[perl #34976] substr uses utf8 length cache incorrectly

p4raw-id: //depot/perl@24270

2 files changed, 9 insertions, 2 deletions

diff --git a/sv.c b/sv.c
index f0a5a0f608..7bfd7a5083 100644
--- a/sv.c
+++ b/sv.c
@@ -6402,7 +6402,7 @@ Perl_sv_pos_u2b(pTHX_ register SV *sv, I32* offsetp, I32* lenp)
 	 if (lenp) {
 	      found = FALSE;
 	      start = s;
-              if (utf8_mg_pos(sv, &mg, &cache, 2, lenp, *lenp + *offsetp, &s, start, send)) {
+              if (utf8_mg_pos(sv, &mg, &cache, 2, lenp, *lenp, &s, start, send)) {
                   *lenp -= boffset;
                   found = TRUE;
               }
diff --git a/t/op/substr.t b/t/op/substr.t
index 9549d29028..2c6a0ed32e 100755
--- a/t/op/substr.t
+++ b/t/op/substr.t
@@ -1,6 +1,6 @@
 #!./perl
 
-print "1..190\n";
+print "1..192\n";
 
 #P = start of string  Q = start of substr  R = end of substr  S = end of string
 
@@ -658,3 +658,10 @@ ok 174, $x eq "\x{100}\x{200}\xFFb";
     substr($a, -1) &= chr(0xfeff);
     ok 190, $a eq "\xbf";
 }
+
+# [perl #34976] incorrect caching of utf8 substr length
+{
+    my  $a = "abcd\x{100}";
+    ok 191, substr($a,1,2) eq 'bc';
+    ok 192, substr($a,1,1) eq 'b';
+}