Add a new warning "Negative repeat count"

for the cases $x x -1.

p4raw-id: //depot/perl@22543

3 files changed, 17 insertions, 1 deletions

diff --git a/pod/perlop.pod b/pod/perlop.pod
index 86cb294fe3..4430fe7609 100644
--- a/pod/perlop.pod
+++ b/pod/perlop.pod
@@ -238,7 +238,9 @@ Binary "x" is the repetition operator.  In scalar context or if the left
 operand is not enclosed in parentheses, it returns a string consisting
 of the left operand repeated the number of times specified by the right
 operand.  In list context, if the left operand is enclosed in
-parentheses, it repeats the list.
+parentheses, it repeats the list.  If the right operand is zero or
+negative, it returns an empty string or an empty list, depending on the
+context.
 
     print '-' x 80;		# print row of dashes
 
diff --git a/pp.c b/pp.c
index d3434c37d6..53c7162915 100644
--- a/pp.c
+++ b/pp.c
@@ -1386,6 +1386,11 @@ PP(pp_repeat)
   dSP; dATARGET; tryAMAGICbin(repeat,opASSIGN);
   {
     register IV count = POPi;
+    if (count < 0) {
+	if (ckWARN(WARN_MISC))
+	    Perl_warner(aTHX_ packWARN(WARN_MISC), "Negative repeat count");
+	count = 0;
+    }
     if (GIMME == G_ARRAY && PL_op->op_private & OPpREPEAT_DOLIST) {
 	dMARK;
 	I32 items = SP - MARK;
diff --git a/t/lib/warnings/pp b/t/lib/warnings/pp
index 5ed7aa0891..db4202739b 100644
--- a/t/lib/warnings/pp
+++ b/t/lib/warnings/pp
@@ -102,3 +102,12 @@ use utf8 ;
 $_ = "\x80  \xff" ;
 reverse ;
 EXPECT
+########
+# pp.c
+use warnings;
+$a = "b" x -1;
+$a = "b" x 0;
+no warnings;
+$a = "b" x -1;
+EXPECT
+Negative repeat count at - line 3.