EEx typings mismatch causing dialyzer errors on 1.19

[dkulchenko]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.1] [source] [64-bit] [smp:28:28] [ds:28:28:10] [async-threads:1] [jit:ns]

Elixir 1.19.0 (compiled with Erlang/OTP 28)

Operating system

Debian 13

Current behavior

I was trying to track down why mjml_eex (https://github.com/akoutmos/mjml_eex) started throwing up Dialyzer errors on 1.19, and got it down to a minimal repro using only stdlib.

The following file (incorrectly) reports a dialyzer error on 1.19 but not on 1.18:

defmodule EExDialyzerBug do
  @moduledoc """
  Minimal reproduction for Elixir 1.19 dialyzer issue with EEx.compile_string/2.

  Running `elixir --version` shows: Elixir 1.19.0 (compiled with Erlang/OTP 28)

  ## Issue

  EEx.compile_string/2 accepts [compile_opt] which includes {:engine, module()},
  but internally passes all options to tokenize/2 which only accepts [tokenize_opt].

  1.19's enhanced type checking detects this spec inconsistency.

  ## To reproduce

  1. Create a new Mix project: `mix new test_eex_bug`
  2. Add dialyxir to mix.exs deps:
     {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false}
  3. Add to mix.exs project/0 function:
     dialyzer: [plt_add_apps: [:eex]]
  4. Place this file in lib/
  5. Run: `mix deps.get && mix compile && mix dialyzer`

  ## Expected result

  No dialyzer warnings (the code works correctly at runtime).

  ## Actual result

  Dialyzer reports:
  "Function MACRO-__using__/2 has no local return."
  "The call 'Elixir.EEx':compile_string(...,[{'engine', ...}]) will never return
  since it differs in the 2nd argument from the success typing arguments"
  """

  defmacro __using__(_opts) do
    # This call inside the macro triggers the dialyzer error
    template_ast = EEx.compile_string("<p>Hello <%= @name %>!</p>", engine: EEx.SmartEngine)

    quote do
      def render(assigns) do
        var!(assigns) = assigns
        unquote(template_ast)
      end
    end
  end
end

defmodule EExDialyzerBug.User do
  use EExDialyzerBug
end

On 1.19, shows:

lib/dialyzer_eex_bug.ex:35:24: The call 'Elixir.EEx':compile_string(<<60,112,62,72,101,108,108,111,32,60,37,61,32,64,110,97,109,101,32,37,62,33,60,47,112,62>>,[{'engine', 'Elixir.EEx.SmartEngine'}]) will never return since it differs in the 2nd argument from the success typing arguments: (binary(),[{'column',non_neg_integer()} | {'file',binary()} | {'indentation',non_neg_integer()} | {'line',non_neg_integer()} | {'trim',boolean()}])

This is because EEx.compile_string/2 accepts [compile_opt] which includes {:engine, module()}, but internally passes all options to tokenize/2 which only accepts [tokenize_opt].

Expected behavior

The same code on 1.18.4, reports:

Total errors: 0, Skipped: 0, Unnecessary Skips: 0
done in 0m1.5s
done (passed successfully)

[josevalim]

@dkulchenko would you like to send a PR?

[dkulchenko]

@dkulchenko would you like to send a PR?

Sure thing, sent :) #14835