/home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib'
    libpth=/lib /usr/lib
    libs= -lcurl -lncurses -lz -lzoslib /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib/celquopt.s.o -lzoslib-supp /jenkins/workspace/Port-Build/perl/perl5/.zoslib_hooks/zoslib_env_hook.c.o
    perllibs=-lcurl -lncurses -lz -lzoslib /home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib/celquopt.s.o -lzoslib-supp /jenkins/workspace/Port-Build/perl/perl5/.zoslib_hooks/zoslib_env_hook.c.o
    libc=
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dllload.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='  /jenkins/workspace/Port-Build/perl/perl5/libperl.x'
    cccdlflags='-c -m64'
    lddlflags='-m64 -L/home/itodoro/zopen/usr/local/zopen/curl/curl/lib -L/home/itodoro/zopen/usr/local/zopen/ncurses/ncurses/lib -L/home/itodoro/zopen/usr/local/zopen/zlib/zlib/lib -L/home/itodoro/zopen/usr/local/zopen/zoslib/zoslib/lib /jenkins/workspace/Port-Build/perl/perl5/libperl.x'


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_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
  Locally applied patches:
    uncommitted-changes
  Built under os390
  Compiled at Jan 23 2025 11:25:38
  %ENV:
    PERL5LIB="/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/os390/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/os390/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/os390/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/:/home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/os390/:"
  @INC:
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7//os390
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/os390/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/os390/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7//os390
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/5.41.7/os390/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/
    /home/itodoro/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/lib/perl5/5.41.7/os390/
    ../lib/site_perl/5.41.7/os390
    ../lib/site_perl/5.41.7
    ../lib/5.41.7/os390
    ../lib/5.41.7

[IgorTodorovskiIBM]

You can also reproduce this by pulling the latest automake code, building it and running automake.

curl -O https://ftp.gnu.org/gnu/automake/automake-1.17.tar.gz
cd automake*
./configure
make -j10
make install
automake

[Grinnz]

If it is intended for !!$val to be used as the integer 0 or 1 to be compared to $neg, this warning could be avoided by making that explicit: 0+!!$val == $neg

The purpose of the warning is to call out a common mistake where !$x == $y is written instead of !($x == $y) - but warnings just produce STDERR output, is it causing some other issue?

[IgorTodorovskiIBM]

If it is intended for !!$val to be used as the integer 0 or 1 to be compared to $neg, this warning could be avoided by making that explicit: 0+!!$val == $neg

The purpose of the warning is to call out a common mistake where !$x == $y is written instead of !($x == $y) - but warnings just produce STDERR output, is it causing some other issue?

Yes, in automake case it's causing automake to exit with rc of 255

 ./bin/automake-1.17 ; echo $?
Possible precedence problem between ! and numeric eq (==) at ./bin/automake-1.17 line 6875.
255

[IgorTodorovskiIBM]

Ah they have: use warnings FATAL => 'all';

[Grinnz]

Using fatal warnings is an assumption of risk of breakage: https://perldoc.perl.org/warnings#Fatal-Warnings

[mauke]

my $neg = ($1 eq '!') ? 1 : 0;
my $val = transform_token($token, $transform, $2);
return (!!$val == $neg) ? '##%' : '';

This code is bizarre. It uses ? 1 : 0 in the initialization of $neg where it is unnecessary (since eq already returns a boolean), but does not use it for $val where it would make sense (since transform_token returns a string).

That is,

my $neg = $1 eq '!';
my $val = transform_token($token, $transform, $2) ? 1 : 0;
return ($val == $neg) ? '##%' : '';

would do the right thing and not trigger a warning.

(Alternatively, parenthesizing the last expression as return (!!$val) == $neg ? '##%' : '' would also avoid the warning.)

---

That said, the warning already has a list of exceptions where it doesn't trigger:

• (!...) == ... (LHS parenthesized) does not warn
• !... == !... (negation on both LHS and RHS) does not warn

It would be fairly easy to add !!... == ... (double negation on LHS) to the list of exceptions. Is this something we want?

[Grinnz]

I think that would risk failing to catch instances where !!($x == $y) was accidentally written without the parentheses, as that is rather similar to the original case it is meant to catch, and that exception should only be added if automake isn't reasonably able to work around the warning; and if that is the case, they should be strongly recommended not to use fatal warnings as this issue will occur again.

[IgorTodorovskiIBM]

I'm also seeing a similar case in texinfo (another GNU package):

makeinfo
Possible precedence problem between ! and string eq at /home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Converter.pm line 1092.
Possible precedence problem between ! and string eq at /home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Info.pm line 92.
makeinfo: missing file argument.
Try `makeinfo --help' for more information.

/home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Converter.pm line 1092.

1091   my $fh;
1092   if (! $self->{'output_file'} eq '') {
1093     $fh = $self->Texinfo::Common::open_out($self->{'output_file'});
1094     if (!$fh) {
1095       $self->document_error(sprintf(__("could not open %s for writing: %s"),
1096                                     $self->{'output_file'}, $!));
1097       return undef;
1098     }
1099   }

/home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Info.pm line 92.

 90
 91   my $fh;
 92   if (! $self->{'output_file'} eq '') {
 93     if ($self->get_conf('VERBOSE')) {
 94       print STDERR "Output file $self->{'output_file'}\n";
 95     }
 96     $fh = _open_info_file($self, $self->{'output_file'});
 97     if (!$fh) {
 98       return undef;
 99     }
100   }

[IgorTodorovskiIBM]

Curl fails at make check with Perl 5.41.7 due to this as well:

make[2]: Leaving directory '/jenkins/workspace/Port-Build/curl/curl-8.11.1/tests'
srcdir=. /jenkins/zopen/usr/local/zopen/perl/perl5-heads.v5.41.7.20250123_102326.zos/bin/perl -I. ./runtests.pl   -a -s 
Possible precedence problem between ! and string eq at ./runtests.pl line 1691.
make[1]: *** [Makefile:898: quiet-test] Error 129
make[1]: Leaving directory '/jenkins/workspace/Port-Build/curl/curl-8.11.1/tests'

[IgorTodorovskiIBM]

I have sent an email to automake@gnu.org <automake@gnu.org> to report the problem.

[Grinnz]

I'm also seeing a similar case in texinfo (another GNU package):

makeinfo
Possible precedence problem between ! and string eq at /home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Converter.pm line 1092.
Possible precedence problem between ! and string eq at /home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Info.pm line 92.
makeinfo: missing file argument.
Try `makeinfo --help' for more information.

/home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Converter.pm line 1092.

1091   my $fh;
1092   if (! $self->{'output_file'} eq '') {
1093     $fh = $self->Texinfo::Common::open_out($self->{'output_file'});
1094     if (!$fh) {
1095       $self->document_error(sprintf(__("could not open %s for writing: %s"),
1096                                     $self->{'output_file'}, $!));
1097       return undef;
1098     }
1099   }

/home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Info.pm line 92.

 90
 91   my $fh;
 92   if (! $self->{'output_file'} eq '') {
 93     if ($self->get_conf('VERBOSE')) {
 94       print STDERR "Output file $self->{'output_file'}\n";
 95     }
 96     $fh = _open_info_file($self, $self->{'output_file'});
 97     if (!$fh) {
 98       return undef;
 99     }
100   }

These are actual logical errors which the warning has pointed out; the code is attempting to check whether $self->{output_file} is non-empty, but instead checks whether $self->{output_file} is true (via checking whether its boolean negation is equal to the empty string).

[IgorTodorovskiIBM]

I'm also seeing a similar case in texinfo (another GNU package):

makeinfo
Possible precedence problem between ! and string eq at /home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Converter.pm line 1092.
Possible precedence problem between ! and string eq at /home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Info.pm line 92.
makeinfo: missing file argument.
Try `makeinfo --help' for more information.

/home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Converter.pm line 1092.

1091   my $fh;
1092   if (! $self->{'output_file'} eq '') {
1093     $fh = $self->Texinfo::Common::open_out($self->{'output_file'});
1094     if (!$fh) {
1095       $self->document_error(sprintf(__("could not open %s for writing: %s"),
1096                                     $self->{'output_file'}, $!));
1097       return undef;
1098     }
1099   }

/home/itodoro/zopen/usr/local/zopen/texinfo/texinfo/share/texinfo/Texinfo/Convert/Info.pm line 92.

 90
 91   my $fh;
 92   if (! $self->{'output_file'} eq '') {
 93     if ($self->get_conf('VERBOSE')) {
 94       print STDERR "Output file $self->{'output_file'}\n";
 95     }
 96     $fh = _open_info_file($self, $self->{'output_file'});
 97     if (!$fh) {
 98       return undef;
 99     }
100   }

These are actual logical errors which the warning has pointed out; the code is attempting to check whether $self->{output_file} is non-empty, but instead checks whether $self->{output_file} is true (via checking whether its boolean negation is equal to the empty string).

Should these warnings have been introduced in a minor or major release rather than a patch release? 5.41.3 did not emit these warnings. I've seen 3 popular open source tools break as a result of these changes.

[Grinnz]

As mentioned, these warnings emit only STDERR in supported operation. If these utilities are using fatal warnings, they have accepted the contract that they must update their code whenever new warnings are introduced. This allows new warnings to be introduced, as otherwise Perl's devotion to backwards compatibility would make it near impossible to do so, major version or not.

In addition, this is not a patch release, this is a development release. These warnings will not be introduced to a stable version of Perl until 5.42.0.