Fix undef %Foo:: to update subclasses

This is something I think I broke with 80ebaca, which made sure
that isa linearisations were cached on subclasses after calls
to mro_isa_changed_in (so the data could be used to delete
isarev entries).

The result is that hv_undef, which calls mro_isa_changed_in before
deleting everything, was updating the subclasses’ isa caches while its
own @ISA was still visible.

1 files changed, 9 insertions, 1 deletions

diff --git a/t/mro/basic.t b/t/mro/basic.t
index c6f25429b8..b42c802b49 100644
--- a/t/mro/basic.t
+++ b/t/mro/basic.t
@@ -3,7 +3,7 @@
 use strict;
 use warnings;
 
-BEGIN { require q(./test.pl); } plan(tests => 51);
+BEGIN { require q(./test.pl); } plan(tests => 52);
 
 require mro;
 
@@ -320,3 +320,11 @@ is(eval { MRO_N->testfunc() }, 123);
     delete $Blength::{ISA};
     ok !Blength->isa("Bladd"), 'delete $package::{ISA}';
 }
+
+{
+    # Undefining stashes
+    @Thrext::ISA = "Thwit";
+    @Thwit::ISA = "Sile";
+    undef %Thwit::;
+    ok !Thrext->isa('Sile'), 'undef %package:: updates subclasses';
+}