Regression in running `mix test`

[dominicletz]

Elixir and Erlang/OTP versions

I've tried 1.19.1 and now main as well. The last good working version for me is 1.18.4

> elixir --version
Erlang/OTP 26 [erts-14.2.5.7] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Elixir 1.20.0-dev (f175f5f) (compiled with Erlang/OTP 26)

Operating system

Linux

Current behavior

Looks like something broke in selecting test files causing this output:

> mix test
...
Compiling 8 files (.ex)
Generated websockex app
** (FunctionClauseError) no function clause matching in String."-inlined-starts_with_string?/3-"/3    
    
    The following arguments were given to String."-inlined-starts_with_string?/3-"/3:
    
        # 1
        "test/support/test_client.ex"
    
        # 2
        27
    
        # 3
        108
    
    (elixir 1.19.1) lib/string.ex:2759: String."-inlined-starts_with_string?/3-"/3
    (elixir 1.19.1) lib/enum.ex:4312: Enum.predicate_list/3
    (mix 1.19.1) lib/mix/tasks/test.ex:808: anonymous fn/4 in Mix.Tasks.Test.classify_test_files/3
    (elixir 1.19.1) lib/enum.ex:2520: Enum."-reduce/3-lists^foldl/2-0-"/3
    (mix 1.19.1) lib/mix/tasks/test.ex:802: Mix.Tasks.Test.classify_test_files/3
    (mix 1.19.1) lib/mix/tasks/test.ex:659: Mix.Tasks.Test.do_run/3
    (mix 1.19.1) lib/mix/task.ex:499: anonymous fn/3 in Mix.Task.run_task/5
    (mix 1.19.1) lib/mix/cli.ex:131: Mix.CLI.run_task/2

The file structure is visible here: https://github.com/witchtails/websockex and can be reproduced with git checkout ...; mix deps.get; mix test

Expected behavior

Same as with elixir 1.18.4: tests should pass:

> mix test
...
Compiling 8 files (.ex)
Generated websockex app
Running ExUnit with seed: 111149, max_cases: 32

.....................................................................................................................................................................................................................
Finished in 2.9 seconds (2.9s async, 0.04s sync)
213 tests, 0 failures

[wojtekmach]

I was able to reproduce it like this:

$ mix new foo
$ cd foo
$ mkdir test/support
$ touch test/support/foo.ex
$ cat<<EOF > mix.exs
defmodule Foo.MixProject do
  use Mix.Project

  def project do
    [
      app: :foo,
      version: "0.1.0",
      elixir: "~> 1.20-dev",
      start_permanent: Mix.env() == :prod,
      elixirc_paths: [~c"lib", ~c"test/support"]
    ]
  end
end
EOF

Changing:

- elixirc_paths: [~c"lib", ~c"test/support"]
+ elixirc_paths: ["lib", "test/support"]

fixes the issue. See https://github.com/Azolo/websockex/blob/4a94f6870528f45d64cdd47bd4374faf52528466/mix.exs#L21.