That’s a scope problem in iex and single file scripts. Since everything is compiled at once (not each file into .beam file inside _build directory) the code must work as custom modules are not defined at compile-time. Using them run-time is definitely not a problem. This behaviour is not best, but very reasonable.

This should do the job:

defmodule Customer do
  defstruct name: "", company: ""
end

defmodule BugReport do
  defstruct owner: nil, details: "", severity: 1

  def new(data \\ %{}, customer \\ nil) do
    owner = case customer do
      struct when is_struct(struct, Customer) -> struct
      map_or_list when is_map(map_or_list) or is_list(map_or_list) -> struct(Customer, map_or_list)
      _customer -> %Customer{}
    end

    map = Map.new(data)
    struct(%__MODULE__{owner: owner}, map)
  end
end

This allows us to write code like:

iex> BugReport.new()
%BugReport{owner: %Customer{name: "", company: ""}, details: "", severity: 1}

iex> BugReport.new(details: "…")
%BugReport{owner: %Customer{name: "", company: ""}, details: "…", severity: 1}

iex> BugReport.new([], [name: "xyz"])
%BugReport{owner: %Customer{name: "xyz", company: ""}, details: "", severity: 1}

In both cases you are able to use Keyword lists as well as Map and in customer case also Customer struct (of course as long as call with %Customer{…} argument is in run-time).