Refuse to create a class if its package already contains a non-empty @isa array

Author: leonerd

class.c

@@ -251,6 +251,17 @@ Perl_class_setup_stash(pTHX_ HV *stash)
             HEKfARG(HvNAME_HEK(stash)));
     }
 
+    {
+        SV *isaname = newSVpvf("%" HEKf "::ISA", HvNAME_HEK(stash));
+        sv_2mortal(isaname);
+
+        AV *isa = get_av(SvPV_nolen(isaname), (SvFLAGS(isaname) & SVf_UTF8));
+
+        if(isa && av_count(isa) > 0)
+            croak("Cannot create class %" HEKf " as it already has a non-empty @ISA",
+                HvNAME_HEK(stash));
+    }
+
     char *classname = HvNAME(stash);
     U32 nameflags = HvNAMEUTF8(stash) ? SVf_UTF8 : 0;
 

pod/perldiag.pod

@@ -713,6 +713,12 @@ no legal conversion from that type of reference to a typeglob.
 (P) Perl detected an attempt to copy a value to an internal type that cannot
 be directly assigned to.
 
+=item Cannot create class %s as it already has a non-empty @ISA
+
+(F) An attempt was made to create a class out of a package that already has
+an C<@ISA> array, and the array is not empty.  This is not permitted, as it
+would lead to a class with inconsistent inheritance.
+
 =item Cannot find encoding "%s"
 
 (S io) You tried to apply an encoding that did not exist to a filehandle,

t/lib/croak/class

@@ -80,3 +80,10 @@ class XXX {}
 push @XXX::ISA, q(Another);
 EXPECT
 Modification of a read-only value attempted at - line 4.
+########
+no warnings 'experimental::class';
+use feature 'class';
+BEGIN { push @XXX::ISA, q(Another); }
+class XXX {}
+EXPECT
+Cannot create class XXX as it already has a non-empty @ISA at - line 4.