Manually revert ‘Remove SvREADONLY_on from op.c:op_const_sv’

This effectively reverts d0a32af375ac806258a97cce0091ce4b3636f8dc.

It was wrong.  And I should know that, too.

I was the one who implemented padtmp swiping and went around turning
on the read-only flag to make it work correctly.

This fails, because the second call to &$sub returns undef, its value
having been stolen by the $y assignment.

use Test::More tests=>1;
my $sub = sub {
    my $x = "x"x2000; sub () {$x};
}->();
$y = &$sub;
$z = &$sub;
is $z, $y;

1 files changed, 1 insertions, 0 deletions

diff --git a/pad.c b/pad.c
index 1533ec5d49..c9e16e55b9 100644
--- a/pad.c
+++ b/pad.c
@@ -2239,6 +2239,7 @@ S_cv_clone_pad(pTHX_ CV *proto, CV *cv, CV *outside, bool newcv)
 	    /* If the lexical is not used elsewhere, it is safe to turn on
 	       SvPADTMP, since it is only when it is used in lvalue con-
 	       text that the difference is observable.  */
+	    SvREADONLY_on(const_sv);
 	    SvPADTMP_on(const_sv);
 	    SvREFCNT_dec_NN(cv);
 	    cv = newCONSTSUB(CvSTASH(proto), NULL, const_sv);