refaliasing breaks refcounting of field variable

This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.43 running under perl 5.37.9.


-----------------------------------------------------------------
<!--[Please describe your issue here]-->

Combining the experimental core class system with experimental
refaliasing\, here\'s an attempt to replace a field variable\:

```
$ perl5.37.9 -Mexperimental=refaliasing,class -lwe 'class Foo { field $aa; method bb { print \$aa; } method cc { print \$aa; \$aa = \(my$x); print \$aa; } } $x = Foo->new; $x->bb; $x->cc; $x->bb;'
SCALAR(0x56013fda95f0)
SCALAR(0x56013fda95f0)
SCALAR(0x56013febfd80)
UNKNOWN(0x56013fda95f0)
zsh: segmentation fault  perl5.37.9 -Mexperimental=refaliasing,class -lwe 
```

How this ought to behave is up for debate\. Maybe the replacement
operation should croak\, maybe it should affect the pad of the running
method without affecting the field variable\, and maybe it should
replace the field variable in the object representation\. But it
certainly shouldn\'t do the above\, prematurely freeing an SV and then
segfaulting\. It looks to me like this is doing a replacement in the pad
but getting confused about the refcounting of the pad entry\.

<!--[Please do not change anything below this line]-->
<!------------------------------------------------------------------- -->


---
**Flags**
- category=core
- severity=medium
---
**Perl configuration**
```
Site configuration information for perl 5.37.9:

Configured by zefram at Wed Mar  1 02:31:43 GMT 2023.

Summary of my perl5 (revision 5 version 37 subversion 9) configuration:
   
  Platform:
    osname=linux
    osvers=5.10.0-21-amd64
    archname=x86_64-linux-thread-multi
    uname='linux barba.rous.org 5.10.0-21-amd64 #1 smp debian 5.10.162-1 (2023-01-21) x86_64 gnulinux '
    config_args='-des -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52 -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -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='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='10.2.1 20210110'
    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 /lib/x86_64-linux-gnu /lib
    libs=-lpthread -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.31.so
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.31'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/5.37.9/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'


---
@INC for perl 5.37.9:
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/site_perl/5.37.9/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/site_perl/5.37.9
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/5.37.9/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/5.37.9

---
Environment for perl 5.37.9:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh
```

• refaliasing breaks refcounting of field variable by zefram via perl5-porters
• Re: refaliasing breaks refcounting of field variable by Paul "LeoNerd" Evans