if ($symtab && !exists $symtab->{$name}) {
      # No typeglob yet, so we can use a reference as space-
      # efficient proxy for a constant subroutine
      # The check in Perl_ck_rvconst knows that inlinable
      # constants from cv_const_sv are read only. So we have to:
      Internals::SvREADONLY($scalar, 1);
      $symtab->{$name} = \$scalar;
      mro::method_changed_in($pkg);
  } else {
      *$full_name = sub () { $scalar };
  }