defimpl for: Any introduces spurious compile-connected dependencies

Elixir: 1.19.5 (compiled with Erlang/OTP 28) Erlang/OTP: 28.4.1

Project structure

lib/
├── presentable.ex          # Protocol with @fallback_to_any true
├── presentable/
│   ├── any.ex              # defimpl for Any
│   ├── cat.ex              # defimpl for Models.Cat
│   └── dog.ex              # defimpl for Models.Dog
├── models/
│   ├── cat.ex              # Simple struct
│   └── dog.ex              # Simple struct
└── protocol_fallback_bug.ex

Reproducing

mix compile
mix xref graph --label compile-connected --fail-above 0

Output

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

The protocol definition (presentable.ex) has a compile dependency on its Any implementation (any.ex). This is inverted; the protocol should not depend on any of its implementations at compile time.

Expected behavior

Zero compile-connected dependencies. Protocol implementations should only have a runtime or exports dependency on the protocol definition, never the reverse.

Why this matters

The compile dependency from the protocol to any.ex means:

1. Any change to any.ex forces presentable.ex to recompile.
2. Since all implementations depend on the protocol (via exports), they transitively recompile too.
3. In a real project with 23+ implementations, a single change to the Any fallback cascades into recompilation of every implementation file.

This defeats the purpose of protocols as a compile-time decoupling mechanism.

Key finding

The trigger is the existence of defimpl for: Any, not @fallback_to_any true itself. Tested all four combinations:

@fallback_to_any true	defimpl for: Any	Compile-connected deps
No	No	0
Yes	No	0
No	Yes	1
Yes	Yes	1

Note on Elixir 1.18.x

On Elixir 1.18.3 (Erlang/OTP 27.3), the scope of this problem was larger. Every defimpl file gained a compile-connected dependency to the protocol (4 edges in this reproduction, not just 1), and all implementations formed a compile cycle with the protocol through the Any implementation:

lib/presentable/any.ex
└── lib/presentable.ex (compile)
    └── lib/presentable/any.ex (compile)
lib/presentable/cat.ex
└── lib/presentable.ex (compile)
lib/presentable/dog.ex
└── lib/presentable.ex (compile)

It appears that 1.19.x partially fixed this; the implementations no longer have direct compile deps to the protocol, but the inverted compile dependency from the protocol to any.ex remains.