Proposal: infer obvious parameters and return types

Feature

Similar to #4409, but also broader, allow for mypy to infer the parameter and return types for a function that's a passthrough to other typed functions.

Pitch

Consider the case in keyring, where the get_password function is a convenience accessor to (a) resolve the backend and (b) invoke the method of the same name on that backend with the same parameters:

https://github.com/jaraco/keyring/blob/db6896acea942a86d3fbee7e2a556fffb38055ba/keyring/core.py#L54-L56

The get_keyring() is annotated and always returns a KeyringBackend. KeyringBackend is annotated and its get_password always demands str parameters and declares its return type.

As a result, it's unambiguous what the required parameters and return type for core.get_password must be.

In pypa/twine#733, we learned that if a downstream consumer of the library enables disallow_untyped_calls, it will fail on core.get_password unless that function is redundantly decorated with the same parameters and return types of KeyringBackend.get_password.

It would be nice if mypy could infer the types from these unambiguous cases like passthrough functions, possibly gated by a feature flag or a decorator on the function (e.g. @typing.passthrough), and avoid the somewhat messy redundancy that results from hand-copying the types.

まず issue #4409 と keyring/core.py の passthrough の例、および参照されている get_keyring() と KeyringBackend の定義を確認します。mypy がこれらの曖昧さのない wrapper をどのように識別できるかを判断し、冗長なアノテーションなしで一致するパラメーター型と戻り値型を推論できることを完了条件として定義し、提案する feature flag または decorator があれば指定します。