Use defelems for (goto) &xsub calls

Before ce0d59f:

$ perl -e '++$#_; &utf8::encode'
Modification of a read-only value attempted at -e line 1.

As of ce0d59f:

$ ./perl -Ilib -e '++$#_; &utf8::encode'
Assertion failed: (sv), function Perl_sv_utf8_encode, file sv.c, line 3581.
Abort trap: 6

Calling sub { utf8::encode($_[0]) } should be more or less equivalent
to calling utf8::encode, but it is not in this case:

$ ./perl -Ilib -we '++$#_; &{sub { utf8::encode($_[0]) }}'
Use of uninitialized value in subroutine entry at -e line 1.

In the first two examples above, an implementation detail is leaking
through.  What you are seeing is not the array element, but a place-
holder that indicates an element that has not been assigned to yet.

We should use defelem magic so that what the XSUB assigns to will cre-
ate an array element (as happens with utf8::encode($_[0])).

All of the above applies to goto &xsub as well.

1 files changed, 12 insertions, 1 deletions

diff --git a/pp_hot.c b/pp_hot.c
index be16eae5ba..3b96643b00 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -2728,9 +2728,20 @@ try_autoload:
 	    const I32 items = AvFILLp(av) + 1;   /* @_ is not tieable */
 
 	    if (items) {
+		SSize_t i = 0;
 		/* Mark is at the end of the stack. */
 		EXTEND(SP, items);
-		Copy(AvARRAY(av), SP + 1, items, SV*);
+		for (; i < items; ++i)
+		    if (AvARRAY(av)[i]) SP[i+1] = AvARRAY(av)[i];
+		    else {
+			SV * const lv = sv_2mortal(newSV_type(SVt_PVLV));
+			SP[i+1] = lv;
+			LvTYPE(lv) = 'y';
+			sv_magic(lv, NULL, PERL_MAGIC_defelem, NULL, 0);
+			LvTARG(lv) = SvREFCNT_inc_simple_NN(av);
+			LvSTARGOFF(lv) = i;
+			LvTARGLEN(lv) = 1;
+		    }
 		SP += items;
 		PUTBACK ;		
 	    }