Background

I am playing around with defguard in macro I am defining. The macro has an opaque type and I think dialyzer is having issues with this:

macro:

defmodule NewType do

  defmacro deftype(name, type) do
    quote do
      defmodule unquote(name) do
        @opaque t :: {unquote(name), unquote(type)}

        @spec new(value :: unquote(type)) :: t
        def new(value) when is_binary(value), do: {unquote(name), value}

        @spec extract(new_type :: t) :: unquote(type)
        def extract({unquote(name), value}), do: value
      end
    end
  end

  defguard is_type?(value, mod)
           when is_tuple(value) and
                  elem(value, 0) == mod and is_binary(elem(value, 1))
end

In theory, an invocation of deftype Name, String.t() would generate:

defmodule Name do
   @opaque t :: {Name, String.t}

  @spec new(value :: String.t()) :: t
  def new(value) when is_binary(value), do: {Name, value}

  @spec extract(new_type :: t) :: String.t()
  def extract({Name, value}), do: value
end

And in fact, this code works as intended:

type.ex (module where I define several types)

defmodule Type do
  import NewType
  
  deftype Name, String.t()
end

test.ex

defmodule Test do
  alias Type.Name

  @spec print(Name.t()) :: binary
  def print(name), do: Name.extract(name)
end

Problem

The problem arises when I try to use the guard:

test.ex

defmodule Test do
  import NewType
  alias Type.Name

  @spec print(Name.t()) :: binary
  def print(name) when is_type?(name, Name), do: Name.extract(name)
end

Everything runs as expected, but dialyzer does not see it the same way:

lib/test.ex:6:no_return
Function print/1 has no local return.
________________________________________________________________________________
lib/test.ex:6:call_without_opaque
Function call without opaqueness type mismatch.

Call does not have expected opaque term of type Type.Name.t() in the 1st position.

Type.Name.extract(_name :: tuple())

________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2

Even though Name.extract does receive a Name.t as a parameter, dialyzer states otherwise.
At this point, I think this might be because I am “exposing” the internal data structure in the guard with is_tuple, but at the same time, I don’t see how else I can have defguard and still use opaque types.

Question

Am I missing something?