Restore autouse’s exemption from redef warnings

This also restores the subroutine redefinition warning for newly-cre-
ated XSUBs outside the autouse package.  See below.

This commit added the exemption to fix a known bug, that loading a
module and importing from it would cause a redefinition warning if
there were an autouse stub:

perl-5.004_03-1092-g2f34f9d
commit 2f34f9d4825ac9262ece854fc4c50479f4838ff8
Author: Ilya Zakharevich <ilya@math.berkeley.edu>
Date:   Mon Mar 2 16:36:02 1998 -0500

    Make autouse -w-safe

    p4raw-id: //depot/perl@781

The subroutine redefinition warning occurs in three places.
This commit removed the autouse exemption from two
of them.  I can’t see how it wasn’t a mistake, as <5104D4DBC598D211B5FE0000F8FE7EB202D49EE9@mbtlipnt02.btlabs.bt.co.uk>
(the apparent source of the patch, makes no mention of it:

perl-5.005_02-2920-ge476b1b
commit e476b1b5c29f354cf8dad61a9fc6d855bdfb5b7d
Author: Gurusamy Sarathy <gsar@cpan.org>
Date:   Sun Feb 20 22:58:09 2000 +0000

    lexical warnings update, ability to inspect bitmask in calling
    scope, among other things (from Paul Marquess)

    p4raw-id: //depot/perl@5170

This commit refactored things to remove some compiler warnings, but
in doing so reversed the logic of the condition, causing redefini-
tion warnings for newly-created XSUBs to apply only to subs from the
autouse package:

perl-5.8.0-5131-g66a1b24
commit 66a1b24beb76ea873ad4caa57ee3ab9df945afbf
Author: Andy Lester <andy@petdance.com>
Date:   Mon Jun 6 05:11:07 2005 -0500

    Random cleanups #47
    Message-ID: <20050606151107.GC7022@petdance.com>

    p4raw-id: //depot/perl@24735

I’ve basically reinstated the changes in 2f34f9d4, but with tests
this time.

It may not make sense for autouse to be exempt for newATTRSUB and
newXS, but keeping the logic surrounding the warning as close as
possible to being the same could allow future refactorings to
merge them.

2 files changed, 44 insertions, 1 deletions

diff --git a/dist/autouse/t/autouse.t b/dist/autouse/t/autouse.t
index 53e1740df9..59374c2057 100644
--- a/dist/autouse/t/autouse.t
+++ b/dist/autouse/t/autouse.t
@@ -8,7 +8,7 @@ BEGIN {
     }
 }
 
-use Test::More tests => 12;
+use Test::More tests => 15;
 
 BEGIN {
     require autouse;
@@ -69,3 +69,34 @@ autouse->import("MyTestModule" => 'test_function');
 my $ret = test_function();
 is( $ret, 'works' );
 
+# Test that autouse is exempt from all methods of triggering the subroutine
+# redefinition warning.
+SKIP: {
+    skip "Fails in 5.15.5 and below (perl bug)", 2 if $] < 5.0150051;
+    use warnings; local $^W = 1;
+    my $w;
+    local $SIG{__WARN__} = sub { $w .= shift };
+    use autouse MyTestModule2 => 'test_function2';
+    *MyTestModule2::test_function2 = \&test_function2;
+    require MyTestModule2;
+    is $w, undef,
+       'no redefinition warning when clobbering autouse stub with new sub';
+    undef $w;
+    import MyTestModule2 'test_function2';
+    is $w, undef,
+       'no redefinition warning when clobbering autouse stub via *a=\&b';
+}
+SKIP: {
+    skip "Fails from 5.10 to 5.15.5 (perl bug)", 1
+	if $] < 5.0150051 and $] > 5.0099;
+    use Config;
+    skip "no B", 1 unless $Config{extensions} =~ /\bB\b/;
+    use warnings; local $^W = 1;
+    my $w;
+    local $SIG{__WARN__} = sub { $w .= shift };
+    use autouse B => "sv_undef";
+    *B::sv_undef = \&sv_undef;
+    require B;
+    is $w, undef,
+      'no redefinition warning when clobbering autouse stub with new XSUB';
+}
diff --git a/dist/autouse/t/lib/MyTestModule2.pm b/dist/autouse/t/lib/MyTestModule2.pm
new file mode 100644
index 0000000000..e2b551b2a7
--- /dev/null
+++ b/dist/autouse/t/lib/MyTestModule2.pm
@@ -0,0 +1,12 @@
+package MyTestModule2;
+use warnings;
+
+@ISA = Exporter;
+require Exporter;
+@EXPORT_OK = 'test_function2';
+
+sub test_function2 {
+  return 'works';
+}
+
+1;