C++11 generated code tries to copy unique_ptr when using `valid` keyword

[fzakaria]

I have valid keywords in my kaitai struct definition.

meta:
  id: nix_nar
  title: Nix Archive (NAR)
  file-extension: nar
  endian: le
doc: |
    Nix Archive (NAR) format. A simple, reproducible binary archive
    format used by the Nix package manager to serialize file system objects.
doc-ref: 'https://nixos.org/manual/nix/stable/command-ref/nix-store.html#nar-format'

seq:
  - id: magic
    type: padded_str
    doc: "Magic string, must be 'nix-archive-1'."
    valid:
      expr: _.body == 'nix-archive-1'
  - id: root_node
    type: node
    doc: "The root of the archive, which is always a single node."

ksc is generating code like this:

void nix_nar_t::_read() {
    m_magic = std::unique_ptr<padded_str_t>(new padded_str_t(m__io, this, m__root));
    {
        std::unique_ptr<padded_str_t> _ = m_magic;
        if (!(_->body() == std::string("nix-archive-1"))) {
            throw kaitai::validation_expr_error<std::unique_ptr<nix_nar_t::padded_str_t>>(m_magic, m__io, std::string("/seq/0"));
        }
    }
    m_root_node = std::unique_ptr<node_t>(new node_t(m__io, this, m__root));
}

The problematic line is std::unique_ptr<padded_str_t> _ = m_magic; which fails with the following error

error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = nix_nar_t::padded_str_t; _Dp = std::default_delete<nix_nar_t::padded_str_t>]’
   17 |         std::unique_ptr<padded_str_t> _ = m_magic;