list assign in list context was over-optimising

GH #17816

This code:

    my $x = 1;
    print (($x, undef) = (2 => $x));

was printing "22" when it should have been printing "21".
An optimisation skips the 'common values on both sides' test
when the LHS of an assign only contains a single var; as the example
above shows, this is not sufficient.

This was broken by v5.23.1-202-g808ce55782

This commit fixes it by counting undef's on the LHS towards the var
count if they don't appear first.

1 files changed, 7 insertions, 3 deletions

diff --git a/op.c b/op.c
index 05f6d9d1a3..49aac853d4 100644
--- a/op.c
+++ b/op.c
@@ -15679,11 +15679,15 @@ S_aassign_scan(pTHX_ OP* o, bool rhs, int *scalars_p)
         goto do_next;
 
     case OP_UNDEF:
-        /* undef counts as a scalar on the RHS:
-         *   (undef, $x) = ...;         # only 1 scalar on LHS: always safe
+        /* undef on LHS following a var is significant, e.g.
+         *    my $x = 1;
+         *    @a = (($x, undef) = (2 => $x));
+         *    # @a shoul be (2,1) not (2,2)
+         *
+         * undef on RHS counts as a scalar:
          *   ($x, $y)    = (undef, $x); # 2 scalars on RHS: unsafe
          */
-        if (rhs)
+        if ((!rhs && *scalars_p) || rhs)
             (*scalars_p)++;
         flags = AAS_SAFE_SCALAR;
         break;