Do not cache utf8 offsets for non-canonical lengths

In particular, if the length is beyond the end, it should not be stored as the end.

1 files changed, 9 insertions, 1 deletions

diff --git a/t/op/index.t b/t/op/index.t
index 2f0834e8d1..a218848851 100644
--- a/t/op/index.t
+++ b/t/op/index.t
@@ -8,7 +8,7 @@ BEGIN {
 }
 
 use strict;
-plan( tests => 414 );
+plan( tests => 415 );
 
 run_tests() unless caller;
 
@@ -358,4 +358,12 @@ $x;
 EOS
     }
 
+    {
+        my $s = "abc";
+        my $len = length($s);
+        utf8::upgrade($s);
+        length($s);
+        is(index($s, "", $len+1), 3, 'Overlong index doesn\'t confuse utf8 cache');
+    }
+
 } # end of sub run_tests