`defimpl for: Any` introduces a compile dependency from the protocol to its implementation

[ribanez7]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.3] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] [dtrace]

Elixir 1.19.5 (compiled with Erlang/OTP 28)

Operating system

any

Current behavior

defimpl for: Any creates an inverted compile dependency from the protocol definition to the Any implementation, forming a compile cycle. This causes changes to any.ex to force recompilation of the protocol, and transitively all other implementations.

Protocol:

defprotocol Presentable do
  @fallback_to_any true

  @spec to_type(t()) :: map() | nil
  def to_type(entity)
end

Struct:

defmodule Models.Cat do
  defstruct [:name]
end

Implementation:

defimpl Presentable, for: Models.Cat do
  def to_type(%Models.Cat{name: name}), do: %{kind: "cat", name: name}
end

Any implementation:

defimpl Presentable, for: Any do
  def to_type(_entity), do: nil
end

Running mix xref graph --label compile-connected --fail-above 0 shows:

lib/presentable.ex
└── lib/presentable/any.ex (compile)
** (Mix) Too many references (found: 1, permitted: 0)

The trigger is the existence of defimpl for: Any, not @fallback_to_any true itself. Removing the Any implementation while keeping @fallback_to_any true results in zero compile-connected dependencies.

You can check the repo created to reproduce the issue here: https://github.com/ribanez7/protocol_fallback_bug

Note: On Elixir 1.18.3 (Erlang/OTP 27.3) the scope was larger; every defimpl file gained a compile-connected dependency to the protocol (4 edges instead of 1). It appears 1.19.x partially fixed this but the inverted dependency from the protocol to any.ex remains.

Expected behavior

defimpl for: Any should not add a compile dependency from the protocol to the implementation. I expect to run mix xref graph --label compile-connected --fail-above 0 and have zero compile-connected dependencies.