Dialyzer issue demo
This project demonstrates an unusual Dialyzer failure, where generated clauses that can't match still trigger errors.
Running mix dialyzer on either master or phoenix-1.5 will give the following error:
Jones-MBP:blog_umbrella mattjones$ mix dialyzer
==> blog_web
Compiling 2 files (.ex)
Finding suitable PLTs
Checking PLT...
[:asn1, :blog, :compiler, :connection, :cowboy, :cowlib, :crypto, :db_connection, :decimal, :ecto, :ecto_sql, :eex, :elixir, :ex_unit, :file_system, :gettext, :jason, :kernel, :logger, :mime, :mix, :phoenix, :phoenix_ecto, :phoenix_html, :phoenix_live_dashboard, :phoenix_live_reload, :phoenix_live_view, :phoenix_pubsub, :plug, :plug_cowboy, :plug_crypto, :postgrex, :public_key, :ranch, :runtime_tools, :ssl, :stdlib, :telemetry, :telemetry_metrics, :telemetry_poller]
PLT is up to date!
No :ignore_warnings opt specified in mix.exs and default does not exist.
Starting Dialyzer
[
check_plt: false,
init_plt: '/Users/mattjones/src/blog_umbrella/_build/dev/dialyxir_erlang-22.2.4_elixir-1.10.0_deps-dev.plt',
files: ['/Users/mattjones/src/blog_umbrella/_build/dev/lib/blog/ebin/Elixir.Blog.Application.beam',
'/Users/mattjones/src/blog_umbrella/_build/dev/lib/blog/ebin/Elixir.Blog.Repo.beam',
'/Users/mattjones/src/blog_umbrella/_build/dev/lib/blog/ebin/Elixir.Blog.beam',
'/Users/mattjones/src/blog_umbrella/_build/dev/lib/blog_web/ebin/Elixir.BlogWeb.Application.beam',
'/Users/mattjones/src/blog_umbrella/_build/dev/lib/blog_web/ebin/Elixir.BlogWeb.Endpoint.beam',
...],
warnings: [:race_conditions, :no_undefined_callbacks, :unknown]
]
Total errors: 1, Skipped: 0, Unnecessary Skips: 0
done in 0m1.99s
lib/blog_web/controllers/page_controller.ex:1:pattern_match
The pattern can never match the type.
Pattern:
_ = %Plug.Conn{:halted => true}
Type:
%{:halted => false, _ => _}
________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2
The offending clause is generated by Plug.Builder:
But that should be subsequently marked as generated: true right below the quote block.
In my production app, this happens in one controller file without any explicit setting of halted: false, but I haven't been able to reproduce that in isolation.