Can not set bitstring value in defstruct in Elixir 1.18, is this a bug?

gsmlg · March 25, 2025, 1:20am

After update elixir to 1.18 gots an error

** (ArgumentError) argument error

This error has no error stack or error line.

After a few hours debug, the error is occurred at this code

defstruct value: <<0::4>>

In version 1.18, bitstring can not be set in defstruct.
This is not in the release note, I’m not sure this is a new feature or bug.

Eiji · March 25, 2025, 3:27am

I have analysed the problem and created a bug report:

github.com/elixir-lang/elixir

Can not set the bitstring value in a struct definition

opened 03:25AM - 25 Mar 25 UTC

Eiji7

### Elixir and Erlang/OTP versions ```bash $ elixir --version Erlang/OTP 27 [er…ts-15.2.3] [source] [64-bit] [smp:32:32] [ds:32:32:10] [async-threads:1] [jit:ns] Elixir 1.18.3 (compiled with Erlang/OTP 27) ``` ### Operating system Gentoo Linux with default/linux/amd64/23.0/desktop/plasma profile and 6.12.16 kernel version. ### Current behavior Defining a struct with default bitstring value raises `ArgumentError` with message `argument error`. It happens only if the size is not a multiple of `8`. The following examples are failing: ```elixir defmodule Example1, do: defstruct value: <<0::1>> defmodule Example2, do: defstruct value: <<0::2>> defmodule Example3, do: defstruct value: <<0::3>> defmodule Example4, do: defstruct value: <<0::4>> defmodule Example5, do: defstruct value: <<0::5>> defmodule Example6, do: defstruct value: <<0::6>> defmodule Example7, do: defstruct value: <<0::7>> defmodule Example9, do: defstruct value: <<0::9>> defmodule Example10, do: defstruct value: <<0::10>> defmodule Example11, do: defstruct value: <<0::11>> defmodule Example12, do: defstruct value: <<0::12>> defmodule Example13, do: defstruct value: <<0::13>> defmodule Example14, do: defstruct value: <<0::14>> defmodule Example15, do: defstruct value: <<0::15>> ``` The following examples are working: ```elixir defmodule Example8, do: defstruct value: <<0::8>> defmodule Example16, do: defstruct value: <<0::16>> defmodule Example24, do: defstruct value: <<0::24>> defmodule Example32, do: defstruct value: <<0::32>> defmodule Example40, do: defstruct value: <<0::40>> defmodule Example48, do: defstruct value: <<0::48>> defmodule Example56, do: defstruct value: <<0::56>> ``` Here is a complete error with stacktrace for `Example1`: ```elixir ** (ArgumentError) argument error (elixir 1.18.3) src/elixir_erl.erl:109: :elixir_erl.elixir_to_erl(<<0::size(1)>>, 0) (elixir 1.18.3) src/elixir_erl.erl:70: :elixir_erl."-elixir_to_erl/2-lc$^1/1-0-"/2 (elixir 1.18.3) src/elixir_erl.erl:71: :elixir_erl.elixir_to_erl/2 (elixir 1.18.3) src/elixir_erl.erl:111: :elixir_erl.elixir_to_erl_cons/2 (elixir 1.18.3) src/elixir_erl.erl:339: :elixir_erl.struct_info/1 (elixir 1.18.3) src/elixir_erl.erl:311: :elixir_erl.add_info_function/6 (elixir 1.18.3) src/elixir_erl.erl:286: :elixir_erl.functions_form/8 iex:15: (file) ``` ### Expected behavior I don't see why bitstring as a default value may be not supported especially that it works in version `1.17.3`. What's interesting is that the below call also does not work: ```elixir iex> :elixir_erl.elixir_to_erl(<<0::size(1)>>, 0) ** (ArgumentError) argument error (elixir 1.17.3) src/elixir_erl.erl:96: :elixir_erl.elixir_to_erl/2 iex:1: (file) ``` which means that either `:elixir_erl.elixir_to_erl/2` does not support non UTF-8 bitstrings or if it's expected then this call is incorrectly used since `Elixir` version `1.18.0-rc.0`. --- I've tried to search for a root issue. I believe I found a commit that introduces this bug: cb2e036. To be even more precise it happened in the first commit of #13984 which is: [792e604](https://github.com/elixir-lang/elixir/pull/13984/commits/792e60421f0aa17fec60a37fd6e5e73789fe8560) 1. Looks like a module paraller checker is now calling a `:elixir_erl.debug_info/4`: https://github.com/elixir-lang/elixir/blob/66c5908619f2ee9b4b1113e0302b00b5a59a5abb/lib/elixir/lib/module/parallel_checker.ex#L65-L79 2. which calls `:elixir_erl.dynamic_form/1`: https://github.com/elixir-lang/elixir/blob/66c5908619f2ee9b4b1113e0302b00b5a59a5abb/lib/elixir/src/elixir_erl.erl#L9-L37 3. which calls `:elixir_erl.functions_form/8`: https://github.com/elixir-lang/elixir/blob/66c5908619f2ee9b4b1113e0302b00b5a59a5abb/lib/elixir/src/elixir_erl.erl#L156-L178 4. (rest is in the stacktrace above) If I'm correct the changes in type system caused a bug in struct definition. Since we want to keep a backwards compatibility we have to add bitstring support for the ` :elixir_erl.elixir_to_erl/2` function. Related forum topic: https://elixirforum.com/t/can-not-set-bitstring-value-in-defstruct-in-elixir-1-18-is-this-a-bug/70121?u=eiji