$lexical = (index($x, y) == -1) optimization returns the right side

[tonycoz]

Module:

Description

7e8d786 optimized code like:

$lexical = (index(...) == -1);

into an index op with flags that makes it assign to $lexical. Unfortunately that new operation returns the result of the == rather than $lexical. For example with 5.18.3:

$ ~/perl/5.18.3-dbg/bin/perl5.18.3 -le 'my $x; ($x = (index($y, $z) == -1)) =~ s/^/x/; print $x'
x

but with blead:

$ ./perl -le 'my $x; ($x = (index($y, $z) == -1)) =~ s/^/x/; print $x'
Modification of a read-only value attempted at -e line 1.

Note that without the index this works as expected:

$ ./perl -le 'my $x; ($x = 1) =~ s/^/x/; print $x'
x1

Expected behavior

The assignment should return the modifiable lvalue for $lexical

Perl configuration

Summary of my perl5 (revision 5 version 31 subversion 11) configuration:
  Commit id: ece42804593a3366ce48ea404a2c0c60063eeb1e
  Platform:
    osname=linux
    osvers=4.19.0-6-amd64
    archname=x86_64-linux
    uname='linux mars 4.19.0-6-amd64 #1 smp debian 4.19.67-2+deb10u2 (2019-11-11) x86_64 gnulinux '
    config_args='-des -Dusedevel -DDEBUGGING'
    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 ='-fwrapv -DDEBUGGING -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 -g'
    cppflags='-fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='8.3.0'
    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/gcc/x86_64-linux-gnu/8/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /lib64 /usr/lib64
    libs=-lpthread -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.28.so
    so=so
    useshrplib=false
    libperl=libperl.a
    gnulibc_version='2.28'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -g -L/usr/local/lib -fstack-protector-strong'

Characteristics of this binary (from libperl): 
  Compile-time options:
    DEBUGGING
    HAS_TIMES
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_DEVEL
    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 Apr 22 2020 14:46:25
  %ENV:
    PERLBREW_BASHRC_VERSION="0.43"
    PERLBREW_HOME="/home/tony/.perlbrew"
    PERLBREW_MANPATH=""
    PERLBREW_PATH="/home/tony/perl5/perlbrew/bin"
    PERLBREW_ROOT="/home/tony/perl5/perlbrew"
    PERLBREW_VERSION="0.67"
  @INC:
    lib
    /usr/local/lib/perl5/site_perl/5.31.11/x86_64-linux
    /usr/local/lib/perl5/site_perl/5.31.11
    /usr/local/lib/perl5/5.31.11/x86_64-linux
    /usr/local/lib/perl5/5.31.11