Warn about defining implicit alias modules with aliases that are taken

[ulissesalmeida]

Environment

• Elixir & Erlang/OTP versions (elixir --version): 1.12.1
• Operating system: MacOS BigSur

Current behaviour

For example, initially, you have:

defmodule Post do
  defmodule Comment do
  end
end

That will generate Post and Post.Comment.

But if later, someone adds an alias to a different module:

defmodule Post do
  alias Utils.Comment

  defmodule Comment do
  end
end

The alias for Post.Comment will not exist anymore.

Expected behaviour

I would expect a failure because Utils.Comment is taken or a warning that Comment is an alias to something else.

The conversation about it started in an Ecto mailing list: https://groups.google.com/g/elixir-ecto/c/rxZT_kzPp2o

[eksperimental]

What is the expected behavior?

[ulissesalmeida]

@eksperimental I would expect a warning for nested module definitions when the given module name is an alias defined previously.

Something like:

"warning: Redefining alias Utils.Comment to Post.Comment:5"

[josevalim]

Moved to https://github.com/elixir-ecto/ecto/issues/4124