assertion failure in pp_methstart

[tonycoz]

use feature "class";
use strict;

class Foo {
  field $fa;
  field $fb;
  field $fc;
  field $fd;
  field $fe;
  field $ff;
  field $fg;
  field $fh;
  field $fi;
  field $fj;
  field $fk;
  field $fl;
  field $fm;
  field $fn;
  field $fo;
  field $fp;
  field $fq;
  field $fr;
  field $fs;
  field $ft;
  field $fu;
  field $fv;
  field $fw;
  field $fx;
  field $fy;
  field $fz;

  method fn { "xx $fa $fb $fc"; }
}

class Bar {
  field $fa;
  field $fb;

  method fn { "yy $fa $fb"; }
}

my $x = Bar->new;
print Foo::fn($x); # deliberately evil, but it should produce a runtime (compile-time?) error not an assertion

perl: class.c:187: Perl_pp_methstart: Assertion `ObjectMAXFIELD(instance)+1 > max_fieldix' failed.
Aborted

[leonerd]

Oooh fun. Yes I suspect this comes from a failure to check the instance is derived from the right class. See the TODO marker in class.c's PP(pp_methstart):

    if(!SvROK(self) ||
        !SvOBJECT((rv = SvRV(self))) ||
        SvTYPE(rv) != SVt_PVOBJ)
        /* TODO: check it's in an appropriate class */
        Perl_croak(aTHX_ "Cannot invoke method on a non-instance");

[leonerd]

Fixed; new test added:

$ git show
commit 1fbf5b2dd3fa178bc59539fba47a9f4e0ab972f1 (HEAD -> feature-class)
Author: Paul Evans <[email protected]>
Date:   Wed Aug 17 15:54:03 2022 +0100

    Check that object instance is a member of the correct class in pp_methstart
    
    Fixes https://github.com/leonerd/perl5/discussions/15
...
diff --git a/t/lib/croak/class b/t/lib/croak/class
index 3df84dc796..82a527a023 100644
--- a/t/lib/croak/class
+++ b/t/lib/croak/class
@@ -23,6 +23,15 @@ XXX::m([])
 EXPECT
 Cannot invoke method on a non-instance at - line 5.
 ########
+# Method calls from a different class
+no warnings 'experimental::class';
+use feature 'class';
+class XXX { method m { } }
+class YYY {}
+YYY->new->XXX::m();
+EXPECT
+Cannot invoke method on an instance of a different class at - line 6.
+########