util.c: Stop ck_index/fbm_compile from abusing readonliness

There is currently a convenient exception in sv_force_normal that
allows read-only scalars to be modified at compile time.

I want to get rid of it, since most code that relies on it is buggy.

So stop ck_index from relying on that when it calls fbm_compile by
stopping fbm_compile from triggering the error by forcing stringifica-
tion of variables that are already strings.

1 files changed, 3 insertions, 1 deletions

diff --git a/util.c b/util.c
index e07ae4d203..11a7863d3b 100644
--- a/util.c
+++ b/util.c
@@ -539,7 +539,9 @@ Perl_fbm_compile(pTHX_ SV *sv, U32 flags)
 	if (mg && mg->mg_len >= 0)
 	    mg->mg_len++;
     }
-    s = (U8*)SvPV_force_mutable(sv, len);
+    if (!SvPOK(sv) || SvNIOKp(sv) || SvIsCOW(sv))
+	s = (U8*)SvPV_force_mutable(sv, len);
+    else s = (U8 *)SvPV_mutable(sv, len);
     if (len == 0)		/* TAIL might be on a zero-length string. */
 	return;
     SvUPGRADE(sv, SVt_PVMG);