[perl #90160] U* gives ‘U0 mode on an empty string’

This is a regression in 5.10 caused by change 23966/08ca2aa38a29,
which added a bit of faulty logic. It was treating U* in the middle of
a pack template as equivalent to U0, if the input string was empty.
(cherry picked from commit c5333953a555847ef4f1457905bf0f111a79eb72)

2 files changed, 6 insertions, 2 deletions

diff --git a/pp_pack.c b/pp_pack.c
index 0ae8afde9d..65adb51352 100644
--- a/pp_pack.c
+++ b/pp_pack.c
@@ -1660,7 +1660,7 @@ S_unpack_rec(pTHX_ tempsym_t* symptr, const char *s, const char *strbeg, const c
 	    break;
 	case 'U':
 	    if (len == 0) {
-                if (explicit_length) {
+                if (explicit_length && howlen != e_star) {
 		    /* Switch to "bytes in UTF-8" mode */
 		    if (symptr->flags & FLAG_DO_UTF8) utf8 = 0;
 		    else
diff --git a/t/op/pack.t b/t/op/pack.t
index 5775cafb53..7b9912aa94 100644
--- a/t/op/pack.t
+++ b/t/op/pack.t
@@ -12,7 +12,7 @@ my $no_endianness = $] > 5.009 ? '' :
 my $no_signedness = $] > 5.009 ? '' :
   "Signed/unsigned pack modifiers not available on this perl";
 
-plan tests => 14699;
+plan tests => 14700;
 
 use strict;
 use warnings qw(FATAL all);
@@ -1993,3 +1993,7 @@ is(unpack('c'), 65, "one-arg unpack (change #18751)"); # defaulting to $_
     my $y = runperl( prog => 'print split( /,/, unpack(q(%32u*), q(#,3,Q)), qq(\n)), qq(\n)' );
     is($y, "0\n", "split /a/, unpack('%32u*'...) didn't crash");
 }
+
+#90160
+is(eval { () = unpack "C0 U*", ""; "ok" }, "ok",
+  'medial U* on empty string');