python / python/typing

Clarification on how Typevars in a generic function should resolve

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topic: documentation
Dominant language
Python
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Description

Hi, regarding the issue opened here:

https://github.com/astral-sh/ty/issues/2255

Can you please provide clarification on how TypeVars in a function like func in the below code should be resolved?

class A:
    pass

class B(A):
    pass

class C(A):
    pass

def func[T: A](x: T, y: T) -> T:
    ...

func(B(), C()) # I think this should raise a type error by type checkers but no type checker (mypy, pyright, etc) raise an error for this

Please look at these comments:
https://github.com/astral-sh/ty/issues/2255#issuecomment-3697606854
https://github.com/astral-sh/ty/issues/2255#issuecomment-3697628015
https://github.com/astral-sh/ty/issues/2255#issuecomment-3698867080
https://github.com/astral-sh/ty/issues/2255#issuecomment-3699067335

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  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Read the linked ty issue #2255 and its cited comments first, then compare their interpretations with the generic-function example here. Document the agreed rule for resolving T in func(B(), C()), including whether the call should be rejected; done means the typing community has a clear, recorded answer.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers, documentation
Issue type
Documentation
Difficulty
5/5
Estimated time
Over a week
Activity status
Quiet
Clarity
Needs clarification
Newbie friendliness
30/100

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