Segfaults when inheriting from empty class

[Ovid]

Description

When using the experimental class syntax in v5.38.0, we segfault if we inherit from an empty base class.

Steps to Reproduce

perl -E 'use experimental "class"; class Animal; class Dog  :isa(Animal); say "Hello"'
Segmentation fault: 11

Defining the body of the Animal class makes the error go away.

perl -E 'use experimental "class"; class Animal {}; class Dog  :isa(Animal); say "Hello"'
Hello

Expected behavior

I would have expected the code above to print "Hello"

Perl configuration

Summary of my perl5 (revision 5 version 38 subversion 0) configuration:
   
  Platform:
    osname=darwin
    osvers=22.5.0
    archname=darwin-2level
    uname='darwin ovids-m1-laptop.local 22.5.0 darwin kernel version 22.5.0: thu jun 8 22:22:20 pdt 2023; root:xnu-8796.121.3~7release_arm64_t6000 arm64 '
    config_args='-de -Dprefix=/Users/ovid/perl5/perlbrew/perls/perl-5.38.0 -Aeval:scriptdir=/Users/ovid/perl5/perlbrew/perls/perl-5.38.0/bin'
    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='cc'
    ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=13.4 -DNO_POSIX_2008_LOCALE -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -I/opt/local/include'
    optimize='-O3'
    cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=13.4 -DNO_POSIX_2008_LOCALE -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -I/opt/local/include'
    ccversion=''
    gccversion='Apple LLVM 14.0.3 (clang-1403.0.22.14.1)'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=8
    longdblkind=0
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -mmacosx-version-min=13.4 -fstack-protector-strong -L/usr/local/lib -L/opt/local/lib'
    libpth=/usr/local/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/14.0.3/lib /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib /opt/local/lib /usr/lib
    libs= 
    perllibs=
    libc=
    so=dylib
    useshrplib=false
    libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=bundle
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags=' '
    lddlflags=' -mmacosx-version-min=13.4 -bundle -undefined dynamic_lookup -L/usr/local/lib -L/opt/local/lib -fstack-protector-strong'


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_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
  Built under darwin
  Compiled at Jul  4 2023 08:24:13
  %ENV:
    PERL5LIB="/Users/ovid/veure/t/tests:/Users/ovid/veure/t/shared:/Users/ovid/veure/t/admin:/Users/ovid/perlcustom/lib"
    PERLBREW_HOME="/Users/ovid/.perlbrew"
    PERLBREW_MANPATH="/Users/ovid/perl5/perlbrew/perls/perl-5.38.0/man"
    PERLBREW_PATH="/Users/ovid/perl5/perlbrew/bin:/Users/ovid/perl5/perlbrew/perls/perl-5.38.0/bin"
    PERLBREW_PERL="perl-5.38.0"
    PERLBREW_ROOT="/Users/ovid/perl5/perlbrew"
    PERLBREW_SHELLRC_VERSION="0.96"
    PERLBREW_VERSION="0.96"
  @INC:
    /Users/ovid/veure/t/tests
    /Users/ovid/veure/t/shared
    /Users/ovid/veure/t/admin
    /Users/ovid/perlcustom/lib
    /Users/ovid/perl5/perlbrew/perls/perl-5.38.0/lib/site_perl/5.38.0/darwin-2level
    /Users/ovid/perl5/perlbrew/perls/perl-5.38.0/lib/site_perl/5.38.0
    /Users/ovid/perl5/perlbrew/perls/perl-5.38.0/lib/5.38.0/darwin-2level
    /Users/ovid/perl5/perlbrew/perls/perl-5.38.0/lib/5.38.0

[johannessen]

Possibly related: #20890

[johannessen]

Just remembered the PSC #102 summary, according to which this should have been documented as a known bug. But I’m afraid we all kind of forgot about that. 😑

[bscan]

• The segfaults on feature-class and refaliasing won't be fixed before 5.38, and should just be documented as "known bugs"

Which segfaults are known, and which are new bugs? I'm also seeing a segfault with this:

class Point {
    field $x: param();
}

[johannessen]

I think that one may be new?

Searching for “class” in the issues here, I see two more – #20947 and #20956.

[jkeenan]

Description

When using the experimental class syntax in v5.38.0, we segfault if we inherit from an empty base class. [snip]

Defining the body of the Animal class makes the error go away.

perl -E 'use experimental "class"; class Animal {}; class Dog  :isa(Animal); say "Hello"'
Hello

Providing a body for at least one of the class definitions appears to avoid the segfault.

$ perl -v | head -2 | tail -1
This is perl 5, version 38, subversion 0 (v5.38.0) built for x86_64-linux

$ perl -E 'use experimental "class"; class Animal; class Dog {}; say "Hello"'
Hello

$ perl -E 'use experimental "class"; class Animal; class Dog; class Entity {}; say "Hello"'
Hello

$ perl -E 'use experimental "class"; class Entity; class Dog {}; class Animal; say "Hello"'
Hello

[Ovid]

Yes, this does look like a duplicate of https://github.com/Perl/perl5/issues/20890. Sorry for that.

[leonerd]

Still seems to fail on current bleadperl:

$ ./perl -Ilib -E 'use experimental "class"; class Animal; class Dog  :isa(Animal); say "Hello"'
perl: op.c:7737: Perl_newSVOP: Assertion `sv' failed.
Aborted