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[perl #123665] 'Useless use of a constant in void context' is compile-time but not syntax warning

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From:
Ed Avis
Date:
January 24, 2015 08:06
Subject:
[perl #123665] 'Useless use of a constant in void context' is compile-time but not syntax warning
Message ID:
rt-4.0.18-23096-1422086782-120.123665-75-0@perl.org
# New Ticket Created by  "Ed Avis" 
# Please include the string:  [perl #123665]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=123665 >


This is a bug report for perl from eda@waniasset.com,
generated with the help of perlbug 1.39 running under perl 5.18.4.


-----------------------------------------------------------------
[Please describe your issue here]

This program warns 'Useless use of a constant ("a") in void context':

#!/usr/bin/perl
use strict;
use warnings;
use 5.018;
use warnings FATAL => 'syntax';
sub x { 'a'; return 0 }
say 'ok';

Since x() is never called the warning must be compile time.  Yet
it is not made FATAL.

Now it is not really a syntax error, more a semantic check, so I can
see how it is not included as 'syntax'.  I would like to make all
compilation warnings fatal without making run-time warnings fatal.
Perhaps that could be done using a BEGIN block and so on.  Could
that feature be added to warnings.pm, as

    use warnings FATAL => 'compilation';

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=low
---
Site configuration information for perl 5.18.4:

Configured by Red Hat, Inc. at Thu Oct 30 14:55:21 UTC 2014.

Summary of my perl5 (revision 5 version 18 subversion 4) configuration:

  Platform:
    osname=linux, osvers=3.16.3-200.fc20.x86_64, archname=x86_64-linux-thread-multi
    uname='linux buildvm-18.phx2.fedoraproject.org 3.16.3-200.fc20.x86_64 #1 smp wed sep 17 22:34:21 utc 2014 x86_64 x86_64 x86_64 gnulinux '
    config_args='-des -Doptimize=-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic -Dccdlflags=-Wl,--enable-new-dtags -Dlddlflags=-shared -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic -Wl,-z,relro  -Dshrpdir=/usr/lib64 -DDEBUGGING=-g -Dversion=5.18.4 -Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red Hat, Inc. -Dprefix=/usr -Dvendorprefix=/usr -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl5 -Dsitearch=/usr/local/lib64/perl5 -Dprivlib=/usr/share/perl5 -Dvendorlib=/usr/share/perl5/vendor_perl -Darchlib=/usr/lib64/perl5 -Dvendorarch=/usr/lib64/perl5/vendor_perl -Darchname=x86_64-linux-thread-multi -Dlibpth=/usr/local/lib64 /lib64 /usr/lib64 -Duseshrplib -Dusethreads -Duseithreads -Dusedtrace=/usr/bin/dtrace -Duselargefiles -Dd_semctl_semun -Di_db -Ui
 _ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio -Dinstallusrbinperl=n -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less -isr -Dd_gethostent_r_proto -Ud_endhostent_r_proto -Ud_sethostent_r_proto -Ud_endprotoent_r_proto -Ud_setprotoent_r_proto -Ud_endservent_r_proto -Ud_setservent_r_proto -Dscriptdir=/usr/bin -Dusesitecustomize'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.8.3 20140911 (Red Hat 4.8.3-7)', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =' -fstack-protector'
    libpth=/usr/local/lib64 /lib64 /usr/lib64
    libs=-lresolv -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lpthread -lc -lgdbm_compat
    perllibs=-lresolv -lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.18'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,--enable-new-dtags'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -Wl,-z,relro '

Locally applied patches:
    Fedora Patch1: Removes date check, Fedora/RHEL specific
    Fedora Patch3: support for libdir64
    Fedora Patch4: use libresolv instead of libbind
    Fedora Patch5: USE_MM_LD_RUN_PATH
    Fedora Patch6: Skip hostname tests, due to builders not being network capable
    Fedora Patch7: Dont run one io test due to random builder failures
    Fedora Patch9: Fix find2perl to translate ? glob properly (RT#113054)
    Fedora Patch10: Update h2ph(1) documentation (RT#117647)
    Fedora Patch11: Update pod2html(1) documentation (RT#117623)
    Fedora Patch12: Disable ornaments on perl5db AutoTrace tests (RT#118817)
    Fedora Patch14: Do not use system Term::ReadLine::Gnu in tests (RT#118821)
    Fedora Patch15: Define SONAME for libperl.so
    Fedora Patch16: Install libperl.so to -Dshrpdir value
    Fedora Patch18: Fix crash with \\&$glob_copy (RT#119051)
    Fedora Patch19: Fix coreamp.t rand test (RT#118237)
    Fedora Patch20: Reap child in case where exception has been thrown (RT#114722)
    Fedora Patch21: Fix using regular expressions containing multiple code blocks (RT#117917)
    Fedora Patch22: Create site paths by cpan for the first time (CPAN RT#99905)
    Fedora Patch200: Link XS modules to libperl.so with EU::CBuilder on Linux
    Fedora Patch201: Link XS modules to libperl.so with EU::MM on Linux

---
@INC for perl 5.18.4:
    /home/eda/lib/perl5/
    /usr/local/lib64/perl5
    /usr/local/share/perl5
    /usr/lib64/perl5/vendor_perl
    /usr/share/perl5/vendor_perl
    /usr/lib64/perl5
    /usr/share/perl5
    .

---
Environment for perl 5.18.4:
    HOME=/home/eda
    LANG=en_GB.UTF-8
    LANGUAGE (unset)
    LC_COLLATE=C
    LC_CTYPE=en_GB.UTF-8
    LC_MESSAGES=en_GB.UTF-8
    LC_MONETARY=en_GB.UTF-8
    LC_NUMERIC=en_GB.UTF-8
    LC_TIME=en_GB.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/eda/bin:/home/eda/bin:/usr/local/bin:/usr/bin:/sbin:/usr/sbin:/sbin:/usr/sbin
    PERL5LIB=/home/eda/lib/perl5/
    PERL_BADLANG (unset)
    SHELL=/bin/bash


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