alias overrriding even with Elixir. prefix

[josevalim]

I wanted to see how this alias overwriting work in case of external submodules and I've found something odd

defmodule Parent do
        alias OtherParent.Child
        def a1, do: IO.puts "Parent " <> inspect(__ENV__.aliases)
        defmodule Elixir.Child do
          def a2, do: IO.puts "Elixir.Child " <> inspect(__ENV__.aliases)
        end
        def a3, do: IO.puts "Parent " <> inspect(__ENV__.aliases)
end

iex(3)> Parent.a1
Parent [{Child, OtherParent.Child}]
:ok
iex(4)> Child.a2
Elixir.Child []
:ok
iex(5)> Parent.a3
Parent []
:ok

So it removes the alias. This does not happen for nested external submodule e.g. Elixir.Child.One

Originally posted by @lukaszsamson in #12451 (comment)

[sabiwara]

I think what's happening is:

• defmodule Elixir.Child starts expanding an alias:

  elixir/lib/elixir/lib/kernel.ex

  Line 4925 in 8a0961c

  unquote(with_alias)

  ({:alias, [defined: Child, context: Parent, line: 4], [Child, [as: nil, warn: false]]})
• we call this clause which cleans the alias ([{'Elixir.Child','Elixir.OtherParent.Child'}] -> [])
• this clause is what makes this alias removal test pass

I'm not sure what the right fix is, we need to be able to distinguish between the legit alias removal and the case where it is triggered by a nested defmodule.

[josevalim]

@sabiwara I would argue that we should not do an alias if the module starts with Elixir., because the point of Elixir. is to bypass aliases altogether. :)

Do you agree @lukaszsamson?

[lukaszsamson]

I would argue that we should not do an alias if the module starts with Elixir., because the point of Elixir. is to bypass aliases altogether. :)

It sounds reasonable.

@josevalim when we are at it let's take a look on another non trivial example. Does this behave correctly? It seems __MODULE__.Child expands to Elixir.Parent.Child and not Parent.Child and does not set an alias {Parent, Parent.Parent}.

defmodule Parent do
    alias OtherParent.Child
    def a1, do: IO.puts "Parent " <> inspect(__ENV__.aliases)
    defmodule __MODULE__.Child do
      def a2, do: IO.puts "__MODULE__.Child " <> inspect(__ENV__.aliases)
    end
    def a3, do: IO.puts "Parent " <> inspect(__ENV__.aliases)
end

iex(2)> Parent.a1
Parent [{Child, OtherParent.Child}]
:ok
iex(3)> Parent.Child.a2
__MODULE__.Child [{Child, OtherParent.Child}]
:ok
iex(4)> Parent.a3
Parent [{Child, OtherParent.Child}]
:ok

[josevalim]

@lukaszsamson it is correct to me, because __MODULE__.Child is also fully qualified.