vivify array elements when putting them on stack

When the elements of an array are put on the stack, by a padav, rv2av,
or padrange op, null pointers in the AvARRAY were being pushed as
&PL_sv_undef, which was OK in rvalue contexts but caused misbehaviour
in lvalue contexts.  Change this to vivify these elements.  There's no
attempt here to limit the vivification to lvalue contexts: the existing
op flags aren't enough to detect \(@a), and attempting to catch all
cases where a new flag needs to be set would be an error-prone process.
Fixes [perl #8910].

1 files changed, 5 insertions, 3 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 8a74f58cb8..d479ecf218 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -1163,15 +1163,17 @@ S_pushav(pTHX_ AV* const av)
     if (UNLIKELY(SvRMAGICAL(av))) {
         PADOFFSET i;
         for (i=0; i < (PADOFFSET)maxarg; i++) {
-            SV ** const svp = av_fetch(av, i, FALSE);
+            SV ** const svp = av_fetch(av, i, TRUE);
             SP[i+1] = svp ? *svp : &PL_sv_undef;
         }
     }
     else {
         PADOFFSET i;
         for (i=0; i < (PADOFFSET)maxarg; i++) {
-            SV * const sv = AvARRAY(av)[i];
-            SP[i+1] = LIKELY(sv) ? sv : &PL_sv_undef;
+            SV *sv = AvARRAY(av)[i];
+	    if (!LIKELY(sv))
+		AvARRAY(av)[i] = sv = newSV(0);
+	    SP[i+1] = sv;
         }
     }
     SP += maxarg;