CompileError in 1.18+ for a defprotocol with a defstruct that derives another protocol

[benstepp]

Elixir and Erlang/OTP versions

Erlang/OTP 27 [erts-15.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Elixir 1.18.1 (compiled with Erlang/OTP 25)

Operating system

macOS 15.2

Current behavior

When compiling a defprotocol with a defstruct that also derives another protocol, there is a compiler error in 1.18+ (1.18.0 and 1.18.1 checked) that did not occur on previous versions.

Example

# hello.exs
defprotocol Hello do
  def name(object)

  # import Kernel
  @derive {Inspect, only: [:name]}
  defstruct [:name]
end

Running elixir hello.exs will cause the following CompileError.

   error: undefined function def/2 (there is no such import)
    │
  6 │   defstruct [
    │   ^^^^^^^^^^^
    │
    └─ hello.exs:6: Inspect.Hello (module)

** (CompileError) hello.exs: cannot compile module Inspect.Hello (errors have been logged)
    hello.exs:6: (file)
    (elixir 1.18.1) hello.exs:6: Inspect.__deriving__/2

A workaround for the CompileError can be to uncomment the import Kernel right before deriving.

This can also be seen with any other derived protocol

defprotocol Hello do
  def name(object)

  @derive JSON.Encoder
  defstruct [:name]
end

---

defimpl inside vs outside defprotocol body

After some more investigation on protocol implementation unrelated to deriving the following does not work in 1.17 or 1.18, and maybe that was always intended to be the case. (unless you import Kernel for the correct def somewhere)

defprotocol Hello do
  def name(object)

  defstruct [:name]

  # import Kernel
  defimpl Inspect do
    # import Kernel
    def inspect(_struct, _opts) do
      "hello"
    end
  end
end

    error: undefined function def/2 (there is no such import)
    │
  8 │     def inspect(_struct, _opts) do
    │     ^
    │
    └─ hello.exs:9:5: Inspect.Hello (module)

** (CompileError) hello.exs: cannot compile module Inspect.Hello (errors have been logged)

Moving the defimpl outside of the defprotocol body causes it to work in both 1.17 and 1.18, so maybe defimpl in the body was supposed to work?

defprotocol Hello do
  def name(object)

  defstruct [:name]
end

defimpl Inspect, for: Hello do
  def inspect(_struct, _opts) do
    "hello"
  end
end

Expected behavior

This was discovered when upgrading from 1.17 to 1.18, and it was found that deriving Inspect inside of defprotocol caused a CompileError. The previous behaviour where it compiled without errors would be expected.

After investigation though, I think a little more with protocol implementation is inconsistent. Right now I think it looks something like:

impl method	works in 1.17	works in 1.18
deriving inside defprotocol body	yes	no
defimpl inside defprotocol body	no	no
defimpl outside of defprotocol body	yes	yes
deriving inside defmodule body	yes	yes
defimpl inside defmodule body	yes	yes
defimpl outside of defmodule body	yes	yes

I think defimpl needs to import to the correct Kernel.def to make all of the ways to implement a protocol consistent when used on a defprotocol instead of a defmodule, but I'm not sure.

[josevalim]

A protocol is meant to be an interface and you are not supposed to define a struct inside of it. Similar to how you can’t define functions.

You should decouple them and we will try to make this clearer in the future. :)

[benstepp]

Thanks! If writing this project today (this is over 5 years old at this point) that's how I would organize the code, so I definitely think forcing the separation is a great design decision.