Module with "use Tesla" causes runtime error in Gradualizer

[japhib]

The Tesla library is a common HTTP client for Elixir. Part of the setup for using Tesla involves declaring a module that looks like this:

defmodule MyHTTPClient do
  use Tesla
  # define middlewares & configs
end

The use Tesla line invokes a macro that generates a sizable amount of code for this module. When that code is typechecked via mix gradient, it results in a runtime error like the following:

Typechecking files...

** (FunctionClauseError) no function clause matching in :gradualizer_lib.pick_value/2

    The following arguments were given to :gradualizer_lib.pick_value/2:

        # 1
        {:type, 0, :any, []}

        # 2
        {:env, ... [edited for brevity]

    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:160: :gradualizer_lib.pick_value/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:186: :gradualizer_lib."-pick_value/2-lc$^0/1-0-"/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:186: :gradualizer_lib."-pick_value/2-lc$^0/1-0-"/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:186: :gradualizer_lib.pick_value/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:196: :gradualizer_lib."-pick_value/2-lc$^2/1-2-"/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:197: :gradualizer_lib."-pick_value/2-lc$^2/1-2-"/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:197: :gradualizer_lib.pick_value/2
    (gradualizer) use_tesla/deps/gradualizer/src/gradualizer_lib.erl:154: :gradualizer_lib."-pick_values/2-lc$^0/1-0-"/2

The same error occurs when doing mix gradient lib/use_tesla.ex, i.e. invoking only the single file rather than the entire Mix project.

I've attempted to reproduce the issue on pure Elixir code (i.e. not macros) using Gradient's decompilation facilities in Gradient.Debug, but so far no luck. This definitely seems like an issue in Gradualizer rather than Gradient, but I'm still working on reproducing it with pure Erlang code.

I've created a PR that adds this case to Gradient as a (failing) unit test: #167

[erszcz]

Hi, @japhib!

Thanks for the report. It's another one in a row about gradualizer.pick_value/2 together with #169 and josefs/Gradualizer#530. We'll take a look into it, as this function ought to be guarded from getting spuriuos, non-normalised types such as this. I have a hunch it's connected with the support for intersection types aka overloaded functions aka functions with multiple clause specs - josefs/Gradualizer#461. We'll look into it 👍