[perl #80098] Bleadperl breaks Attribute::Lexical

If

@UNIVERSAL::ISA = "a";

and

@a::ISA = "b";

then methods are searched for in these orders:

main UNIVERSAL a b
UNIVERSAL a b UNIVERSAL a b
a b UNIVERSAL a b
b UNIVERSAL a b

For method lookup, looking at a stash twice causes no problems (except
for a SUPER bug I’ve just found--to be dealt with separately).

My fix to next::method in a5cd004 which made it do a second pass with
UNIVERSAL the way gv_fetchmeth does did not take into account that it
might return its caller (sub a::foo { return shift->next::can }),
causing an infinite loop.

This patch makes it check, on the second pass, whether each stash is
the stash at the start of the MRO list and breaks if that is the case,
so the MROs are effectively:

main UNIVERSAL a b
UNIVERSAL a b
a b UNIVERSAL
b UNIVERSAL a

(which is what they are effectively already for method lookup).

1 files changed, 14 insertions, 2 deletions

diff --git a/t/mro/next_edgecases.t b/t/mro/next_edgecases.t
index e77ce7be31..3840a4b882 100644
--- a/t/mro/next_edgecases.t
+++ b/t/mro/next_edgecases.t
@@ -5,7 +5,7 @@ use warnings;
 
 BEGIN { chdir 't'; require q(./test.pl); @INC = qw "../lib lib" }
 
-plan(tests => 14);
+plan(tests => 17);
 
 {
 
@@ -94,13 +94,15 @@ plan(tests => 14);
 
 }
 
-# Test next::method with UNIVERSAL methods
+# Test next::method/can with UNIVERSAL methods
 {
     package UNIVERSAL;
     sub foo { "foo" }
+    sub kan { shift->next::can }
     our @ISA = "a";
     package a;
     sub bar { "bar" }
+    sub baz { shift->next::can }
     package M;
     sub foo { shift->next::method }
     sub bar { shift->next::method }
@@ -108,4 +110,14 @@ plan(tests => 14);
 
     is eval { M->foo }, "foo", 'next::method with implicit UNIVERSAL';
     is eval { M->bar }, "bar", 'n::m w/superclass of implicit UNIVERSAL';
+
+    is baz a, undef,
+     'univ superclasses next::cannot their own methods';
+    is kan UNIVERSAL, undef,
+     'UNIVERSAL next::cannot its own methods';
+
+    @a::ISA = 'b';
+    sub b::cnadd { shift->next::can }
+    is baz b, \&a::baz,
+      'univ supersuperclass noxt::can method in its immediate subclasses';
 }