Improve type inference for recursive protocols

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#3,829 1 comment 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Stale
Tech stack
python
Domain
compilers

Research direction

Start by reproducing the recursive Protocol and Generic example in the issue and confirm the inferred UninhabitedType. Read the discussion and context from follow-up issue #3132, then trace the type-inference path responsible for structural inference cycles. Done means the example infers int without regressing related recursive protocol cases.

Written by the indexing model from the issue text.

Description

bug priority-1-normal topic-protocols topic-recursive-types

Consider this example:

class P(Protocol[T_co]):
    def meth(self) -> P[T_co]: ...

class C(Generic[T]):
    def meth(self) -> C[T]: ...

def fun(arg: P[T]) -> T: ...
x: C[int]
reveal_type(f(x))  # I think this should be 'int'

But currently the inferred type is UninhabitedType, since we don't find any constraints for T due to a purely structural inference cycle. Unfortunately, it looks like this doesn't have simple solutions.

This is a follow-up for #3132

Dominant language
Python
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