PATCH: [perl #121816] Add warning for repetition x < 0

I consider this experimental, so that if code breaks as a result, we
will remove it.

I chose the numeric warnings category.  But misc or a new subcategory of
numeric might be better choices.

There is also the issue if someone is calculating the repeat count in
floating point and gets something that would be 0 if there were infinite
precision, but ends up being a very small negative number.  The current
implementation will warn on that, but probably shouldn't.  I suspect that
this would be extremely rare in practice.

1 files changed, 8 insertions, 6 deletions

diff --git a/pp.c b/pp.c
index f544c39a94..c240b22f26 100644
--- a/pp.c
+++ b/pp.c
@@ -1656,23 +1656,25 @@ PP(pp_repeat)
 	      else
 		   count = uv;
 	 } else {
-	      const IV iv = SvIV_nomg(sv);
-	      if (iv < 0)
-		   count = 0;
-	      else
-		   count = iv;
+	      count = SvIV_nomg(sv);
 	 }
     }
     else if (SvNOKp(sv)) {
 	 const NV nv = SvNV_nomg(sv);
 	 if (nv < 0.0)
-	      count = 0;
+              count = -1;   /* An arbitrary negative integer */
 	 else
 	      count = (IV)nv;
     }
     else
 	 count = SvIV_nomg(sv);
 
+    if (count < 0) {
+        count = 0;
+        Perl_ck_warner(aTHX_ packWARN(WARN_NUMERIC),
+                                         "Negative repeat count does nothing");
+    }
+
     if (GIMME == G_ARRAY && PL_op->op_private & OPpREPEAT_DOLIST) {
 	dMARK;
 	static const char* const oom_list_extend = "Out of memory during list extend";