Compile-time check for nonexistent ${^MAGIC} variables

[epa]

Description Although with use strict Perl will report the use of a variable that hasn't been declared, it doesn't catch mistakes in builtin variables that have a long name such as ${^PREMATCH}. They are silently accepted and return undef.

I understand that this is "the way it has always been", but it's unfortunate. Firstly for the programmer who doesn't get the quick certainty of not having made a typo somewhere.

(But also, it's in principle a forward compatibility problem: code might be using ${^WHATEVER} and, whether accidentally or deliberately, relying on treating it as an ordinary variable. If a later Perl release creates ${^WHATEVER} as a builtin variable, the meaning of existing code will change.)

Steps to Reproduce perl -cE 'use strict; say ${^MUTCH}'

Expected behavior "No such builtin variable ${^MUTCH}"

Perl configuration

Summary of my perl5 (revision 5 version 26 subversion 3) configuration:

  Platform:
    osname=linux
    osvers=4.18.0-305.25.1.el8_4.x86_64
    archname=x86_64-linux-thread-multi
    uname='linux x86-64-02.build.eng.rdu2.redhat.com 4.18.0-305.25.1.el8_4.x86_64 #1 smp mon oct 18 14:34:11 edt 2021 x86_64 x86_64 x86_64 gnulinux '
    config_args='-des -Doptimize=none -Dccflags=-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc\
-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protectio\
n -Dldflags=-Wl,-z,relro  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -Dccdlflags=-Wl,--enable-new-dtags -Wl,-z,relro  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -D\
lddlflags=-shared -Wl,-z,relro  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -Dshrpdir=/usr/lib64 -DDEBUGGING=-g -Dversion=5.26.3 -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 -Dvendo\
rlib=/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 -Dusesh\
rplib -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 -U\
bincompat5005 -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_pr\
oto -Ud_setservent_r_proto -Dscriptdir=/usr/bin -Dusesitecustomize -Duse64bitint'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='gcc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switch\
es -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fwra\
pv -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='  -g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switch\
es -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fwra\
pv -fno-strict-aliasing -I/usr/local/include'
    ccversion=''
    gccversion='8.5.0 20210514 (Red Hat 8.5.0-18)'
    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='gcc'
    ldflags ='-Wl,-z,relro -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib64 /lib64 /usr/lib64 /usr/local/lib /usr/lib /lib/../lib64 /usr/lib/../lib64 /lib
    libs=-lpthread -lresolv -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc -lgdbm_compat
    perllibs=-lpthread -lresolv -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.28.so
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.28'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,--enable-new-dtags -Wl,-z,relro -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld'
    cccdlflags='-fPIC'
    lddlflags='-lpthread -shared -Wl,-z,relro -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -L/usr/local/lib -fstack-protector-strong'

[haarg]

This was discussed in PPC 14, which would create many more variables of this form. The general consensus was that adding strictness to these variables would be good. While implementing that strictness is separate from the PPC from a language design perspective, it is likely to have a lot of overlap in implementation. I believe someone is working on implementing PPC 14, so this is likely something to be looked at after that work is done.