fix: Preserve the types of annotated async functions wrapped with on_exception or on_predicate

[MeisQuietude]

What changed

• Preserve the complete callable type through on_exception and on_predicate while giving the internal async retry wrappers accurate coroutine contracts.
• Add exact sync and async typing regression coverage for mypy, ty, and pyrefly, including strict disallow_any_decorated and unannotated sync functions.

Why

Version 2.4.0 reconstructs decorated async callables in a way that exposes Coroutine[Any, Any, T] to mypy and fails disallow_any_decorated. An identity transformation preserves the public callable contract without an overlapping async overload that can make pyrefly misclassify unannotated sync functions.

Fixes #200

[read-the-docs-community]

Documentation build overview

📚 backoff | 🛠️ Build #34282625 | 📁 Comparing c186219 against latest (0430374)

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1 file changed

± api/reference.html

[edgarrmondragon]

Thanks @MeisQuietude, just a couple of comments

[edgarrmondragon]

Does this actually exercise something? i.e. does this fail somehow without the decorator typing changes? Otherwise, it's not clear to me what this tests.

[MeisQuietude]

This covers the sync side of the decorator typing change. Pyrefly infers str for these unannotated sync functions, and the decorated calls must preserve that inferred type instead of becoming coroutines. The strict mypy test cannot cover this case because it requires explicit return annotations.

it guards against an alternative overload-based implementation of this fix.

For this sync function:

@backoff.on_exception(backoff.expo, ValueError)
def load_value(value: str):
    return value

load_value("hello").upper()

The results are:

• Callable[[_CallableT], _CallableT]: Pyrefly preserves the inferred sync return type, so this passes.
• Async-first + generic-sync overloads: Pyrefly resolves the call as a coroutine and reports Coroutine has no attribute upper.

This test prevents fixing the async case by accidentally breaking unannotated sync functions.

--

But ofc if you're willing to remove it, just say and I do it. I appeciate your work :)

[edgarrmondragon]

Ah, I see. I did consider exploring the @overload option, so it's good to have in case I forget that it won't work 😅.

Thanks!