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.

Start by reviewing issue #4409 and the passthrough example in keyring/core.py, along with the referenced get_keyring() and KeyringBackend definitions. Determine how mypy could identify these unambiguous wrappers and define completion as inferring matching parameter and return types without redundant annotations, with any proposed feature flag or decorator specified.