[perl #118691] Allow defelem magic with neg indices

When a nonexistent array element is passed to a subroutine, a special
‘deferred element’ scalar (implemented using something called defelem
magic) is passed to the subroutine instead, which delegates to the
array element.  This allows some_benign_function($array[$nonexistent])
to avoid autovivifying unnecessarily.

Whether this magic would be triggered was based on whether the element
was within the range 0..$#array.  Since arrays can contain nonexistent
elements before $#array, this logic is incorrect.  It also makes sense
to allow $array[$neg] where the negative number points before the
beginning of the array to create a deferred element and only croak if
it is assigned to.

This commit fixes the logic for when deferred elements are created
and implements these deferred negative elements.

Since we have to be able to store negative values in xlv_targoff, it
is convenient to make it a union (with two types--signed and unsigned)
and use LvSTARGOFF for defelem array indices.

1 files changed, 8 insertions, 2 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 58a30831d8..571cd63b0a 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -2795,7 +2795,7 @@ PP(pp_aelem)
     IV elem = SvIV(elemsv);
     AV *const av = MUTABLE_AV(POPs);
     const U32 lval = PL_op->op_flags & OPf_MOD || LVRET;
-    const U32 defer = (PL_op->op_private & OPpLVAL_DEFER) && (elem > av_len(av));
+    const U32 defer = PL_op->op_private & OPpLVAL_DEFER;
     const bool localizing = PL_op->op_private & OPpLVAL_INTRO;
     bool preeminent = TRUE;
     SV *sv;
@@ -2836,14 +2836,20 @@ PP(pp_aelem)
 #endif
 	if (!svp || !*svp) {
 	    SV* lv;
+	    IV len;
 	    if (!defer)
 		DIE(aTHX_ PL_no_aelem, elem);
+	    len = av_len(av);
 	    lv = sv_newmortal();
 	    sv_upgrade(lv, SVt_PVLV);
 	    LvTYPE(lv) = 'y';
 	    sv_magic(lv, NULL, PERL_MAGIC_defelem, NULL, 0);
 	    LvTARG(lv) = SvREFCNT_inc_simple(av);
-	    LvTARGOFF(lv) = elem;
+	    /* Resolve a negative index now, unless it points before the
+	       beginning of the array, in which case record it for error
+	       reporting in magic_setdefelem. */
+	    LvSTARGOFF(lv) =
+		elem < 0 && len + elem >= 0 ? len + elem : elem;
 	    LvTARGLEN(lv) = 1;
 	    PUSHs(lv);
 	    RETURN;