list assign in list context: honour LHS undef

GH #16685

In

    @a = ($x, undef, undef) = (1))

@a should have 3 elements. v5.25.6-79-gb09ed995ad broke this and was
returning one element.

The fix is simple: that previous commit made it so that elements were
pushed back onto the stack only if they weren't immortal, so
&PL_sv_undef was getting skipped. Make it so they always are.

(cherry picked from commit 282d9dfeb4cea3c2d0335ba78faa3a9db931f1ec)

2 files changed, 10 insertions, 2 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 004dfba6b0..95f6d64411 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -2738,8 +2738,8 @@ PP(pp_aassign)
 	    if (!SvIMMORTAL(lsv)) {
                 sv_set_undef(lsv);
                 SvSETMAGIC(lsv);
-                *relem++ = lsv;
             }
+            *relem++ = lsv;
 	    break;
         } /* switch */
     } /* while */
diff --git a/t/op/aassign.t b/t/op/aassign.t
index 9128f9fd98..aa1f2c722c 100644
--- a/t/op/aassign.t
+++ b/t/op/aassign.t
@@ -595,7 +595,7 @@ SKIP: {
 }
 
 {
-    # GH #17816
+    # GH #16685
     # don't use the "1-arg on LHS can't be common" optimisation
     # when there are undef's there
     my $x = 1;
@@ -603,5 +603,13 @@ SKIP: {
     is("@a", "2 1", "GH #17816");
 }
 
+{
+    # GH #17816
+    # honour trailing undef's in list context
+    my $x = 1;
+    my @a = (($x, undef, undef) = (1));
+    is(scalar @a, 3, "GH #17816");
+}
+
 
 done_testing();