thread: limit scope of monkey patch

Future versions of Mail::Thread may be released to fix this bug.
However, since it's been about 8 years since the bug was reported..

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/PublicInbox/Thread.pm b/lib/PublicInbox/Thread.pm
index a2553da7..7dabf243 100644
--- a/lib/PublicInbox/Thread.pm
+++ b/lib/PublicInbox/Thread.pm
@@ -7,7 +7,10 @@ package PublicInbox::Thread;
 use strict;
 use warnings;
 use base qw(Mail::Thread);
-sub _container_class { 'PublicInbox::Thread::Container' }
+
+if ($Mail::Thread::VERSION <= 2.55) {
+        eval q(sub _container_class { 'PublicInbox::Thread::Container' });
+}
 
 package PublicInbox::Thread::Container;
 use strict;