module T::Private::Methods

def self.register_forwarder(from_method, to_method, mode: Modes.overridable, remove_first_param: false)
  # Normalize the method (see comment in signature_for_key).
  from_method = from_method.owner.instance_method(from_method.name)
  from_key = method_to_key(from_method)
  maybe_run_sig_block_for_key(from_key)
  if @signatures_by_method.key?(from_key)
    raise "#{from_method} already has a method signature"
  end
  from_params = from_method.parameters
  if from_params.length != 2 || from_params[0][0] != :rest || from_params[1][0] != :block
    raise "forwarder methods should take a single splat param and a block param. `#{from_method}` " \
          "takes these params: #{from_params}. For help, ask #dev-productivity."
  end
  # If there's already a signature for to_method, we get `parameters` from there, to enable
  # chained forwarding. NB: we don't use `signature_for_key` here, because the normalization it
  # does is broken when `to_method` has been clobbered by another method.
  to_key = method_to_key(to_method)
  maybe_run_sig_block_for_key(to_key)
  to_params = @signatures_by_method[to_key]&.parameters || to_method.parameters
  if remove_first_param
    to_params = to_params[1..-1]
  end
  # We don't bother trying to preserve any types from to_signature because this won't be
  # statically analyzed, and the types will just get checked when the forwarding happens.
  from_signature = Signature.new_untyped(method: from_method, mode: mode, parameters: to_params)
  @signatures_by_method[from_key] = from_signature
end