Stop creating defelems for undef in foreach(@_)

This is part of ticket #119433.  This particular bug is triggered by
Data::Dump’s test suite.

Commit ce0d59f changed arrays to use NULL for nonexistent elements,
instead of &PL_sv_undef (the special scalar returned by Perl’s ‘undef’
operator).

‘foreach’ was not updated to account.  It was still treating
&PL_sv_undef as a nonexistent element.  This was causing ‘Modifica-
tion of non-creatable array value attempted, subscript 0’, due to a
similar bug in vivify_defelem, which the next commit will fix.

(Fixing vivify_defelem without fixing foreach will make the test pass,
but for foreach to create a defelem to begin with is inefficient and
should be addressed anyway.)

1 files changed, 3 insertions, 4 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 82c8e12a06..4faa738122 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -1918,10 +1918,7 @@ PP(pp_iter)
                 SvREFCNT_inc_simple_void_NN(sv);
             }
         }
-        else
-            sv = &PL_sv_undef;
-
-        if (!av_is_stack && sv == &PL_sv_undef) {
+        else if (!av_is_stack) {
             SV *lv = newSV_type(SVt_PVLV);
             LvTYPE(lv) = 'y';
             sv_magic(lv, NULL, PERL_MAGIC_defelem, NULL, 0);
@@ -1930,6 +1927,8 @@ PP(pp_iter)
             LvTARGLEN(lv) = (STRLEN)UV_MAX;
             sv = lv;
         }
+        else
+            sv = &PL_sv_undef;
 
         oldsv = *itersvp;
         *itersvp = sv;