The guard clause can never succeed (with `ExUnit` `assert`)

Precheck

[x] Take a look at the open issues and be sure that your issue is not already covered.
[x] Be sure your versions of Dialyxir and Erlex are up to date.

Environment

Elixir & Erlang/OTP versions (elixir --version):

Erlang/OTP 24 [erts-12.2.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit]

Elixir 1.13.3 (compiled with Erlang/OTP 24)

Which version of Dialyxir are you using? (cat mix.lock | grep dialyxir):

cat mix.lock | grep dialyxir
  "dialyxir": {:hex, :dialyxir, "1.1.0", "c5aab0d6e71e5522e77beff7ba9e08f8e02bad90dfbeffae60eaf0cb47e29488", [:mix], [{:erlex, ">= 0.2.6", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm", "07ea8e49c45f15264ebe6d5b93799d4dd56a44036cf42d0ad9c960bc266c0b9a"},

Current behavior

This code:

https://github.com/etalab/transport-site/blob/2c34dcd6d18a0d354b4697bec80a5136d7454303/apps/transport/test/support/s3_test_utils.ex#L14-L20

Leads to this warning:

test/support/s3_test_utils.ex:14:guard_fail
The guard clause:

when _ :: %{:http_method => :get, :path => <<_::8>>, :service => :s3, _ => _} === false

can never succeed.

Expected behavior

I was expecting it would work, even after looking a bit closer at how exunit handles matches (https://github.com/elixir-lang/elixir/blob/feb9884a3e7196aac6c8ba2132cb41515adb1c5e/lib/ex_unit/lib/ex_unit/assertions.ex#L84-L99), and switching from require ExUnit.Assertions to import ExUnit.Assertions.

I wonder if my understanding is incorrect, or if this is a trouble in the dialyzer run, not very sure yet, but I thought it would be worth opening to discuss this here.

My intuition is that it's unhappy that the expression passed to assert can never evaluate to nil or false, because instead of a comparison you are passing a non-refutable pattern match. So instead of nil/false you will get an exception, and the guard clause in the expanded macro will indeed never be used.

This isn't really a question about dialyxir since its a question about the results of the analysis which isn't done in this project; but for it to be an OTP issue you'd have to reproduce it in valid Erlang. The best place to ask for help is the Elixir forum. You might have a tough time getting good results dialyzing tests just because almost no one does it, so idiomatic expressions may yield warnings that you can't solve.

jeremyjh / dialyxir