Inconsistency between %Mod{} and Mod.__struct__() when delegating

[mguimas]

Environment

$ elixir --version
Erlang/OTP 21 [erts-10.2.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Elixir 1.8.1 (compiled with Erlang/OTP 21)

Current behavior

Given the file xx.ex

defmodule XX do
  defstruct [:field]
end

the following holds for XX

iex(2)> %XX{}
%XX{field: nil}
iex(3)> XX.__struct__()
%XX{field: nil}
iex(4)> struct(XX)
%XX{field: nil}
iex(5)> %XX{} == XX.__struct__()
true

but given the file yy.ex

defmodule YY do
  defdelegate __struct__, to: XX
  defdelegate __struct__(x), to: XX
end

the follwing happens for YY

iex(6)> %YY{}
%YY{field: nil}
iex(7)> YY.__struct__()
%XX{field: nil}
iex(8)> struct(YY)
%XX{field: nil}
iex(9)> %YY{} == YY.__struct__()
false

in which different idioms that are expected to be equivalent return inconsistent results.

Expected behavior

The expected behavior is for %Mod{} to be equivalent to Mod.__struct__(), consequently the function Kernel.SpecialForms.%/2 must be fixed to return the value of Mod.__struct__/0 (and of Mod.__struct__/1 as applicable).

The defdelegate construct is working properly because it returns the result of XX.__struct__/{0,1} as the result of YY.__struct__/{0,1}, as expected for delegation.

This means that somehow Kernel.SpecialForms.%/2 is overwritting the result returned by YY.__struct__/{0,1}, and it must not, and instead pass it "as is".

Note that fixing Kernel.SpecialForms.%/2 additionally solves the case when XX is an Ecto schema. For example, given

defmodule XX do
  use Ecto.Schema
  schema "table" do
    field :field
  end
end

the following is happening now

iex(11)> %YY{}
%YY{__meta__: #Ecto.Schema.Metadata<:built, "table">, field: nil, id: nil}
iex(12)> %YY{} |> IO.inspect(structs: false, pretty: true)
%{
  __meta__: %{
    __struct__: Ecto.Schema.Metadata,
    context: nil,
    prefix: nil,
    schema: XX,
    source: "table",
    state: :built
  },
  __struct__: YY,
  field: nil,
  id: nil
}
%YY{__meta__: #Ecto.Schema.Metadata<:built, "table">, field: nil, id: nil}

where the __meta__ refers to XX, and not YY.

The solution should be to not fix in Ecto also, but to fix in Kernel.SpecialForms.%/2 instead, as doing the latter solves all the reported cases.

Thanks

[fertapric]

I'm currently working on this one 🙂