I’m using Dialyzer to check over some code that uses the mongodb package. I’ve got a function that abstracts the Mongo.insert_one/4 function and adds in some extra logging for certain conditions:

  def insert(collection, args) do
    case Mongo.insert_one(mongo_pid(), collection, args) do
      {:ok, %Mongo.InsertOneResult{acknowledged: true, inserted_id: id}} ->
        {:ok, %{"_id" => id}}

      # Check for 11000 error (usually this means a duplicate)
      {:error, %Mongo.WriteError{write_errors: [%{"code" => 11_000} | _]} = err} -> {:error, "Duplicate entry"}

      {:error, err} -> {:error, err}
    end
  end

When I run dialyzer, I get the following error:

The pattern can never match the type.

Pattern:
{:error, _err = %Mongo.WriteError{:write_errors => [%{<<99, 111, 100, 101>> => 11000} | _]}}

Type:

  :ok
  | {:error,
     %Mongo.Error{
       :__exception__ => _,
       :code => number(),
       :host => binary(),
       :message => binary()
     }}
  | {:ok,
     %Mongo.InsertOneResult{
       :acknowledged => false,
       :inserted_id => nil | %BSON.ObjectId{:value => <<_::96>>}
     }}

After looking at the mongodb source code I can see that it appears to miss a viable return type:

@type result(t) :: :ok | {:ok, t} | {:error, Mongo.Error.t()}

should be (I think) something like:

@type result(t) :: :ok | {:ok, t} | {:error, Mongo.Error.t()} | {:error, Mongo.WriteError.t()}

My questions are:

1. Am I correct in thinking that the dialyzer error in my app is resulting from what appears to be an incorrect type definition in a dependency (mongodb in this case)?
2. Apart from filing an issue, what are the ways to configure dialyzer to ignore that particular error in that specific place?
3. Why doesn’t Dialyzer catch this problem in mongodb? I cloned the repo, I fired up Dialyzer, and it shows no errors. Is that expected? Common?