Make tr/a/b/ croak on read-only null COWs

$ ./perl -Ilib -e 'use constant nullrocow => (keys%{{""=>undef}})[0]; for(nullrocow) { y/a/b/ }'
$ ./perl -Ilib -e 'use constant nullro => ""; for(nullro) { y/a/b/ }'
Modification of a read-only value attempted at -e line 1.

It should croak on COW scalars that are read-only, even if they are
zero-length, just as it does on non-COW scalars.

This logic is left over from when READONLY+FAKE meant COW.

1 files changed, 0 insertions, 1 deletions

diff --git a/doop.c b/doop.c
index faf74c59a8..5a819a7f52 100644
--- a/doop.c
+++ b/doop.c
@@ -632,7 +632,6 @@ Perl_do_trans(pTHX_ SV *sv)
     PERL_ARGS_ASSERT_DO_TRANS;
 
     if (SvREADONLY(sv) && !(PL_op->op_private & OPpTRANS_IDENTICAL)) {
-        if (!SvIsCOW(sv))
             Perl_croak_no_modify();
     }
     (void)SvPV_const(sv, len);